NIST Special Publication 800-81-2

Secure Domain Name System(DNS) Deployment Guide
Ramaswamy ChandramouliScott Rose

http://dx.doi.org/10.6028/NIST.SP.800-81-2

C O M P U T E R

S E C U R I T Y

NIST Special Publication 800-81-2

Secure Domain Name System(DNS) Deployment Guide
Ramaswamy ChandramouliComputer Security Division
Information Technology Laboratory
Scott Rose
Advanced Network Technology Division
Information Technology Laboratory

http://dx.doi.org/10.6028/NIST.SP.800-81-2

September 2013

U.S. Department of Commerce
Penny Pritzker, Secretary
National Institute of Standards and Technology
Patrick D. Gallagher, Under Secretary of Commerce for Standards and Technology and Director

Authority
This publication has been developed by NIST to further its statutory responsibilities under theFederal Information Security Management Act (FISMA), Public Law (P.L.) 107-347. NIST is
responsible for developing information security standards and guidelines, including minimum
requirements for Federal information systems, but such standards and guidelines shall not apply
to national security systems without the express approval of appropriate Federal officials
exercising policy authority over such systems. This guideline is consistent with the requirementsof the Office of Management and Budget (OMB) Circular A-130, Section 8b(3), Securing Agency
Information Systems, as analyzed in Circular A-130, Appendix IV: Analysis of Key Sections.
Supplemental information is provided in Circular A-130, Appendix III, Security of Federal
Automated Information Resources.
Nothing in this publication should be taken to contradict the standards and guidelines mademandatory and binding on Federal agencies by the Secretary of Commerce under statutory
authority. Nor should these guidelines be interpreted as altering or superseding the existing
authorities of the Secretary of Commerce, Director of the OMB, or any other Federal official.
This publication may be used by nongovernmental organizations on a voluntary basis and is not
subject to copyright in the United States. Attribution would, however, be appreciated by NIST.
National Institute of Standards and Technology Special Publication 800-81-2
Natl. Inst. Stand. Technol. Spec. Publ. 800-81-2 126 Pages (September 2013)
CODEN: NSPUE2

http://dx.doi.org/10.6028/NIST.SP.800-81-2
Certain commercial entities, equipment, or materials may be identified in this document in order todescribe an experimental procedure or concept adequately. Such identification is not intended to imply
recommendation or endorsement by NIST, nor is it intended to imply that the entities, materials, or
equipment are necessarily the best available for the purpose.
There may be references in this publication to other publications currently under development by NIST
in accordance with its assigned statutory responsibilities. The information in this publication, including
concepts and methodologies, may be used by Federal agencies even before the completion of suchcompanion publications. Thus, until each publication is completed, current requirements, guidelines,
and procedures, where they exist, remain operative. For planning and transition purposes, Federal
agencies may wish to closely follow the development of these new publications by NIST.
Organizations are encouraged to review all draft publications during public comment periods andprovide feedback to NIST. All NIST Computer Security Division publications, other than the ones
noted above, are available at http://csrc.nist.gov/publications.

ii

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

Reports on Computer Systems Technology
The Information Technology Laboratory (ITL) at the National Institute of Standards and Technology(NIST) promotes the U.S. economy and public welfare by providing technical leadership for the Nation’s
measurement and standards infrastructure. ITL develops tests, test methods, reference data, proof ofconcept implementations, and technical analyses to advance the development and productive use of
information technology. ITL’s responsibilities include the development of management, administrative,
technical, and physical standards and guidelines for the cost-effective security and privacy of other thannational security-related information in Federal information systems. The Special Publication 800-series
reports on ITL’s research, guidelines, and outreach efforts in information system security, and its
collaborative activities with industry, government, and academic organizations.

Abstract
The Domain Name System (DNS) is a distributed computing system that enables access to Internet
resources by user-friendly domain names rather than IP addresses, by translating domain names to IP
addresses and back. The DNS infrastructure is made up of computing and communication entities called
Name Servers each of which contains information about a small portion of the domain name space. The
domain name data provided by DNS is intended to be available to any computer located anywhere in theInternet.This document provides deployment guidelines for securing DNS within an enterprise. Because
DNS data is meant to be public, preserving the confidentiality of DNS data. The primary security goals
for DNS are data integrity and source authentication, which are needed to ensure the authenticity of
domain name information and maintain the integrity of domain name information in transit. This
document provides extensive guidance on maintaining data integrity and performing source
authentication. DNS components are often subjected to denial-of-service attacks intended to disrupt
access to the resources whose domain names are handled by the attacked DNS components. This
document presents guidelines for configuring DNS deployments to prevent many denial-of-service
attacks that exploit vulnerabilities in various DNS components.

Keywords
Authoritative Name Server; Caching Name Server; Domain Name System (DNS); DNS Query/Response;
DNS Security Extensions (DNSSEC); Resource Record (RR); Trust Anchor; Validating Resolveriii

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

Acknowledgements

The authors, Ramaswamy Chandramouli and Scott Rose of the National Institute of Standards andTechnology (NIST), wish to thank their colleagues who reviewed drafts of this document. Special thanks
are due for some members of Government DNSSEC working Group who provided useful feedback andpointers to some of the documents referred to in this document. We also thank Tim Grance, program
manager of the Cyber and Network Security program and Doug Montgomery of the Advanced Network
Technologies Division for their leadership and guidance throughout this project. Last but not the least,
we are grateful to Douglas Maughan of the Department of Homeland Security for the sponsorship of thiseffort. The authors would also like to thank those that provided valuable feedback on the original
revision of this Special Publication.

iv

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDETable of Contents
Executive Summary ................................................................................................................. 1
Changes in this Document ...................................................................................................... 4
1.

Introduction .................................................................................................................... 1-1
1.1
1.2
1.3
1.4

2.

Securing Domain Name System.................................................................................... 2-1
2.1
2.2
2.3
2.4

3.

3.3

DNS Query/Response ............................................................................................4-4
Zone Transfer .........................................................................................................4-4
Dynamic Updates ...................................................................................................4-5
DNS NOTIFY..........................................................................................................4-6

DNS Hosting Environment—Threats, Security Objectives, and Protection Approaches
5-1
5.1
5.2
5.3
5.4
5.5
5.6
5.7

6.

DNS Zone Data ......................................................................................................3-1
Name Servers (DNS Software) ...............................................................................3-1
3.2.1 Authoritative Name Servers ....................................................................... 3-2
3.2.2 Caching Name Servers .............................................................................. 3-2
Resolvers (DNS Software)......................................................................................3-2

DNS Transactions .......................................................................................................... 4-4
4.1
4.2
4.3
4.4

5.

What is the Domain Name System (DNS)? ............................................................2-1
DNS Infrastructure ..................................................................................................2-2
DNS Components and Security Objectives.............................................................2-6
Focus of the Document ..........................................................................................2-6

DNS Data and DNS Software ......................................................................................... 3-1
3.1
3.2

4.

Authority .................................................................................................................1-1
Purpose and Scope ................................................................................................1-1
Audience ................................................................................................................1-1
Document Structure................................................................................................1-2

Host Platform Threats.............................................................................................5-1
DNS Software Threats ............................................................................................5-2
Threats Due to DNS Data Contents ........................................................................5-2
Security Objectives .................................................................................................5-3
Host Platform Protection Approach .........................................................................5-3
DNS Software Protection Approach ........................................................................5-3
DNS Data Content Control – Protection Approach..................................................5-3

DNS Transactions—Threats, Security Objectives, and Protection Approaches ....... 6-1
6.1

6.2
6.3
6.4
6.5

DNS Query/Response Threats and Protection Approaches....................................6-1
6.1.1 Forged or Bogus Response ....................................................................... 6-1
6.1.2 Removal of Some RRs............................................................................... 6-2
6.1.3 Incorrect Expansion Rules Applied to Wildcard RRs .................................. 6-2
6.1.4 Protection Approach for DNS Query/Response Threats—DNSSEC .......... 6-2
Zone Transfer Threats and Protection Approaches ................................................6-4
Dynamic Updates Threats and Protection Approaches ...........................................6-5
DNS NOTIFY Threats and Protection Approaches .................................................6-5
Threats Summary ...................................................................................................6-6v

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

7.

Guidelines for Securing DNS Hosting Environment .................................................... 7-1
7.1
7.2

7.3
7.4
8.

Guidelines for Securing DNS Transactions.................................................................. 8-1
8.1

8.2

8.3
9.

Securing DNS Host Platform ..................................................................................7-1
Securing DNS Software ..........................................................................................7-1
7.2.1 Running the Latest Version of Name Server Software ............................... 7-1
7.2.2 Turning off the Version Query .................................................................... 7-2
7.2.3 Running Name Server Software with Restricted Privileges ........................ 7-3
7.2.4 Isolating the Name Server Software ........................................................... 7-3
7.2.5 Dedicated Name Server Instance for Each Function .................................. 7-3
7.2.6 Removing Name Server Software from Nondesignated Hosts ................... 7-4
7.2.7 Network and Geographic Dispersion of Authoritative Name Servers .......... 7-4
7.2.8 Limiting Information Exposure through Partitioning of Zone files ................ 7-5
7.2.9 Limiting Information Exposure Through Separate Name Servers for Different
Clients 7-6
Content Control of Zone File ...................................................................................7-6
Recommendations Summary .................................................................................7-6
Restricting Transaction Entities Based on IP Address ............................................8-1
8.1.1 Restricting DNS Query/Response Transaction Entities .............................. 8-3
8.1.2 Restricting Zone Transfer Transaction Entities ........................................... 8-7
8.1.3 Restricting Zone Transfer in NSD............................................................... 8-8
8.1.4 Restricting Zone Transfer in Windows Server ............................................ 8-8
8.1.5 Restricting Dynamic Update Transaction Entities ....................................... 8-8
8.1.6 Restricting BIND DNS NOTIFY Transaction Entities .................................. 8-9
8.1.7 Restricting NSD DNS NOTIFY Transaction Entities ................................. 8-10
Transaction Protection Through Hash-Based Message Authentication Codes (TSIG)8-10
8.2.1 Key Generation ........................................................................................ 8-13
8.2.2 Defining the Keys in the Communicating Name Servers .......................... 8-13
8.2.3 Defining the Keys in a NSD Configuration File ......................................... 8-14
8.2.4 Instructing Name Servers to Use Keys in All Transactions ....................... 8-14
8.2.5 Checklists for Key File Creation and Key Configuration Process .............. 8-14
8.2.6 Securing Zone Transfers using TSIG ....................................................... 8-15
8.2.7 Securing Dynamic Updates Using TSIG or SIG(0) ................................... 8-15
8.2.8 Configuring Dynamic Update Forwarding Restrictions Using TSIG Keys . 8-16
8.2.9 Configuring Fine-Grained Dynamic Update Restrictions Using TSIG/SIG(0)Keys 8-16
Recommendations Summary ...............................................................................8-18

Guidelines for Securing DNS Query/Response ........................................................... 9-1
9.1
9.2

Enabling DNSSEC processing in BIND & NSD .......................................................9-1
DNSSEC Mechanisms and Operations ..................................................................9-1
9.2.1 Sign and Serve .......................................................................................... 9-1
9.2.2 Verify Signature ......................................................................................... 9-2
9.3 Generation of Public Key-Private Key Pair (DNSSEC-OP1) ...................................9-3
9.3.1 Key Pair Generation—Illustrative Example ................................................. 9-6
9.4 Secure Storage of Private Keys (DNSSEC-OP2) ...................................................9-8
9.5 Publishing the Public Key (DNSSEC-OP3) and Setting Up Trust Anchors (DNSSECOP7) 9-9
9.6 Zone Signing (DNSSEC-OP4) ..............................................................................9-10
9.6.1 Zone Signing—Illustrative Example.......................................................... 9-10vi

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

9.7

Establishing Trust Chain and Signature Verification (DNSSEC-OP8) ...................9-11
9.7.1 Recording and Communicating Results of Signature Verification ............. 9-13
9.8 Additional Protection Measures for DNS Query/Response ...................................9-14
9.9 Dynamic Updates in a DNSSEC-aware Zone .......................................................9-15
9.10 Recommendations Summary ...............................................................................9-18
10. Guidelines for Minimizing Information Exposure Through DNS Data Content Control
10-1
10.1 Choosing Parameter Values in SOA RR ..............................................................10-1
10.2 Information Leakage from Informational RRTypes ................................................10-2
10.3 Using RRSIG Validity Periods to Minimize Key Compromise................................10-2
10.4 Hashed Authenticated Denial of Existance ...........................................................10-3
10.5 Resource Record TTL Value Recommendations ..................................................10-5
10.6 Recommendations Summary ...............................................................................10-6
11. Guidelines for DNS Security Administration Operations ......................................... 11-1
11.1 Organizational Key Management Practices ..........................................................11-1
11.2 Scheduled Key Rollovers (Key Lifetimes) .............................................................11-1
11.2.1 Key Rollover in a Locally Secure Zone ..................................................... 11-2
11.2.2 Key Rollover in a Chained Secure Zone ................................................... 11-3
11.2.3 ZSK Key Rollover in a Chained Secure Zone ........................................... 11-3
11.2.4 KSK Key Rollover in a Chained Secure Zone (Manually without Revoke Bit)
11-3
11.2.5 KSK Key Rollover in a Chained Secure Zone (using the Revoke Bit) ....... 11-5
11.3 Emergency Key Rollovers ....................................................................................11-5
11.3.1 Emergency ZSK Rollover ......................................................................... 11-5
11.3.2 Emergency KSK Rollover ......................................................................... 11-6
11.4 Re-Signing a Zone ...............................................................................................11-6
11.5 DNSSEC Algorithm Migration ...............................................................................11-7
11.5.1 Special Considerations in Key Rollovers with Multiple Signing Algorithms in
Use 11-8
11.6 Special Consideration When Transitioning from NSEC Signed Zones to NSEC3
Signed Zones ................................................................................................................11-9
11.7 DNSSEC in a Split-Zone Deployments ...............................................................11-10
11.7.1 Ideal Solution: Internal Delegation............................................................. 11-10
11.7.2 Deploying DNSSEC in the Ideal Solution Scenario ...................................... 11-11
11.7.3 Usual Solution: Same Zone Name with Different Files ................................. 11-11
11.7.4 Deploying DNSSEC in the Usual Solution Scenario ..................................... 11-11
11.7.5 Considerations for Mobile End Users .......................................................... 11-12
11.8 Recommendations Summary .............................................................................11-12
12. Guidelines on Securing Recursive Servers (Resolver) & Stub Resolvers ............ 12-14
12.1 Setting up the Recursive Service ........................................................................12-14
12.1.1 Threats to Recursive Servers/Resolvers ................................................ 12-14
12.1.2 Securing the Host Platform for Recursive Servers/Resolvers ................. 12-14
12.2 Guidelines for Establishing Servers ....................................................................12-15
12.2.1 Aggregate Caches ................................................................................. 12-15
12.3 Setting up the Root Hints File .............................................................................12-17
12.5 Recommendations Summary ..............................................................................12-18
13. Guidelines on Securing Validating Resolvers ......................................................... 13-20

vii

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

13.1 Enabling DNSSEC Validation .............................................................................13-20
13.1.1 On the Stub Client .................................................................................. 13-20
13.1.2 Configuring DNSSEC on a Windows 7 or Windows 8 System ................ 13-20
13.1.3 Using a Validating Recursive Server ...................................................... 13-20
13.1.4 Recommendations for Providing Service to Mobile Hosts ...................... 13-21
13.2 Establishing Initial Trust Anchors ........................................................................13-22
13.3 Maintaining Trust Anchors ..................................................................................13-23
13.4 Recommendations Summary .............................................................................13-24

List of Appendices
Appendix A— Definitions of Important Terms .................................................................... A-1
Appendix B— Vendor Specific Steps to Meet Checklist Items .......................................... B-1
Appendix C— Acronyms ...................................................................................................... C-1
Appendix D— Bibliography .................................................................................................. D-1

List of Figures
Figure 2-1. Partial DNS Name Space Hierarchy..................................................................... 2-3
Figure 2-2. Name Resolution Process (without cache search) ............................................... 2-5
Figure 9-1. RRSIG RR’s RDATA Field Layout........................................................................ 9-2
Figure 9-2. A Mapping of Islands of Security .......................................................................... 9-9
Figure 10-1. NSEC3 Hash Name Space .............................................................................. 10-4
Figure 11-1: Key Lifecycles Using Multiple Algorithms .......................................................... 11-9
Figure 12-1: Example of Resolver Architecture Using an Aggregate Cache. ....................... 12-16

List of Tables
Table 6-1. DNS Transaction Threats and Security Objectives ................................................ 6-6
Table 8-1. BIND Access Control Statement Syntax for DNS Transactions ............................. 8-2
Table 9-1. Digital Signature Algorithms, Min. Key Sizes, and Crypto Periods ......................... 9-6
Table 9-2. Impact of Trust Anchor in Labeling Responses ................................................... 9-10

viii

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

Executive Summary
The Internet is the world’s largest computing network, with hundreds of million of users. From theperspective of a user, each node or resource on this network is identified by a unique name—the domain
name—such as www.nist.gov. However, from the perspective of network equipment that routes
communications across the Internet, the unique identifier for a resource is an Internet Protocol (IP)address, such as 172.30.128.27. To access Internet resources by user-friendly domain names rather thanIP addresses, users need a system that translates domain names to IP addresses and back. This translationis the primary task of an engine called the Domain Name System (DNS).
The DNS infrastructure is made up of computing and communication entities that are geographically
distributed throughout the world. There are more than 250 top-level domains, such as .gov and .com, andseveral million second-level domains, such as nist.gov and ietf.org. Accordingly, there are many nameservers in the DNS infrastructure, which each contain information about a small portion of the domainname space. The DNS infrastructure functions through collaboration among the various entities involved.
The domain name data provided by DNS is intended to be available to any computer located anywhere in
the Internet.
This document provides deployment guidelines for securing DNS within an enterprise. Because DNS data
is meant to be public, preserving the confidentiality of DNS data pertaining to publicly accessible IT
resources is not a concern. The primary security goals for DNS are data integrity and source
authentication, which are needed to ensure the authenticity of domain name information and maintain the
integrity of domain name information in transit. This document provides extensive guidance on
maintaining data integrity and performing source authentication. Availability of DNS services and data isalso very important; DNS components are often subjected to denial-of-service attacks intended to disrupt
access to the resources whose domain names are handled by the attacked DNS components. This
document presents guidelines for configuring DNS deployments to prevent many denial-of-service
attacks that exploit vulnerabilities in various DNS components.
DNS is susceptible to the same types of vulnerabilities (platform, software, and network-level) as anyother distributed computing system. However, because it is an infrastructure system for the global
Internet, it has the following special characteristics not found in many distributed computing systems:••

No well-defined system boundaries—participating entities are not subject to geographic or
topologic confinement rules
No need for data confidentiality—the data should be accessible to any entity regardless of theentity’s location or affiliation.

Because of these characteristics, conventional network-level attacks such as masquerading and message
tampering, as well as violations of the integrity of the hosted and disseminated data, have a completely
different set of functional impacts, as follows:••

A masquerader that spoofs the identity of a DNS node can deny access to services for the set ofInternet resources for which the node provides information (i.e., domains served by the node).
This denial is not only for a limited set of clients but for the entire universe of all clients needing
access to those resources
Bogus DNS information provided by a masquerader or intruder can poison the information cacheof the DNS node providing that subset of DNS information (i.e., the name server providing

ES-1

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEInternet access service to the enterprise’s users), resulting in a denial of service to the resources
serviced by it••

Violation of the integrity of DNS information resident on its authoritative source or the
information cache of an intermediary that has accumulated information from several historicalqueries may break the chained information retrieval process of DNS. This could result in either adenial of service for DNS name resolution function or misdirection of users to a harmful set ofillegitimate resources.
If the name resolution data hosted by the DNS system violates content requirements as defined in
DNS standards, it could have adverse impacts such as increased workload on the DNS system, orserving obsolete data that could result in denial of service to Internet resources. In most software,
program data independence (as in conventional Database Management Systems (DBMS))
provides a degree of buffer against adverse impacts due to erroneous data. In the case of DNS, thedata content determines the integrity of the entire system.

Based on these functional impacts, the deployment guidelines for secure DNS presented in this document
broadly consist of the following generic and DNS-specific recommendations:•••

Implement appropriate system and network security controls for securing the DNS hosting
environment, such as operating system and application patching, process isolation, and network
fault tolerance.
Protect DNS transactions such as update of DNS name resolution data and data replication thatinvolve DNS nodes within an enterprise’s control. The transactions should be protected using
hash-based message authentication codes based on shared secrets, as outlined in Internet
Engineering Task Force’s (IETF) Transaction Signature (TSIG) specification.
Protect the ubiquitous DNS query/response transaction that could involve any DNS node in the
global Internet using digital signatures based on asymmetric cryptography, as outlined in IETF’s
Domain Name System Security Extension (DNSSEC) specification.

Enforce content control of DNS name resolution data using a set of integrity constraints that are able toprovide the right balance between performance and integrity of the DNS system. This guide contains
recommendations for securing a DNS name server. Part of those recommendations is deployment of the
DNS Security Extensions (DNSSEC) for zone information. The basic steps to accomplish that part of
security are below:•

Install a DNSSEC capable name server implementation (See Section 7.2.1)•

Check zone file(s) for any possible integrity errors (See Section 10)•

Generate asymmetric key pair for each zone and include them in the zone file (See Section 9.2
and 9.3)•

Sign the zone (See Section 9.6)•

Load the signed zone onto the server•

Configure name server to turn on DNSSEC processing (See Section 9.1)

ES-2

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE•(OPTIONAL) send copy of public key to parent for secure delegation

Note that this guide focuses on authoritative name servers for the most part, but the basic steps of
DNSSEC deployment for caching name servers are below:•

Install a DNSSEC capable resolver implementation (See Section 7.2.1)•

Obtain one or more trust anchors for zones administrator wants validated (See Section 9.7)•

Configure resolver to turn on DNSSEC processing (See Section 9.1)

The rest of the guide covers recommendations for secure configuration and operations of name servers.

ES-3

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

Changes in this Document
The following changes appear in this revision over the initial release of NIST Special Publication 800-81:•

Updated recommendations for cryptographic parameters based on previous NIST Special
Publication 800-57 [800-57P1].

•

Included discussion of NSEC3 Resource Records in DNSSEC•

Discussion of DNSSEC in split view deployments•

Minor fixes of examples and text•

Inclusion of examples based on NSD as well as BIND software packages

ES-4

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

1.

Introduction

1.1

Authority

The National Institute of Standards and Technology (NIST) developed this document in furtherance of its
statutory responsibilities under the Federal Information Security Management Act (FISMA) of 2002,
Public Law 107-347.
NIST is responsible for developing standards and guidelines, including minimum requirements, forproviding adequate information security for all agency operations and assets; but such standards andguidelines shall not apply to national security systems. This guideline is consistent with the requirementsof the Office of Management and Budget (OMB) Circular A-130, Section 8b(3), “Securing Agency
Information Systems,” as analyzed in A-130, Appendix IV: Analysis of Key Sections. Supplemental
information is provided in A-130, Appendix III.
This guideline has been prepared for use by Federal agencies. It may be used by nongovernmentalorganizations on a voluntary basis and is not subject to copyright, though attribution is desired.
Nothing in this document should be taken to contradict standards and guidelines made mandatory andbinding on Federal agencies by the Secretary of Commerce under statutory authority, nor should theseguidelines be interpreted as altering or superseding the existing authorities of the Secretary of Commerce,
Director of the OMB, or any other Federal official.
1.2

Purpose and Scope

This publication seeks to assist organizations in understanding the secure deployment of Domain Name
System (DNS) services in an enterprise. It provides practical, real-world guidance on securing each facetof DNS within an organization based on an analysis of the operating environment and associated threats.
Currently, the DNS is not the target of most attacks, but as hosts become more security aware, andapplications begin to rely on the DNS infrastructure for network operations, the DNS infrastructure will
become a more tempting target. The ultimate goal for DNSSEC is full deployment across the entire
domain tree on the infrastructure side, and implementation in applications that can demand the services
provided by DNSSEC. At present there are no operational nodes in the DNS domain tree that provides
DNSSEC capabilities. Hence the first step towards fully deployment is to provide DNSSEC capability fordomain subtrees that have high security needs. Once DNSSEC capabilities become widely available in
the infrastructure, application developers will be able to develop DNSSEC-aware applications and thus
use DNSSEC as a means for network security.
In this guide, DNSSEC deployment is targeting the DNS infrastructure, not individual hosts. However, as
the infrastructure becomes more secure, DNSSEC will naturally push down to the individual clients thatmake DNS queries. DNSSEC was designed with backward compatibility in mind, so that current
network applications can gain some security from DNSSEC, provided that servers upstream are using
DNSSEC, but in the future, it is hoped all systems (DNS name servers and clients) will be able to perform
at least some of the operations detailed in the DNSSEC specifications and this document.
1.3

Audience

This document has been created for the administrators of DNS deployments, as well as computer security
staff and system administrators who are responsible for performing duties related to DNS.

1-1

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

1.4

Document Structure

The remainder of this document is organized into the following ten major sections:

•
•
•
•
•

Section 2 provides an introduction to DNS and DNS infrastructures. It also discusses the security
objectives of DNS and provides an overview of the aspects of DNS addressed by this document.
Section 3 introduces some basic DNS components, such as the zone file that holds DNS data, andthe name servers and resolvers that provide DNS services.
Section 4 defines the different types of DNS transactions.
Section 5 discusses the threats, security objectives, and protection approaches involving the DNS
hosting environment. Section 6 provides the same types of information for DNS transactions.
Sections 7 and 8 present guidelines for securing the DNS hosting environment and DNS
transactions (except DNS query/response), respectively.

•

Section 9 provides recommendations for securing DNS query/response Transaction.

•

Section 10 focuses on guidance for minimizing information exposure through DNS.

•

Section 11 presents guidelines for DNS security administration operations.

The document also contains appendices with supporting material. Appendix A presents definitions ofimportant DNS security terminology. Appendix B gives information for vendor specific information.
Appendix C contains an acronym list, and Appendix D contains a bibliography.

1-2

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

2.

Securing Domain Name System

This document provides deployment guidelines for securing the Domain Name System (DNS) in an
organization. The deployment guidelines follow from an analysis of security objectives and consequent
protection approaches for all DNS components. The rationale for security objectives and mechanics fordevelopment of deployment guidelines are given below:••

Security objectives for each DNS component are developed on the basis of analysis of operating
environment and associated threats.
Secure deployment guidelines for each DNS component are provided through a combination ofconfiguration options and checklists that are based on policies or best practices.

The primary security specifications (with associated mechanisms) for which this document provides
deployment guidelines are as follows:

•
•
2.1

Internet Engineering Task Force (IETF) Domain Name System Security Extensions (DNSSEC)
specifications, covered by Request for Comments (RFC) 4033, 4034, 4035, and 3833 [RFC4033],[RFC4034], [RFC4035], [RFC3833].
IETF Transaction Signature (TSIG) specifications, covered by RFCs 2845 and 3007 [RFC2845],[RFC3007].
What is the Domain Name System (DNS)?

The Internet is the world’s largest computing network, with more than 580 million users. From theperspective of a user, each node or resource on this network is identified by a unique name: the domainname. Some examples of Internet resources are:•

Web servers—for accessing Web sites•

Mail servers—for delivering e-mail messages•

Application servers—for accessing software systems and databases remotely.

From the perspective of network equipment (e.g., routers) that routes communication packets across theInternet, however, the unique resource identifier is the Internet Protocol (IPv4 or IPv6) address,
represented as a series of four numbers separated by dots (e.g., 123.67.43.254). To access Internet
resources by user-friendly domain names rather than these IP addresses, users need a system thattranslates these domain names to IP addresses and back. This translation is the primary task of an engine
called the Domain Name System (DNS).
Users access an Internet resource (e.g., a Web server) through the corresponding client or user program(e.g., a Web browser) by typing the domain name. To contact the Web server and retrieve the appropriate
Web page, the browser needs the corresponding IP address. It calls DNS to provide this information. Thisfunction of mapping domain names to IP addresses is called name resolution. The protocol that DNS uses
to perform the name resolution function is called the DNS protocol.
The DNS function described above includes the following building blocks. First, DNS should have a datarepository to store the domain names and their associated IP addresses. Because the number of domainnames is large, scalability and performance considerations dictate that it should be distributed. The

2-1

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

domain names may even need to be replicated to provide fault tolerance. Second, there should be softwarethat manages this repository and provides the name resolution function. These two functions (managing
the domain names repository and providing name resolution service) are provided by the primary DNS
component, the name server. There are many categories of name servers, distinguished by type of dataserved and functions performed. To access the services provided by a DNS name server on behalf of user
programs, there is another component of DNS called the resolver. There are two primary categories ofresolvers (caching/recursive/resolving name server and stub resolver),1 distinguished by functionality.
The communication protocol; the various DNS components; the policies governing the configuration ofthese components; and procedures for creation, storage, and usage of domain names constitute the DNS
infrastructure.
2.2

DNS Infrastructure

The DNS infrastructure is made up of computing and communication entities that are geographically
distributed throughout the world. To understand this DNS infrastructure, it is necessary to examine firstthe structure behind the organization of domain names. The domain name space (the universe of alldomain names) is organized in the form of a hierarchy. The topmost level in the hierarchy is the root
domain, which is represented as a dot (“.”). The next level in the hierarchy is called the top-level domain(TLD). There is only one root domain, but there are many TLDs. Each TLD is called a child domain of
the root domain. In this context, the root domain is the parent domain because it is one level above a
TLD. Each TLD, in turn, can have many child domains. The children of TLDs are called second-level orenterprise-level domains.
In a domain name representation, the symbol for the root domain usually is omitted. For example,
consider the domain name marketing.example.com. The rightmost label in this domain name (“com.”) is a
TLD. The next label to the left (“example”) is the second-level or enterprise-level domain. The leftmost
label (“marketing”) is the third-level domain. It also is possible to have a fourth-level domain, fifth-leveldomain, and so forth. Because each of the labels in marketing.example.com is called a domain (TLD,
second-level domain, third-level domain, etc.), the concatenation of all these labels from the current levelto the TLD is a fully qualified domain name (FQDN). In this document, however, the FQDN is referred toas simply a domain name, and the level name is used to identify individual labels.
There is only one root domain. There are several hundreds (possibly soon to be thousands) of TLD's,
categorized into the following three types:•••

Country-code TLDs (ccTLDs)—domains associated with countries and territories. There are
more than 240 ccTLDs. Examples include .uk, .in, and .jp.
Sponsored generic TLDs (gTLDs)—specialized domains with a sponsor representing acommunity of interest. These TLDs include .edu, .gov, .int, .mil, .aero, .coop, and .museum.
Unsponsored generic TLDs (gTLDs)—domains without a sponsoring organization. The list of
unsponsored gTLDs includes .com, .net, .org, .biz, .info, .name, and .pro.

There are several million enterprise-level (second-level or lower) domains. In fact, as of December 2008
there were more than 77 million registered domain names in the .com gTLD alone. A partial DNS namespace hierarchy is shown in Figure 2-1.

1

The caching/recursive/resolving name server is included under the category of resolver since it plays the dual role of being a
resolver and a name server.

2-2

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

Root “.”.gov

nist.gov.com

dhs .gov

example.com

.uk

ac.uk

.arpa

co.u kin -addr.arpa

TLD

Enterpri s eLevel
DomainFigure 2-1. Partial DNS Name Space Hierarchy

There are many name servers in the DNS infrastructure. Each name server contains information about aportion of the domain name space. Name servers are associated with levels as far as the first three levelsof domain name space are concerned. There are 13 name servers associated with the root level; they arecalled root servers. Two of the root servers are currently run by the U.S private-sector corporation
VeriSign; the rest are operated by other organizations around the world as a service to the Internet
community. The organizations that run name servers associated with a TLD are called registries.
Generally, ccTLDs are run by designated registries in the respective countries, and global registries run
gTLDs. For example, VeriSign currently manages the name servers for the .com and .net TLDs, anonprofit entity called Public Internet Registry (PIR) manages the name servers for the .org TLD, andanother nonprofit organization called EDUCAUSE manages the name servers for the .edu TLD. All ofthese registry organizations are subject to change, however. Domain registrants should be familiar withthe points of contact (called registrars) for their particular registry. The name servers associated withenterprise-level domains and below are either run directly by the organizations that own those domains or
outsourced to Internet service providers (ISP) or other service providers.
The DNS infrastructure functions through collaboration among the various entities involved(organizations that manage root servers, registries that run TLDs, etc.) A nonprofit, private-sector
corporation, the Internet Corporation for Assigned Names and Numbers (ICANN), acts as the technicalcoordination body for aspects of DNS. For example, ICANN formulates policies for management of root
servers. ICANN also is the authority for creation of new TLDs. ICANN was established in 1998 by the
U.S. Department of Commerce.
A user (i.e., an individual or corporation) wishing to register a domain name (which can only be an
enterprise-level domain under a TLD) must contact an authorized entity called a registrar (which may
charge a fee, depending on the TLD in question). Registrars are companies that are authorized to registerdomain names in a particular TLD (or even in a sub-domain of a TLD e.g. co.uk.) to end-users. There areregistrars all over the world. When the registrar receives the user’s registration request, the registrar
verifies that the name is available by checking with the registry that manages the corresponding TLD (orsub-domain under the TLD). If the name is available, the registrar registers the name with the appropriate
registry. The registry then adds the new name to its registry database and publishes the new name in DNS.
Note that in some domains (some country codes and gTLDs for example), the same organization acts as
the registry and registrar for names in the domain. There is no middle organization that registers domainnames on behalf of the domain holder.

2-3

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

Organizations that register and obtain an enterprise-level domain name often have to create child domains
to properly identify Internet resources associated with various functional units. For example, the owner ofthe domain name example.com might create the subdomain shipping to create and identify resources
associated with the shipping department of the organization. Similarly, many other subdomains (in thiscontext, third-level domains) may be created to properly identify all of the Internet resources of theorganization. Often, however, in any one organization (that is, the owner of a second-level domain) there
will be many third-level domains but few Internet resources (Web servers, application servers, etc.) ineach of these domains. Hence, it is not economical to assign a unique name server for each of these thirdlevel and lower-level domains. Furthermore, it is administratively convenient to group all information
pertaining to an organization’s primary domain (i.e., a second-level or enterprise-level domain) and all its
subdomains into a single resource and manage it as a unit.
To facilitate this grouping, the DNS defines the concept of a zone. A zone may be either an entire domain
or a domain with one or more subdomains. A zone is a configurable entity within a name server underwhich information on all Internet resources pertaining to a domain and a selected set of subdomains is
described. Thus, zones are administrative building blocks of the DNS name space just as domains are thestructural building blocks. As a result, the term zone commonly is used even to refer to a domain that is
managed as a standalone administrative entity (e.g., the root zone, the .com zone). This document uses theterm zone to refer to a resource entity that contains domain name information about one or more domains
and is managed as a single administrative entity. In other words, the zone is the configurable resource
inside a deployed name server installation that contains the domain name information.
With this overall knowledge of DNS infrastructure (name servers and resolvers), domain names, zones,
name servers of various levels (i.e., root servers and TLD servers) and resolvers, the name resolutionfunction can now be defined in more detail. When a user types the URL www.example.com into a Web
browser, the browser program contacts a type of resolver called a stub resolver that then contacts a local
name server (called a recursive name server or resolving name server). The resolving name server will
check its cache to determine whether it has valid information (the information is determined to be valid
on the basis of criteria described later in this document) to provide IP address for the accessed Internet
resource (i.e., www.marketing.example.com). If not, the resolving name server checks the cache todetermine whether it has the information regarding the name server for the zone marketing.example.com(since this is the zone that is expected to contain the resource www.marketing.example.com). If the nameserver’s IP address is in the cache, the resolver’s next query will be directed against that name server. If
the IP address of the name server of marketing.example.com is not available in the cache, the resolver
determines whether it has the name server information for a zone that is one level higher thanmarketing.example.com (i.e., example.com). If the name server information for example.com is notavailable, the next search will be for the name server of the .com zone in the cache.
If the complete search of the cache (as described above) does not yield the required information, theresolving name server has no alternative but to start its search by querying the name server in the topmost
zone in the DNS name space hierarchy (i.e., the root server). If the cache search is successful, theresolving name server has to query the name servers in one of the levels below the root zone (in thiscontext, either marketing.example.com, example.com, or .com). Because the set of iterative queries
starting from the root server subsumes the set starting from any of the lower-level servers, this section
describes the name resolution process starting from the query sent to the root server by the resolving
name server at the enterprise-level.
Contact with the root servers is enabled by a file called the “root hints” file that is usually present in every
name server in DNS. The root hints file contains the IP address of one or more of the 13 root servers. The
root server will contain information about the name servers for its child zones (i.e., TLDs). A TLD (e.g.,.com) will contain name server information about its child zones (e.g., example.com). The name server

2-4

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

information about its child zones that is carried in a zone is called delegation information. The delegation
information is the one that is used by a zone to refer name resolution requests for a resource lower than itin the domain name hierarchy. Since the name resolution request in this example pertains to a resource inthe third-level domain, the root server must refer the request to a lower-level name server. The responseto the resolving name server that involves sending this delegation information is called the referral. The
referral provides the name and IP address for the name servers for the TLD zone that is relevant to therequest (i.e., the .com zone) (since the query is for a resource in marketing.example.com). Using thisreferral, the resolving name server then formulates and sends a query to the .com zone name server. This
server will provide the referral for example.com’s name server. If the marketing.example.com domain is
included in example.com’s zone, querying the name server for example.com will provide the IP address
for the resource www.marketing.example.com. A diagram of the name resolution process (without cachesearch operations) is given in Figure 2-2.

Figure 2-2. Name Resolution Process (without cache search)

As the description of the name resolution process makes clear, a name server performs the following
functions:•••

It provides a referral to a child zone.
It provides a mapping from domain name to IP address, called domain name resolution, or IP
address to domain name, sometimes called an inverse resolution, but is actually a standard query
for a different type of data.
It provides an error message if the query is for a DNS entry that does not exist.

2-5

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEThe name server performs these three functions with the same DNS database, which is called a zone file.
A class of information called delegation information contains the name server information for child zones
in a parent zone and is used in performing the referral function. The mapping function is performed by aclass of information in a zone file called authoritative information and is provided by a set of records thatlist the resources in that zone, along with its domain name and its corresponding IP address. Because theresources belong to that zone, the information provided is deemed authoritative. Thus, a zone file contains
two categories of information: authoritative information (information about all resources for all domains
in the zone) and delegation information (information about name servers for child zones). The locations in
the zone file where delegation information appears are called delegation points. The level of a zone file isthe level of the topmost domain for which it contains authoritative information. In the previous example,
the zone file in the name server of example.com is the enterprise-level zone file, and the corresponding
name server is called the enterprise-level name server.
2.3

DNS Components and Security Objectives

Before security objectives can be determined, the building blocks of the DNS need to be specified. DNS
includes the following entities:•

The platform (hardware and operating system) on which the name server and resolvers reside•

The name server and resolver software•

DNS transactions•

DNS database (zone files)•

Configuration files in the name server and resolver.

The media access-level network technology (i.e., Ethernet network connecting a stub resolver and the
local resolving name server) is not included in the definition of DNS.
Confidentiality, integrity, availability, and source authentication are security goals that are common toany electronic system. However, the DNS is expected to provide name resolution information for any
publicly available Internet resource. Hence except for DNS data pertaining to internal resources (e.g.,servers inside a firewall etc), that is provided by internal DNS name servers through secure channels, theDNS data provided by public DNS name servers is not deemed confidential. Hence confidentiality is not
one of the security goals of DNS. Ensuring the authenticity of information and maintaining the integrityof information in transit is critical for efficient functioning of the Internet, for which the DNS provides thename resolution service. Hence, integrity and source authentication are the primary DNS security goals.
Because of the sheer diversity of name server platforms and the underlying networks in which DNS
components (such as name server software and resolver software) reside, preventing all types of denialof-service attacks is not feasible. Some baseline guidelines must be observed, however, to prevent denialof-service attacks that exploit vulnerabilities in the name server platform, zone file data content, andaccess control configuration for certain DNS transactions.
2.4

Focus of the Document

There are three main levels of name servers: root name servers, TLD name servers, and enterprise-level
name servers. This document provides deployment guidelines for securing name servers for the .gov
domain (zone) at the TLD level and for all enterprise-level name servers below the .gov TLD. Thus, the

2-6

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

guidelines cover secure configuration and operation of all name servers in all civilian agencies of the U.S.
Federal Government. The target audience consists of zone (and to some extent, system) administrators
who are responsible for the configuration and operation of these name servers. The guidelines are equallyapplicable, however, to any enterprise-level zone (e.g., mit.edu).
The security mechanisms to which the guidelines in this document apply conform to IETF’s DNSSEC
and TSIG specifications. The guidelines in this document cover policies, configuration options, andchecklists for the following DNS components/associated operations):

•

•

•

DNS hosting environment–

Host platform (O/S, file system, communication stack)–

DNS software (name server, resolver)–

DNS data (zone file, configuration file)

DNS transactions–

DNS query/response–

Zone transfers–

Dynamic updates–

DNS NOTIFY

Security administration–

Choice of algorithms and key sizes (TSIG and DNSSEC)–

Key management (generation, storage, and usage)–

Public key publishing and setting up trust anchors–

Key rollovers (scheduled and emergency).

2-7

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEThis page has been left blank intentionally.

2-8

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE3.

DNS Data and DNS Software

The two primary software components of DNS are the name server and the resolver. The primaryfunctions of the name server are to host the database (called the zone file) containing domain informationand to provide responses to name resolution queries through authoritative responses or referrals. The
primary function of the resolver software is to formulate a name resolution query or series of queries. The
primary DNS data is the zone file (the configuration file is another type of DNS data). Section 3.1
discusses the composition of the zone file, and Sections 3.2 and 3.3 address the functions of various typesof name servers and resolvers, respectively. The discussion of name servers and resolvers is in theenterprise-level context and may not apply to corresponding entities in the root and TLD levels.
3.1

DNS Zone Data

The zone file contains information about various resources in that zone. The information about eachresource is represented in a record called a Resource Record (RR). Because a zone may contain several
domains and several types of resources within each domain, the format of each RR contains fields formaking this identification. Specifically, the RR consists of the following major fields:•

Owner name: the domain name or a resource name•

TTL: time to live in seconds•

Class: at present only one class, IN (denoting Internet), is used•

RRType: type of resource•

RData: information about the resource (depends upon RRType)

Some of the common RRTypes are:•••

A: Address RRType. An RR of this type provides the IP address for a host name (identified usinga FQDN).
MX: Mail Exchanger RRType. An RR of this type provides the mail server host name for adomain.
NS: Name Server RRType. An RR of this type provides a name server host name for a domain.

IETF RFC 1035 provides the complete format of valid RRTypes in DNS [RFC1035]. Because there could
be multiple resources of a given RRType (e.g., several hosts acting as name servers) under the sameowner name and since there is only one class (CLASS) (i.e., IN), the information of interest (e.g., all mailservers (RRType = MX) in a domain) in a zone file is on a combination of RRs that contain the sameowner name, TTL, class, and RRType. The set of RRs with the same owner name, class, and RRType iscalled an RRSet. Hence, logically a zone file is made up of several RRSets.
3.2

Name Servers (DNS Software)

There are two main types of name servers: authoritative name servers and caching name servers. The term
authoritative is with respect to a zone. If a name server is an authoritative source for RRs for a particularzone (or zones), it is called an authoritative name server for that zone (or zones). An authoritative nameserver for a zone provides responses to name resolution queries for resources for that zone, using the RRs

3-1

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

in its own zone file. A caching name server (also called a resolving/recursive name server), by contrast,
provides responses either through a series of queries to authoritative name servers in the hierarchy of
domains found in the name resolution query or from a cache of responses built by using previous queries.
3.2.1

Authoritative Name Servers

There are two types of authoritative name servers: master (or primary) name server and slave (orsecondary) name server. To improve fault tolerance, there could be several slave name servers in anenterprise. A master name server contains zone files that are created and edited manually by the zone
administrator. Sometimes a master name server allows the zone file to be dynamically updated byauthorized DNS clients. A master name server that is configured with this feature usually is called theprimary master name server. A slave (secondary) name server also contains authoritative information fora zone, but its zone file is a replication of the one in the associated master name server. The replication isenabled through a transaction called zone transfer that transfers all RRs from the zone file of a mastername server to the slave name server. Because a name resolution query is for a specific RR, a zonetransfer actually is treated as a category of name resolution query with type code AXFR, which means“all RRs for the zone that is being queried.” Whenever the contents of a zone file are changed in themaster name servers, the slave name servers are notified of the change through a transaction called DNSNOTIFY. When a slave name server receives this request, it initiates a zone transfer request to the mastername server.
3.2.2

Caching Name Servers

A caching name server generally is the local name server in the enterprise that performs the nameresolution function on behalf of the various enterprise clients. A caching name server also is called aresolving/recursive name server. The name resolution function is performed by a caching name server in
response to queries from a stub resolver. The search process involved in name resolution may involve
searching its own cache, recursively querying various authoritative name servers through a set of iterative
queries, or a combination of these methods.
A specific name server can be configured to be both an authoritative and a recursive name server. In thisconfiguration, the same name server provides authoritative information for queries pertaining toauthoritative zones while it performs the resolving functions for queries pertaining to other zones. To
perform the resolving function, it has to support recursive queries. Any server that supports recursive
queries is more vulnerable to attack than a server that does not support such queries (see following
sections). As a result, authoritative information might be compromised. Hence, it is not a good security
practice to configure a single name server to perform both authoritative and recursive functions.
3.3

Resolvers (DNS Software)

Software such as Web browsers and e-mail clients that require access to Internet resources make use of
the DNS client, called the client resolver or stub resolver. The stub resolver formulates a name resolution
query for the resource sought by the Internet-accessing software and sends it to a caching (resolving)name server in the enterprise. Stub resolvers generally are configured to send queries to two or more
resolving name servers to provide some fault tolerance for their operation. A stub resolver often isreferred to as simply a resolver. A caching (resolving) name server that receives a query from a stub
resolver also formulates queries for sending them to authoritative name servers (if it is not able to respondto the query from its cache) and hence also sometimes is referred to as a resolver because it has aresolving component and a caching (name server) component.

3-2

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEA resolver (either stub or part of a caching name server) that performs DNSSEC validation is called avalidating resolver (sometimes validator for short). A validating resolver performs all the same DNS
operations (i.e., formulating name resolution queries) as a stub/caching name server but also attempts tovalidate the responses in the form of DNSSEC signed data when available. In order for a validator toperform its validation, it must first be configured with one or more public keys as trust anchors. See
Section 13 for more information about DNSSEC validation.

3-3

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE4.

DNS Transactions

The most common types of DNS transactions are the following:•

DNS query/response•

Zone transfer•

Dynamic updates•

DNS NOTIFY.

This section describes each of these transaction types.
4.1

DNS Query/Response

This is the most common transaction in DNS. A DNS query originates from a resolver; the destination is
an authoritative or caching name server. The most common query is a lookup for an RR, based on its
owner name or RRType. The response may consist of a single RR, an RRSet, or an appropriate error
message.
As discussed previously, there are two types of queries: iterative and recursive. DNS resolvers that send
iterative queries tend to be more robust with regard to the types of responses they provide because they
may have to follow multiple referrals to obtain the final answer to a given query. If they also have a DNS
cache, they can build up a global view of name servers in the DNS and the responses from previous
queries. This can be used to shorten the turnaround time for future queries. Recursive queries usually aresent from stub resolvers that do not have the capability to handle complex DNS operations. Instead, they
rely on an upstream DNS entity (usually a name server with a cache that sends iterative queries on behalfof a collection of stub resolvers) to perform the query and return the ultimate answer.
DNS queries are sent in a single UDP packet. The response usually is a single UDP packet as well, butdata size may result in truncation—in which case the normal procedure is to reissue the query using TCP.
UDP is preferred because of its lower overhead in consuming resources, and DNS administrators shouldmake sure the zone data in responses do not result in a large percentage of truncated DNS responses.
DNS queries are sent in the clear, and it is assumed that the response received is correct and from anauthentic source. As a result, it is possible for an active attacker to intercept (and alter) or forge responses
back to a querying client. Section 6.1 provides a more detailed examination of threats to the DNS
query/response transaction. Sections 8.1.1 and 9 provide security solutions proposed to address thesethreats.
4.2

Zone Transfer

A zone transfer refers to the way a (secondary) slave server refreshes the entire contents of its zone filefrom the (primary) master servers. This process enables a secondary name server to keep its zone file insynch with its primary name server. A zone transfer transaction starts as a query from a secondary nameserver to a primary name server. A zone transfer query—in contrast to a DNS Query—requests all RRs
from a zone instead of requesting RRs of a given owner name or RRType. A zone transfer query
originates from a secondary name server either in response to a DNS NOTIFY message (see Section 4.4)
or on the basis of the value of the Refresh data item in the RData field of the zone’s Start of Authority(SOA) RR.

4-4

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEA zone transfer process has different security implications because it reveals a lot more information than anormal DNS Query and because of the increased resource (bandwidth and response time) usage of themessage. The threats to a zone transfer transaction are discussed in Section 6.2. The protection
mechanisms are discussed in sections 8.1.2 and 8.2.5.
4.3

Dynamic Updates

As enterprises add, delete, and move around IP-network-based resources (i.e., database servers, Web
servers, mail servers, and even name servers), corresponding changes may need to be made to the zone
file that carries information about the domains where the resources are located. In the early days of the
DNS, DNS zone administrators made such changes manually. When such changes became larger involume and more frequent, however, the manual update process was found to be inadequate, and theconcept of dynamic updates was introduced. Apart from volume and frequency, there are some
applications that require instant automatic updates to the DNS zone file through application programs.
Examples of these applications include Certificate Authority (CA) servers, Dynamic Host Configuration
Protocol (DHCP) servers, and Internet multicast address servers. Dynamic Updates are always made to
the zone file of the primary (master) name server.
In a few instances, the DNS server is used as a repository for public key certificates. In these instances, a
CA server receives a public key from a user, signs it with its private key to generate a certificate, andstores this certificate as a CERT or TLSA [DANE] RR in a DNS server. A DHCP server dynamicallyassigns an IP address to a requesting host, and then adds this information (host and newly assigned IP
address) as an A-type RR in the DNS authoritative name server. The DHCP server also deletes this Atype RR after the IP address is returned by the host. An Internet multicast server selects a new multicast
IP address from the Internet multicast IP address space and assigns it to the newly generated multicast
group. Then the server adds this information (domain name of multicast group, newly selected multicast
address) as an A-type RR in the DNS database so that users can join the multicast group using a user
friendly domain name rather than an IP address.
RFC 2136 [RFC2136] outlines the roadmap for the dynamic update mechanism, which subsequently was
implemented in BIND 8 version and has continued in all versions since.
The dynamic update facility provides operations for addition and deletion of RRs in the zone file.
Because updates to the contents of an existing RR can be accomplished through deletion of the old recordand addition of the new record with the changed contents, no separate update operation is provided.
The suite of operations included in the dynamic update facility consists of the following:

•
•
•
•

Add or delete individual RRs for an existing domain
Delete specific RRsets (a set of resource records with the same owner name, class, and RRType[e.g., a set of RRs with RRType NS for the domain/owner name example.com with the common
class IN of course]) for an existing domain
Delete an existing domain (all resource records for a given domain name [e.g., all RRs for thedomain example.com])
Add a new domain (one or more RRs for a new domain [e.g., adding an A-type RR for a new
domain NYBranch.example.com]).

4-5

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEDNS zones that allow dynamic update open themselves to a host of malicious attacks. The full list of
potential attacks is detailed in Section 6.3. The proposed solution to limit or prevent these attacks is given
in sections 8.1.4, 8.2.7, 8.2.8 and 8.2.9.
4.4

DNS NOTIFY

Whenever changes occur in the zone file of the primary (master) DNS server, the secondary (slave) DNS
servers that are supposed to carry identical data as the primary DNS server must be notified of thechanges. This notification is accomplished through the DNS NOTIFY message, which signals a secondary
DNS server to initiate a zone transfer (see Section 4.2). The DNS NOTIFY message is a much more
efficient and faster way of keeping secondary servers in sync with the primary server than the alternative
approach of secondary servers polling the primary server for changes via the SOA Refresh value timeout.
Sending the DNS NOTIFY message, called a notify operation, is a default operation in BIND 9.x. The
next question that arises is: How does the DNS name server software know to which servers the DNSNOTIFY message must be sent? The default in BIND 9.x is to notify servers that are defined in the NS
RRs for the zone. If there are any additional servers to which the zone administrator wants the DNSNOTIFY message to be sent (e.g., stealth slave server), the DNS administrator can add the IP addresses ofthe other entities in the BIND configuration file. There are configuration options to stop the server fromsending DNS NOTIFY messages to a particular zone or to all zones served by this name server.
Once a secondary server receives a DNS NOTIFY message, it resets the relevant zone’s refresh value tozero, prompting a zone transfer attempt. As in any zone refresh, if the zone serial number in the SOA RR
has not increased (see Section 10.1), the zone transfer does not take place. This procedure allows changes
to the zone to propagate to all name servers more quickly.
Since a DNS NOTIFY message triggers zone transfer, spurious DNS NOTIFY messages could result in
unnecessary zone transfers and hence potential denial of service. The proposed solution for minimizingthese spurious notifications is given in Section 8.1.5 and 8.1.6.

4-6

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEThis page has been left blank intentionally.

4-7

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE5.

DNS Hosting Environment—Threats, Security Objectives, and Protection
Approaches

The DNS hosting environment consists of the following elements:•

Host platform (operating system [OS], file system, communication stack)•

DNS software (name server, resolver)•

DNS data (zone file, configuration file)

This section describes threats and recommended protection approaches for these portions of the hosting
environment.
5.1

Host Platform Threats

Threats to the platform that hosts DNS software are no different from threats that any host in the Internet
faces. These generic threats and their impact—viewed specifically from the point of view of DNS hosts—
are as follows:

•
•

•
•

•

2

Threat T1: The OS, any system software, or any other application software on the DNS host
could be vulnerable to attacks such as buffer overflows, resulting in denial of name resolution
service.
Threat T2: The TCP/IP stack in DNS hosts (stub resolver, caching/resolving/recursive nameserver, authoritative name server, etc.) could be subjected to packet flooding attacks (such as
SYNC and smurf), resulting in disruption of communication. An application layer counterpart ofthis attack is to send a large number of forged DNS queries to overwhelm an authoritative or
resolving name server.
Threat T3: A malicious insider who has access to local area network (LAN) segments where
DNS hosts reside could launch an Address Resolution Protocol (ARP) spoofing attack that
disrupts DNS message flows. 2
Threat T4: The platform-level configuration file that enables communication (e.g., resolv.confand host.conf in Unix platforms) can be corrupted by viruses and worms or subject tounauthorized modifications due to inadequate file-level protections, resulting in breakdown ofcommunication among DNS hosts (e.g., between a stub resolver and a resolving name server,
between a resolving name server and an authoritative name server).
Threat T5: The DNS-specific configuration files (named.conf, root.hints, etc.), data files (zonefile), and files containing cryptographic keys could be corrupted by viruses and worms or
subjected to unauthorized modifications due to inadequate file-level protections, resulting in
improper functioning of name resolution service.

This is not strictly a host threat, but rather a network threat, which is mitigated by placing DNS servers within their own
restricted LAN segments (e.g., via VLANs). Since generic network level threats are outside the scope of this document, thisthreat has been included since it involves a DNS parameter (i.e., IP address).

5-1

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE•

5.2

Threat T6: A malicious host on the same LAN as a DNS client may be able to intercept and/or
alter DNS responses. This would allow an attacker to redirect a client to a different site. This
could be the first action in an attack on a client host.
DNS Software Threats

Threats to the DNS software itself can have serious security impacts. The most common software
problems and the impact of threats against them are as follows:

•
•

5.3

Threat T7: DNS software (name server or resolver) could have vulnerabilities such as buffer
overflows that result in denial of service.
Threat T8: DNS software does not provide adequate access control capabilities for its
configuration files (e.g., named.conf), its data files (e.g., zone file) and files containing signingkeys (e.g., TSIG, DNSKEY) to prevent unauthorized read/update of these files. These capabilities
are provided on top of O/S-file level protection referred to in threats T4 and T5 and may depend
upon the latter.
Threats Due to DNS Data Contents

DNS data is made up of two types: zone files and configuration files. The content of both these types of
DNS data has security ramifications. All the security deployment options discussed in this document
relate to configuration file contents. Security implications due to zone file content are discussed in thesection titled “Guidelines for Minimizing Information Exposure through DNS Content Control” (Section10) and are mostly due to the following aspects of zone data:•

Parameter values for certain key fields in RRs of various RRTypes•

Presence of certain RRs in the zone file.

The various types of undesirable contents in the zone file results in different security exposures andconsequent potential threats as follows:

•

•

•

Threat T9—Lame Delegation: This error occurs when FQDN and/or IP addresses of nameservers have been changed in the child zone but the parent zone has not updated the delegation
information (NS RRs and glue records). In this situation, the child zone becomes unreachable(denial of service).
Threat T10—Zone Drift and Zone Thrash: If the Refresh, and Retry, fields in the SOA RR ofthe primary name server are set too high and the zone file is changed frequently, there may be amismatch of data between the primary and secondary name servers. This error is called zone drift;
it results in incorrect zone data at the secondary name servers. If the Refresh and Retry fields in
the SOA RR are set too low, the secondary server will initiate zone transfers frequently. Thiserror is called zone thrash; it results in more workload on both the primary and secondary nameservers. Such incorrect data or increased workload may result in denial of service.
Threat T11—Information for Targeted Attacks: RRs such as HINFO and TXT provide
information about software name and versions (e.g., for resources such as Web servers and mailservers) that will enable the well-equipped attacker to exploit the known vulnerabilities in those
software versions and launch attacks against those resources.

5-2

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

5.4

Security Objectives

Common objectives with respect to protection of the DNS host platform, DNS software, and DNS data
are integrity and availability.
5.5

Host Platform Protection Approach

The protection/threat mitigation approaches for the DNS host platform consist of the following:•

Running a secure OS•

Secure configuration/deployment of OS.

These approaches are discussed in Section 7.1.
5.6

DNS Software Protection Approach

Best practice protection approaches for DNS software are as follows:•

Running the latest version of name server software, or an earlier version with appropriate patches•

Running name server software with restricted privileges•

Isolating name server software•

Setting up a dedicated name server instance for each function•

Removing name server software from nondesignated hosts•

Creating a topological and geographic dispersion of authoritative name servers for fault tolerance•

Limiting IT resource information exposure through two different zone files in the same physicalname server (termed as split DNS) or through separate name servers for different client classes.

These approaches are described in Sections 7.2.1 through 7.2.9.
5.7

DNS Data Content Control – Protection Approach

Control of undesirable content in the zone file is accomplished by analyzing the contents for security
implications, formulating integrity constraints that will check for the presence of such contents andverifying the zone file data for satisfaction of those constraints. Therefore, the only protection approach is
to develop the zone file integrity checker software that contains the necessary constraints and can be run
against the zone file to flag those contents that violate the constraints. To aid in formulation of
constraints, desirable field values (ranges or lists) in the various RRs of zone file are required. These
constraints need to be developed not only for RRs in an unsigned zone but also for additional RRs in asigned zone (zones that have implemented the DNSSEC specification). Hence, the recommendations forcontrol of content of zone files are deferred to Section 10 after discussion of deployment guidelines for
DNSSEC in Section 9 so that they cover those additional RRs as well.

5-3

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEThe services provided by DNS also face threats resulting from vulnerabilities in network infrastructurecomponents such as routers. Network configuration issues are outside the scope of this guidance
document, however.

5-4

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEThis page has been left blank intentionally.

5-5

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE6.

DNS Transactions—Threats, Security Objectives, and Protection
Approaches

The threats to a DNS transaction depend on the type of transaction. Name resolution queries andresponses (DNS query/response) between DNS clients (stub resolver or resolving name server) and DNS
servers (caching/resolving name server or authoritative name server) could involve any nodes in theInternet; hence, the threats against them are much greater in number and severity compared to those forzone transfer, dynamic update, and DNS NOTIFY transactions. In general, the nodes involved in zonetransfer, dynamic update, and DNS NOTIFY transactions are all within the administrative domain of asingle organization. The only exceptions are instances in which the primary or secondary name servers ofan organization are run on its behalf by ISPs or other organizations. There usually is a preexisting trust
relationship in these cases, however, so it is not difficult to set up a mutual authentication system for DNS
zone transfers.
This section seeks to identify protocol-based threats to the operation and administration of the DNS. Thissection also lists DNS protocol methods to address these threats. Non-DNS protocol based solutions suchas IPsec are beyond the scope of this document but could be a more appropriate solution for an
organizations based on its infrastructure.
6.1

DNS Query/Response Threats and Protection Approaches

DNS name resolution queries and responses (DNS query/response) generally involve single, unsigned,
and unencrypted UDP packets. The known threats to DNS query/response transactions have been
documented in IETF RFC 3833 [RFC3833] and can be classified as follows:•

Threat T12: Forged or bogus response•

Threat T13: Removal of some RRs from the response•

Threat T14: Incorrect expansion rules applied to wildcard RRs in a zone file.

6.1.1

Forged or Bogus Response

A forged or bogus response is a response that is different from the one that is expected from a legitimate
authoritative name server. A bogus response can originate from:•

A compromised authoritative name server (for queries originating from a resolving name server)•

A poisoned cache of a resolving name server (for queries originating from a stub resolver).

An authoritative name server could be compromised by a platform-level attack on its OS orcommunication stack (see Section 5.1).
The cache of a resolving (caching) name server could be poisoned by the following attacks:•

Packet Interception. In this type of attack, the attacker eavesdrops on a request and is able togenerate and send a response by spoofing an authoritative name server before the real response
from the legitimate authoritative name server reaches the resolving name server.

6-1

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE••ID Guessing and Query Prediction. In this type of attack, the attacker guesses the ID field in
the header of the DNS request message (because this field is only 16 bits long, brute force
guessing is possible) and possibly the QNAME and QTYPE (owner name and RRType,
respectively). The attacker then injects bogus data into the network as a response by spoofing a
name server.
Responses Accumulated from a Compromised Authoritative Name Server. A compromised
authoritative name server is directed by a controlling adversary to send out bogus responses toqueries from resolving name servers.

The impacts on a system serviced by a resolving name server that has a poisoned cache are as follows:

•
•

6.1.2

Denial of Service. If some crucial RRs such as address records (A RRs) are forged, the systemthat requires this information can never establish connectivity with the intended node.
Client Redirection through Cache Poisoning. Client redirection is performed by selective
poisoning of DNS RRs whose RDATA element contains a name. Examples of such RRs are
CNAME, NS, and MX. The name resolution (i.e., IP address) information for these names isfound in a set of additional information (or glue records when discussing a delegation response).
Normally the resolving name server obtains these necessary A/AAAA RRs through follow-up
queries (also called triggered queries). The responses flowing into the network from these followup queries present yet another opportunity for the attacker to insert bogus records. First theattacker can introduce arbitrary names of the attacker’s choosing in the RDATA portion of
selected RRs; then the attacker can insert the IP addresses of servers (chosen by the attacker) inassociated glue records that are transmitted as an answer to follow-up queries. This type of attack
on two sets of related responses is called a name chaining attack. The overall effect of poisoning
the cache of a resolving name server this way is to misdirect several clients who are making useof the services of that resolving name server. Redirecting the users to nodes of the attacker’s
choosing may enable the attacker to capture sensitive information such as passwords.
Removal of Some RRs

Apart from injecting bogus or forged data in a response, an attacker also could remove RRs from a
response. This action might result in a name resolution query failure and consequent denial of service.
6.1.3

Incorrect Expansion Rules Applied to Wildcard RRs

Many zones use wildcard RRs to economize on the volume of data in the zone file. The wildcard patterns
are used for synthesizing RRs on the fly in generating responses for name resolution queries. (Thesynthesis rules are outlined in section 4.3.2 of IETF RFC 1034 [RFC1034].) If synthesis rules are applied
incorrectly in a name server, the RRs associated with resources existing in an organization may not be
generated and made available in a DNS response. This fault also results in denial of service.
6.1.4

Protection Approach for DNS Query/Response Threats—DNSSECThe underlying feature in the major threat associated with DNS query/response (i.e., forged response or
response failure) is the integrity of DNS data returned in the response. Hence, the security objective is toverify the integrity of each response received. An integral part of integrity verification is to ensure thatvalid data has originated from the right source. Establishing trust in the source is called data origin
authentication. Hence, the security objectives—and consequently the security services—that are required

6-2

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

for securing the DNS query/response transaction are data origin authentication and data integrity
verification.
These services could be provided by establishing trust in the source and verifying the signature of the data
sent by that source. The specification for a digital signature mechanism in the context of the DNS
infrastructure is in IETF’s DNSSEC standard. The objectives, additional RRs, and DNS message contents
involved in the DNSSEC are specified through RFCs 4033, 4034, and 4035 [RFC4033], [RFC4034],[RFC4035]. In DNSSEC, trust in the public key (for signature verification) of the source is established
not by going to a third party or a chain of third parties (as in public key infrastructure [PKI] chaining), but
by starting from a trusted zone (such as the root zone) and establishing the chain of trust down to thecurrent source of response through successive verifications of signature of the public key of a child by its
parent. The public key of the trusted zone is called the trust anchor.
After authenticating the source, the next process DNSSEC calls for is to authenticate the response. Thisrequires that responses consist of not only the requested RRs but also an authenticator associated withthem. In DNSSEC, this authenticator is the digital signature of an RRSet. The digital signature of an
RRSet is encapsulated through a special RRType called RRSIG. The DNS client using the trusted public
key of the source (whose trust has just been established) then verifies the digital signature to detect if the
response is valid or bogus.
To ensure that RRs associated with a query are really missing in the zone file and have not been removed
in transit, the DNSSEC mechanism provides a means for authenticating the nonexistence of an RR. It
generates a special RR called an NSEC (or NSEC3) RR that lists the RRTypes associated with an ownername as well as the next name in the zone file. It sends this special RR, along with its signature, to theresolving name server. By verifying this signature, a DNSSEC-aware resolving name server candetermine which authoritative owner name exists in a zone and which authoritative RRTypes exist at
those owner names.
To protect against the threat of incorrect application of expansion rules for wildcard RRs, the DNSSEC
mechanism provides a means of comparing the validated wildcard RR against an NSEC (or NSEC3) RR
and thereby verifying that the name server applied the wildcard expansion rules correctly in generating ananswer.
DNSSEC can guarantee the integrity of name resolution responses to DNS clients acting on behalf ofInternet-based resources, provided the clients perform the DNSSEC signature verification. In many cases,
however, these DNS clients are stub resolvers that are not DNSSEC-aware. If signature verification is
performed by the resolving name server providing name resolution service for the clients that are stub
resolvers, the end-to-end integrity of the response data can be guaranteed only by protecting thecommunication channel between the resolving name server and the stub resolver.
IETF’s design criteria consider DNS data to be public; hence, confidentiality is not one of the security
goals of DNSSEC. DNSSEC is not designed to directly protect against denial-of-service threats, although
it does so indirectly by providing message integrity and source authentication. DNSSEC also does notprovide communication channel security because name resolution queries and responses travel overmillions of nodes of the public Internet. DNSSEC also can lead to a new type of weakness that did notexist in DNS before. An artifact of how DNSSEC performs negative responses allows a client to map all
the names in a zone. This is called Zone Walking. Zone Walking provides an attacker with a “map” of atarget zone with all domain names and IP addresses in the zone and enables him/her to determine theconfiguration of the internal network and launch some targeted attacks on some key hosts. Therefore, it isadvisable that a zone only contains zone data that the administrator wants to be made public or use the

6-3

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDENSEC3 RR option for providing authenticated denial of existence. For internal DNS, something like splitDNS (see Section 7.2.8) could be deployed. For a discussion of NSEC3, see Section 10.4.

6.2

Zone Transfer Threats and Protection Approaches

Zone transfers are performed to replicate zone files in multiple servers to provide a degree of fault
tolerance in the DNS service provided by an organization. Threats from zone transfers have not been
documented formally through any IETF RFCs. A few threats could be expected, however: the first threat,
denial of service, is common for any network transaction. The second threat is based on exploitation ofknowledge gained from the information provided by zone transfers. The latter threat is common to any
network packet.

••

Threat T15—Denial of Service: Because zone transfers involve the transfer of entire zones, theyplace substantial demands on network resources relative to normal DNS queries. Errant ormalicious frequent zone transfer requests on the name servers of the enterprise can overload themaster zone server and result in denial of service to legitimate users.
Threat T16—The zone transfer response message could be tampered.

The denial-of-service threat (T15) can be minimized if servers allowed to make zone transfer requests are
restricted to a set of known entities. To configure this restriction into the primary name server, there
should be a means of identifying those entities. Name server software such as BIND initially provided aconfiguration feature to restrict zone transfer requests to a set of designated IP addresses. Because IP
addresses can be spoofed, however, this mode of configuration does not provide an adequate means ofrestricting zone transfer access.
The IETF developed an alternate mechanism called a transaction signature (TSIG), whereby mutual
identification of servers is based on a shared secret key. Because the number of servers involved in zonetransfer is limited (generally restricted to name servers in the same administrative domain of anorganization), a bilateral trust model that is based on a shared secret key may be adequate for mostenterprises (except for very large ones). TSIG specifies that the shared secret key be used not only formutual authentication but also for signing zone transfer requests and responses. Hence, it provides
protection against tampering of zone transfer response messages (threat T16). Protection of DNS dataalone (the payload) in a zone transfer message also can be ensured through verification of signaturerecords accompanying RRs from a DNSSEC-signed zone. These signatures, however, do not cover all theinformation in a zone file (e.g., delegation information). Furthermore, they enable verification of only theindividual RRsets and not the entire zone transfer response message.
There is also another method to authenticate DNS transactions by using asymmetric cryptography (i.e.
public key cryptography). The format of the SIG(0) RR is similar to the resource record signature(RRSIG) RR (see Section 9.2.1), and can be validated using a public key stored in the DNS (instead of ashared secret key). SIG(0) can be more computational expensive to use, but offer an advantage in that aprevious trust relationship may not be necessary to use SIG(0) signed messages. However, since mostzone transfers occur between parties that have a previously established relationship, it is considered easier
to implement TSIG for authenticating zone transfer transactions.
Another possibility is to rely on lower level network layer to provide security such as IPSec. This would
remove the need for authentication at the DNS (application) layer. How to set up this level of security isbeyond the scope of this guide.

6-4

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

6.3

Dynamic Updates Threats and Protection Approaches

Dynamic updates involve DNS clients making changes to zone data in an authoritative name server in realtime. Clients typically performing dynamic updates are CA servers, DHCP servers, or Internet Multicast
Address servers. As with zone transfer transaction, the threats associated with dynamic update transaction
have not been officially documented by the IETF through an RFC. The following are some common
threats that could be expected, based on the fact that dynamic updates involve a data update request
transiting a network.

•••

Threat T17—Unauthorized Updates: Unauthorized updates could have several harmful
consequences for the content of zone data. Some harmful data operations include: (a) adding
illegitimate resources (new FQDN and new RRs to a valid zone file), (b) deleting legitimate
resources (entire FQDN or specific RRs), and (c) altering delegation information (NS RRspointing to child zones).
Threat T18: The data in a dynamic update request could be tampered.
Threat T19—Replay Attacks: Update request messages could be captured and resubmitted
later, thus causing inappropriate updates.

Threats T17 and T18 could be countered by authenticating the entities involved and providing a means todetect tampering of the messages. Because these security objectives in the case of zone transfer are metby the TSIG/SIG(0) mechanism, the same TSIG/SIG(0) mechanism is specified for protecting dynamic
updates. Although the dynamic update message contains some replay attack (Threat T19) protection in
the prerequisite field of the message, TSIG/SIG(0) provides an additional mechanism to protect against
replay attacks by including a timestamp field in the dynamic update request. This signed timestamp
enables a server to determine whether the timing of the dynamic update request is within the acceptable
time limits specified in the configuration.
It sometimes makes more sense to use SIG(0) protection mechanisms for dynamic update than for zonetransfer. Dynamic update transactions may happen between parties that do not always have a priorsecurity relationship or may be part of a bootstrapping operation. Therefore it may be impractical to use
TSIG with a shared secret, but SIG(0) authentication using keys stored in the DNS may be a possibility.
Another possibility is to rely on lower level network layer to provide security such as IPSec. This would
remove the need for authentication at the DNS (application) layer. How to set up this level of security isbeyond the scope of this guide.
6.4

DNS NOTIFY Threats and Protection Approaches

DNS NOTIFY is a message sent by primary (master) name servers to secondary (slave) name servers,
causing the secondary servers to start a refresh operation (i.e. query for SOA RR to check the serial
number, etc.) and perform a zone transfer if an update to the zone has occurred. Because the NOTIFY
message is only a signal, there are only minor security risks in dealing with the message. The primary
security risk to consider is the following:•Threat T20—Spurious NOTIFY Messages: Secondary name servers would receive spurious
DNS NOTIFY messages from sources other than the primary name server.

The only impact of receiving spurious DNS NOTIFY message is the increase in workload in secondaryname servers since a zone transfer will only occur when an updated zone is on the primary server.

6-5

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

Because this threat is low impact, the only protection approach required is to configure the secondaryname servers to receive DNS NOTIFY message only from the enterprise’s primary name server.
However, if TSIG is set up for use for all communication between a set of hosts, TSIG will be used withNOTIFY messages as well.
6.5

Threats Summary

Table 6-1 summarizes the DNS transactions and their associated threats, security objectives, and IETF
security mechanism specifications to meet those goals.
Table 6-1. DNS Transaction Threats and Security Objectives
DNS Transaction

Threats

Security Objectives

DNS Query/Response(a) Forged or bogus response(b) Removal of records (RRs) in
responses(c) Incorrect application of
wildcard expansion rules(a) Data origin authentication(b) Data integrity verification

DNSSEC

Zone Transfer(a) Denial of service(b) Tampering of messages(a) Mutual authentication(b) Data integrity verification

TSIG

Dynamic Update(a) Unauthorized Updates(b) Tampering of messages(c) Replay attack(a) Mutual authentication(b) Data integrity verification(c) Signed timestamps

TSIG, GSS-TSIG
or SIG(0)DNS NOTIFY(a) Spurious notifications(a) To prevent denial of service
through increase in workload

Specify hosts fromwhich this message
can be received
TSIG or SIG(0)6-6

DNS Based
Security
Specifications

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

7.

Guidelines for Securing DNS Hosting Environment

The guidelines for secure configuration of the DNS hosting environment are classified under thefollowing headings:•

Securing DNS host platform•

Securing DNS software•

Content control of zone file.

These guidelines are provided mostly in the context of BIND DNS Name Server software. Where
possible, similar guidelines will be provided for other DNS authoritative software packages such as NSD(from NLnet Labs) and Microsoft Windows Server.
7.1

Securing DNS Host Platform

The platform on which the name server software is hosted should be running an adequately secured OS.
Most of the DNS installations run either on a flavor of Unix or Windows. Given this scenario, it isnecessary to ensure the following:

•
•

7.2

The latest OS patches are installed.
Recommended OS configuration practices as issued by CERT®//CC [CERT] and NIST’s NVD
metabase [NVD], based on identified vulnerabilities that pertain to the application profile into
which the name server software fits, are followed. In particular, hosts that run the name server
software should not provide any other services and therefore should be configured to respond to
DNS traffic only. In other words, the only allowed incoming ports/protocols to these hosts should
be 53/udp and 53/tcp. Outgoing DNS messages should be sent from a random port to minimize
the risk of an attacker guessing the outgoing message port and sending forged replies.
Securing DNS Software

Protection approaches for DNS software include choice of appropriate version, installation of patches,running it with restricted privileges, restricting other applications in the execution environment,
dedicating instances for each function, controlling the set of hosts where software is installed, placement
within the network, and limiting information exposure by logical/physical partitioning of zone file data or
running two name server software instances for different client classes.
7.2.1

Running the Latest Version of Name Server Software

Each newer version of the name server software, especially the BIND software, generally is devoid ofvulnerabilities found in earlier versions because it has design changes incorporated to take care of thosevulnerabilities. Of course, these vulnerabilities have been exploited (i.e., some form of attack waslaunched), and sufficient information has been generated with respect to the nature of those exploits.
Thus, it makes good business sense to run the latest version of name server software because theoretically
it is the safest version. Even if the software is the latest version, it is not safe to run it in default mode. The
security administrator should always configure the software to run in the recommended secure mode ofoperation after becoming familiar with the new security settings for the latest version.

7-1

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEIn some installations, it may not be possible to switch over to the latest version of name server software
immediately. In these situations, the administrator should keep pace with vulnerabilities identified in theoperational version and associated security patches [BINDSEC].

Checklist item 1: When installing the upgraded version of name server software, the administrator
should make necessary changes to configuration parameters to take advantage of new security features.
Checklist item 2: Whether running the latest version or an earlier version, the administrator should beaware of the vulnerabilities, exploits, security fixes, and patches for the version that is in operation in theenterprise. The following actions are recommended (for BIND deployments):

7.2.2

–

Subscribe to ISC’s mailing list called “bind-announce” or “nsd-users” for NSD–

Periodically refer to the BIND vulnerabilities page at
http://www.isc.org/products/BIND/bind-security.html–

Refer to CERT®/CC’s Vulnerability Notes Database at http://www.kb.cert.org/vuls/ and the
NIST NVD metabase at http://nvd.nist.gov/.

Turning off the Version Query

There is a feature in BIND that returns the version number of the server daemon running if a special
query is sent to the server. This query is for the string “version.bind” with query type TXT and queryclass Chaos (CH). This information may be of use to attackers who are looking for a specific version of
BIND with a discovered weakness. BIND can be configured to refuse this type of query by having thefollowing command in the BIND configuration file (/etc/named.conf).
options {version none;};
There is a similar feature in NSD to refuse version queries. In the configuration file for NSD(/etc/nsd/nsd.conf).
server:hide-version: yes

For Windows, the DNS version number can be changed using the dnscmd tool with the /config option
and changing the EnableVersionQuery property3.

Checklist item 3: To prevent information about which version of name server software is running on asystem, name servers should be configured to refuse queries for its version information..

3

http://msdn.microsoft.com/en-us/library/cc422472(v=prot.10).aspx

7-2

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

7.2.3

Running Name Server Software with Restricted Privileges

If the name server software is run as a privileged user (e.g., root in Unix systems), any break-in into thesoftware can have disastrous consequences in terms of resources resident in the name server platform.
Specifically, a hacker who breaks into the software acquires unrestricted access and therefore canpotentially execute any commands or modify or delete any files. Hence, it is necessary to run the nameserver software as a nonprivileged user with access restricted to specified directories to contain damages
resulting from break-in.
The user name under which the name server software needs to run can be specified by using the chroot
command in BIND. This is why this approach is known as running the DNS server in a chroot jail. An
example command (entered on the command line) is the following:>named -u named –t /var/named/chroot
where the options specify the following:–u specifies the userID to which the name server software will change after starting. This user
account should be created prior to issuing the chroot command.
–t specifies the directory in which the name server software owner will use as the root “jail” andshould have the appropriate privileges. This directory will have to be created by the systemadministrator before starting the server process.
This ability is also found in NSD as part of the NSD startup scripts. The default userID under which NSD
runs is nsd and the default directory is /etc/nsd/ Both of these defaults can be changed in the NSD
configuration file by adding:server:username: <userID>chroot: <dir>
7.2.4

Isolating the Name Server Software

Even if the DNS software (e.g., BIND) is run on a secure OS, the vulnerabilities of other software
programs on that platform can breach the security of DNS software. Hence, it is recommended that theplatform on which the DNS software runs contains no programs other than those needed for OS andnetwork support. If this is not possible due to resource constraints, care should be taken to ensure to limit
the number of services running on the same platform.
7.2.5

Dedicated Name Server Instance for Each Function

An authoritative name server serves RRs from its own zone file; this function is called an authoritative
function. Serving RRs either from its cache (directly or by building up its cache dynamically throughiterative queries) is called a resolving function; this is how a resolving name server provides responses. A
name server instance can be configured as an authoritative name server, a resolving name server, or both.
Because of attacks such as cache poisoning (see Section 6.1), however, a resolving name server has to be

7-3

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

run under security policy that is different from that of an authoritative name server. Hence, a name server
instance should always be configured as either an authoritative name server or a resolving name server.
An authoritative name server is only intended to provide name resolution for the zones for which it has
authoritative information. Hence, the security policy should have recursion turned off for this type ofname server. Disabling recursion prevents an authoritative name server from sending queries on behalf ofother name servers and building up a cache using responses. Disabling this function eliminates the cachepoisoning threat on authoritative serves and prevents their use as reflectors for DDoS attacks [BCP140].
In BIND, recursion is disabled by using the options statement in the BIND configuration file as follows:
options {recursion no;};
NSD can only operate as an authoritative only server. NSD cannot act as a recursive resolver andtherefore does not have a comparable option, as it is NSD’s default behavior.
A resolving name server is only intended to provide resolving services (processing resolving queries onbehalf of clients) for internal clients. Thus, protection of resolving name servers can be ensured by
restricting their types of interactions (also called transactions) to designated hosts through various
configuration options in the configuration file of the name server software.
7.2.6

Removing Name Server Software from Nondesignated Hosts

DNS software should not be running or present in hosts that are not designated as name servers. The
possibility arises in the case of DNS BIND software because of the fact that many versions of Unix
(including Solaris and Linux versions) come installed with BIND as default. Hence, while taking an
inventory of software in workstations and servers of the enterprise as part of the security audit, it isnecessary to look for BIND installations and remove them from hosts that are not functioning as nameservers.
7.2.7

Network and Geographic Dispersion of Authoritative Name Servers

Most enterprises have an authoritative primary server and a host of authoritative secondary name servers.
It is essential that these authoritative name servers for an enterprise be located on different network
segments. This dispersion ensures the availability of an authoritative name server not only in situations inwhich a particular router or switch fails but also during events involving an attack on an entire network
segment. In addition to network-based dispersion, authoritative name servers should be dispersed
geographically as well. In other words, in addition to being located on different network segments, theauthoritative name servers should not all be located within the same building. One approach that someorganizations follow is to locate some authoritative name servers in their own premises and others in their
ISPs’ data centers or in partnering organizations.
Additionally, a network administrator may choose to use a “hidden” master authoritative server and only
have secondary servers visible on the network. A hidden master authoritative server is an authoritative
DNS server whose IP address does not appear in the name server set for a zone. All of the name serversthat do appear in the zone database as designated name servers all get their zone data from the hidden
master via a zone transfer request. In effect, all visible name servers are actually secondary slave servers.
This prevents potential attackers from targeting the master name server, as its IP address may not appear
in the zone database. A hidden master only accepts zone transfer requests from the set of valid
secondaries and refuses all other DNS queries.

7-4

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEChecklist item 4: The authoritative name servers for an enterprise should be both network andgeographically dispersed. Network-based dispersion consists of ensuring that all name servers are notbehind a single router or switch, in a single subnet, or using a single leased line. Geographic dispersion
consists of ensuring that not all name servers are in the same physical location, and hosting at least asingle secondary server off-site.
Checklist item 5: If a hidden master is used, the hidden authoritative master server should only accept
zone transfer requests from the set of secondary zone name servers and refuse all other DNS queries. The
IP address of the hidden master should not appear in the name server set in the zone database.

7.2.8

Limiting Information Exposure through Partitioning of Zone files

Authoritative name servers for an enterprise receive requests from both external and internal clients. In
many instances, external clients need to receive RRs that pertain only to public services (public Web
server, mail server, etc.) Internal clients need to receive RRs pertaining to public services as well asinternal hosts. Hence, the zone information that serves these RRs can be split into different physical files
for these two types of clients: one for external clients and one for internal clients. This type of
implementation of the zone file is called split DNS.
Split DNS does have some drawbacks. First, remote hosts (travelers using laptops to connect back to anorganization, for example), may not be using an internal resolving DNS server, and therefore may not beable to see internal hosts. Second, internal host information may leak to outside the firewall (by accident
or attack), defeating the purpose of having a split DNS, or causing confusion of an internal and external
host have the same FQDN, but different IP addresses. Split DNS should not be seen as a replacement for
proper access control techniques.

Checklist item 6: For split DNS implementation, there should be a minimum of two physical files orviews. One should exclusively provide name resolution for hosts located inside the firewall. It also cancontain RRsets for hosts outside the firewall. The other file or view should provide name resolution only
for hosts located outside the firewall or in the DMZ, and not for any hosts inside the firewall.

To set up split DNS using BIND, the view statement is used in the BIND configuration file. For example,
to set up an authoritative view of the zone “sales.mycom.com” for internal clients:
view “insider” {match-clients { internal_hosts; };recursion no;zone sales.mycom.com {type master;
file “sales_internal.db”;
};
};

7-5

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEIn this statement, the file “sales_internal.db” contains the authoritative information for zone
“sales.mycom.com” and restricts that information to queries coming from the address list named“internal_hosts”. Section 8.1.1 provides details on how to set up this list. To set up a view of the samezone, but with only external hosts for queries coming from outside the network, the following viewstatement is used in the same configuration file:
view “outsider” {match-clients { any; };match-destinations { public_hosts; };recursion no;zone sales.mycom.com {type master;
file “sales_external.db”;};};
This statement is very similar to the previous statement, except that the file with the zone view is“sales.external.db” and the client list is given as “any”, meaning that queries coming from both outside
and inside the network can see the same external view of “sales.mycom.com.” Together, these statementsallow internal clients to view both the internal and external hosts in “sales.mycom.com,” whereas external
hosts (not on the same network) can see only DNS information contained in the external view of the zone
where the destination matches the address list “public_hosts.”
7.2.9

Limiting Information Exposure Through Separate Name Servers for Different
Clients

Instead of having the same set of authoritative name servers serve different types of clients, an enterprise
could have two different sets of authoritative name servers. One set, called external name servers, can belocated within a DMZ; these would be the only name servers that are accessible to external clients andwould serve RRs pertaining to hosts with public services (Web servers that serve external Web pages orprovide B2C services, mail servers, etc.) The other set, called internal name servers, is to be locatedwithin the firewall and should be configured so they are not reachable from outside and hence provide
naming services exclusively to internal clients. The purpose of both architecture options (i.e., twodifferent sets of name servers and split DNS) is to prevent the outside world from knowing the IP
addresses of internal hosts. This configuration may be the only available option for enterprises that use
DNS server software that does not have the view feature found in BIND or organizations that use NSD as
their authoritative DNS server.
7.3

Content Control of Zone FileAs stated in Section 5.7, the only protection approach for content control of DNS zone file is the use of azone file integrity checker. The effectiveness of integrity checking using a zone file integrity checker
depends upon the database of constraints built into the checker. Hence, the deployment process consistsof developing these constraints with the right logic and the only determinant of the truth value of theselogical predicates are the parameter values for certain key fields in the format of various RRTypes. The
choice of these parameter values forms the deployment guidelines and is discussed in Section 10.
7.4

Recommendations Summary

The following items provide a summary of the major recommendations from this section:

7-6

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE••Checklist item 1: When installing the upgraded version of name server software, theadministrator should make necessary changes to configuration parameters to take advantage of
new security features.
Checklist item 2: Whether running the latest version or an earlier version, the administrator
should be aware of the vulnerabilities, exploits, security fixes, and patches for the version that is
in operation in the enterprise. The following actions are recommended:–

Subscribe to ISC’s mailing list called “bind-announce” or NLnet Labs mailing list “nsdusers”–

Periodically refer to the BIND vulnerabilities page at
http://www.isc.org/products/BIND/bind-security.html–

Refer to CERT®/CC’s Vulnerability Notes Database at http://www.kb.cert.org/vuls/ and the
NIST NVD metabase at http://nvd.nist.gov/.

For other implementations (e.g., MS Windows Server), other announcement lists may exist.

•
•

•

•

Checklist item 3: To prevent information about which version of server software is running on asystem, name servers should be configured to refuse queries for its version.
Checklist item 4: The authoritative name servers for an enterprise should be both network andgeographically dispersed. Network-based dispersion consists of ensuring that all name servers are
not behind a single router or switch, in a single subnet, or using a single leased line. Geographic
dispersion consists of ensuring that not all name servers are in the same physical location, andhosting at least a single secondary server off-site.
Checklist item 5: If a hidden master is used, the hidden authoritative master server should only
accept zone transfer requests from the set of secondary zone name servers and refuse all other
DNS queries. The IP address of the hidden master should not appear in the name server set in thezone database.
Checklist item 6: For split DNS implementation, there should be a minimum of two physicalfiles or views. One should exclusively provide name resolution for hosts located inside thefirewall. It also can contain RRsets for hosts outside the firewall. The other file or view should
provide name resolution only for hosts located outside the firewall or in the DMZ, and not for any
hosts inside the firewall.

7-7

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

8.

Guidelines for Securing DNS Transactions

Section 6 outlines threats, security objectives, and protection approaches for various DNS transactions.
This section provides the steps involved in implementing those approaches, as well as operational bestpractices that go with those implementations. The DNS protocol protection approaches in Table 6-1 areaggregated, indexed by type of approach, and elaborated here as follows:

•

•

•

8.1

Restricting Transaction Entities Based on IP Address. In this type of implementation, theDNS name servers and clients participating in a DNS transaction are restricted to a trusted set ofhosts by specifying their IP addresses in appropriate access control statements provided by thename server software. The protection provided by these IP-based access control statements can be
circumvented by attacks such as IP spoofing. Hence, this solution is not recommended as the sole
protection mechanism for DNS query/response, zone transfer, and dynamic update transactions
that have high threat impact. However, for the DNS NOTIFY transaction, where the only threat isspurious notification (which may not even trigger a zone transfer), an access control based on IP
address will suffice. Although this solution is not recommended generally, a description of themechanics of access control using IP addresses is provided in Section 8.1 because the samestatements are used to identify hosts based on named keys while implementing transaction
protection using hash-based message authentication codes. This approach has been implemented
for all DNS transactions.
Transaction Protection through Hash-Based Message Authentication Codes (TSIG
Specification). In this approach, transaction protection is enabled through generation andverification of hash-based message authentication codes (HMAC). Because these codes areembedded within a special RR of RRType TSIG, the specifications that outline protection of
DNS transactions using HMAC are called TSIG in the DNS community. TSIG specifications aredescribed in RFC 2845 and 3007 [RFC2845], [RFC3007]. Application of TSIG specifications forprotection of zone transfer and dynamic update transactions is described in Section 8.2.
Transaction Protection through Asymmetric Digital Signatures (DNSSEC Specification).
This approach, which goes by the name DNS security extensions (DNSSEC), is describedthrough a family of RFCs [RFC4033], [RFC4034], and [RFC4035]. The core services provided
by DNSSEC are data origin authentication and integrity protection. DNSSEC is used mainly forsecuring DNS information obtained from DNS query/response transactions. The deployment
issues in DNSSEC are described in Section 9.
Restricting Transaction Entities Based on IP Address

Some DNS name server implementations, such as BIND 9.x, provide access control statements throughwhich it is possible to specify hosts that can participate in a given DNS transaction. The hosts can be
identified by their IP address or IP subnet reference (called IP prefix) in these statements.
The list containing these IP addresses and/or IP prefixes is called an address match list. (An address
match list can be made up of other things besides IP addresses and IP prefixes, as described in Section
8.1.1). The address match list is used as an argument in various access control statements that areavailable for use in BIND configuration files. There are separate access control statements for each typeof DNS transaction. The syntax of the various access control statements and the DNS transaction forwhich each is used are given in Table 8.1.

8-1

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDETable 8-1. BIND Access Control Statement Syntax for DNS Transactions
Access Control Statement Syntax

DNS Transaction

allow-query { address_match_list }DNS Query/Response

allow-recursion { address_match_list }Recursive Queryallow-transfer { address_match_list }Zone Transfer

allow-update { address_match_list }Dynamic Updateallow-update-forwarding { address_match_list }Dynamic Updateallow-notify { address_match_list }DNS NOTIFY

blackhole { address_match_list }Blacklisted Hosts

The purpose of each of these access control statements is as follows:

•
•
•
•
•
•
•

allow-query: specifies the list of hosts allowed to query the name server as a whole or aparticular zone within the name server
allow-recursion: specifies the list of hosts allowed to submit recursive queries to the name server
as a whole or to a particular zone served by the name server
allow-transfer: specifies the list of hosts allowed to initiate zone transfer requests to the nameserver as a whole or to a particular zone within the name server. This statement is predominantly
required for configuration of master name servers.
allow-update: specifies the list of hosts allowed to initiate dynamic update requests
allow-update-forwarding: specifies the list of hosts allowed to forward dynamic update requests(regardless of the originator of the requests)
allow-notify: specifies the list of hosts from which to accept DNS NOTIFY messages indicatingchanges in the zone file. This list is relevant only for configuration of secondary slave nameservers.
blackhole: specifies the list of hosts that are blacklisted (barred) from initiating any transaction
with this name server. Used only in an “options” server wide ACL statement.

The foregoing access control statements are, in fact, substatements that can be used in the context of
options and zone statements in the BIND 9.x configuration file (with the exception of blackhole). Whenthey are used within the zone statement, they specify access control restrictions for the corresponding
DNS transaction for that specific zone. When they are used as part of the options statement, they specify
access control restrictions for the corresponding DNS transaction for the name server as a whole (becausea name server could host multiple zones).
NSD has a similar set of configuration options for certain transactions. NSD has a more limited sets ofoptions and currently only restrict zone transfers. In the following sections, if a comparable set of options
exists for NSD, they will be listed. Otherwise it should be noted that a comparable option does not exist
at the time of writing.

8-2

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

8.1.1

Restricting DNS Query/Response Transaction Entities

An example of the usage of the allow-query substatement (to specify restrictions for the DNS
query/response transaction stating the IP addresses/subnets from which DNS queries are accepted) both at
the server level and at the zone level (for the zone example.com) is given below:
options {allow-query { 254.10.20.10; 239.10.30.29/25; };};zone “example.com.” {type master;
file “zonedb.example.com”;
allow-query { 192.249.249.1; 192.249.249.4; };};Specifying the list of IP addresses and IP prefixes within the options and zone statements could clutter theconfiguration file. Furthermore, the list of IP addresses and IP prefixes could be the same for many of theaccess control statements within a name server, and errors could be introduced if any additions or
subtractions are made for that list. To avoid these problems, BIND provides a means to create named
address match lists, which are called access control lists (ACL). These ACLs can be used in place of thelist of IP addresses/IP prefixes (in the address match list argument) in the access control statements.
The ACLs are created by using the acl statement in BIND 9.x. The general syntax of the acl statement
is as follows:
acl acl-list-name {address_match_list};
The acl-list-name is a user-defined string (e.g., internal_hosts). The address_match_list can be a list of IP
addresses, IP address prefixes (denoting subnets), or cryptographic keys. An example of an acl statementthat uses an IP address and a subnet reference in address_match_list is given below. In the example,
254.10.20.10 denotes the IP address of a host, and the IP prefix 239.10.30.0/24 denotes a class C subnet.
acl “internal_hosts” {254.10.20.10;
239.10.30.0/24;};
The use of ACL – “internal_hosts” in place of the list of IP addresses/IP prefix in the options and zone
statement given above is as follows:
options {allow-query { internal_hosts; };};zone “example.com.” {type master;
file “zonedb.example.com”;
allow-query { internal_hosts; };};8-3

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEThe address match list parameter in an access control statement can contain any of the following values:•

An IP address or list of IP addresses•

An IP prefix or list of IP prefixes

•

ACLs

•

A combination of the above three.

The definition of ACLs forms a critical element in the configuration of DNS transaction restrictions.
Hence, it is a good operational practice for the DNS administrator to define and create ACLs pertaining todifferent DNS transactions.

Checklist item 7: It is recommended that the administrator create a named list of trusted hosts (orblacklisted hosts) for each of the different types of DNS transactions. In general, the role of the following
categories of hosts should be considered for inclusion in the appropriate ACL:–DMZ hosts defined in any of the zones in the enterprise–All secondary name servers allowed to initiate zone transfers–

Internal hosts allowed to perform recursive queries.

In addition to IP address, IP prefix, or ACL, the address match list parameter in the access control
statements can take on any of the following special values:•

none: matches no hosts•

any: matches all hosts•

localhost: matches all IP addresses of the server on which the name server is running•

localnets: matches all IP addresses and subnet masks of the server on which the name server isrunning.

Following are a few more examples of commands for creating ACLs and the use of ACLs within options
and zone statements:
acl “local_hosts” {254.10.20.10;
239.10.30.29/25;};
acl “fake-net” {0.0.0.0/8;
1.0.0.0/8;};

8-4

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

options {allow-query { any; };blackhole { fake-net; };};zone “example.com.” {type master;
file “zonedb.example.com”;
allow-query { local_hosts; };};In the named.conf snippet above, two ACLs, local_hosts and fake-net, have been specified. DNS queries
from any hosts are allowed at the server level. No transactions are permitted from the hosts included
under fake-net. Queries to the zone example.com can be initiated only by the hosts included under the
ACL local_hosts because any restriction specified under the zone (zone-specific) statement overrides the
restriction specified under the options (server-wide) statement.
Key material can also be used in ACL statements. This would indicate that only hosts knowing (andusing) the shared key (or key pair) would be able to communicate. How a key is use in an ACL is
discussed in Section 8.2.2.
NSD does not have the feature to define ACLs as a means of only allowing queries from designated set ofhosts.
Administrators using Microsoft Windows Server as their DNS server can query the Microsoft TechNet 4
website for information on configuring access control lists for their server version.
8.1.1.1 Restricting Recursive Queries (a special case under DNS Query/Response)
Authoritative name servers provide name resolution service from their own data and are supposed toprovide this service for any DNS client. Hence, configuring an authoritative name server to accept queries
from a restricted set of hosts does not make sense. The practical security protection for an authoritative
name server is to turn off the query recursion feature so that the authoritative name server does not poison
its cache by querying other (possibly compromised) name servers. A local resolving/recursive nameserver can be configured to accept queries only from internal hosts, to protect it from denial-of-service
attacks as well as cache poisoning. However, there may be situations in which it is economicallyinfeasible to dedicate separate servers for authoritative service and resolution service, and the resolving
name server has to perform as authoritative server for one or more zones. In this situation, the following
strategies are possible within the BIND 9.x name server:

•
•
•
4

Restricting all queries accepted by the server to a specified set of IP addresses of internal clientsand then overriding this set only for authoritative zones so that any DNS client can obtain
information for resources in that zone.
Restricting recursive queries to a specified set of IP addresses of internal clients through a directconfiguration option
Serving different responses (data) to different clients by defining views.

http://technet.microsoft.com/en-US/

8-5

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

Restriction at server level with override for authoritative zones:
In this strategy, the allowable set of internal clients who can submit queries to the name server is specified
through the acl statement as follows:
acl internal_hosts {192.158.43.3; 192.158.43.6; 192.158.44.56;};
The server-wide option would be to restrict all queries to the list of clients:
options {allow-query { internal_hosts; };- or –
allow-recursion { internal_hosts; };};The option can be overridden by specifying zones for which this name server is authoritative (thusallowing queries to that zone from all clients):
zone “example.com” {type master;
file “zonedb.example.com”;
allow-query { any; };};Restricting all recursive queries to a specified set of IP addresses:
Server-wide restriction:
options {allow-recursion { internal_hosts; };};Restricting recursion through views:
The purpose of creating views is to create a logical partition made up of a combination of clients (basedon IP addresses) and zones for which recursive queries will be supported and those for which they will
not be supported. In the following example, the view recursion_view is enabled to define the scope of IP
addresses and zones that are permitted to submit recursive queries; no_recursion_view is meant for
disallowing recursion.
view recursion_view {match-clients { internal_hosts; };recursion yes;};view no_recursion_view {match-clients { any; };recursion no;};
It should be noted that NSD (as of the time of writing) is an authoritative only DNS server. Therefore, an
NSD server will never act as a recursive server and only serve authoritative information from the zones it
is configured to serve.

8-6

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

8.1.2

Restricting Zone Transfer Transaction Entities

Authoritative name servers (especially primary name servers) should be configured with an allow-transfer
access control substatement designating the list of hosts from which zone transfer requests can be
accepted. These restrictions address the denial-of-service threat and potential exploits from unrestricteddissemination of information about internal resources. Based on the need to know, the only name serversthat need to refresh their zone files periodically are the secondary name servers. Hence, zone transferfrom primary name servers should be restricted to secondary name servers. The zone transfer should becompletely disabled in the secondary name servers. The address match list argument for the allowtransfer substatement should consist of IP addresses of secondary name servers and stealth secondaryname servers.
The command to create an ACL “valid_secondary_NS” with the IP addresses of three secondary nameservers is as follows:acl “valid_secondary_NS” {224.10.229.5;
224.10.235.6;
239.10.245.25;};
The allow-transfer substatement can be used in a zone statement and in an options statement. When it isused in a zone statement, it can restrict zone transfer for that zone; when it is used in an options statement,
it can restrict zone transfer for all zones in the name server.
The allow-transfer substatement at the server level is as follows:
options {allow-transfer { “valid_secondary_NS”; };};The allow-transfer substatement at the zone level is as follows:zone “example.com” {type master;
file “zonedb.example.com”;
allow-transfer { “valid_secondary_NS”; };};The foregoing statements apply to primary name servers. In the secondary and stealth secondary nameservers, zone transfer should be disabled as shown below:
zone “example.com” {type slave;
masters { 224.239.5.1; } ;file “zonedb_bak.example.com”;
allow-transfer { none; };};8-7

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

8.1.3

Restricting Zone Transfer in NSD

NSD has a similar set of tools to restrict zone transfers to only a chosen set of slave servers. Like BIND,
the administrator should learn and use the options available in the NSD configuration file. There is no
way to create access control lists (ACLs), but an administrator can list the individual IP address of slave
servers in the zone statements in the NSD configuration file.
In the configuration file, the provide-xfr statement is used in the zone statement block of the nsd.conf file
much like a combined masters statement and allow-transfer statement in BIND configuration files:
zone:#allow transfer from subnet
provide-xfr: 169.192.85.0/24#prevent transfer from specific IP address in blockprovide-xfr: 169.192.85.66 BLOCKED
Only one IP address should appear in a provide-xfr statement, but the address can be an entire subnet.
The provide-xfr statement allows transfers; all other transfer requests are rejected by default.
8.1.4

Restricting Zone Transfer in Windows Server

Microsoft Windows DNS Server can also restrict zone transfers based on different criteria for increased
operational security. Administrators can deny all zone transfer requests, or restrict zone transfers to a setof server (the servers in the NS set for example). Microsoft's TechNet Library 5 contains the current
information on how to use the administrator tools to restrict zone transfers.
8.1.5

Restricting Dynamic Update Transaction Entities

Dynamic updates on a zone file can be directed only to the copy of the zone file that resides on the
primary name server for the zone (i.e., where the master zone file resides). By default, dynamic update isturned off in both BIND 8 and BIND 9. Dynamic updates are enabled or restricted by using one of thefollowing two statements in BIND:•

allow-update•

update-policy (available only in BIND 9 versions).

These statements can be specified only at the zone level, not at the server level. Hence, these statements
are substatements within the zone statement. The allow-update substatement enables specification of
dynamic update restrictions based on IP addresses and a shared secret (also called a TSIG key 6). The useof the allow-update statement using IP addresses alone is addressed in this section. The use of the allowupdate statement using TSIG keys is described in Section 8.2.6.
The update-policy statement enables specification of dynamic update restrictions based on TSIG keys
only, but it enables specification of update restrictions at a finer level of granularity. The allow-update
substatement implies update access rights to all records of a zone; the update-policy substatement can beused to restrict update access rights to one or more designated RRTypes (e.g., A RRs).
5
6

http://technet.microsoft.com/en-us/library/cc771652.aspx
The term TSIG Key (while commonly used) is not technically correct, as it refers to a shared secret string and not a
cryptographic key.

8-8

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

To use the allow-update statement, an address match list must be created. The command to create an ACL
DU_Allowed_List with one IP address is as follows:acl “DU_Allowed_List” {192.249.12.21;};
The ACL DU_Allowed_List (consisting of IP addresses of hosts allowed to send dynamic update requests
for updating the contents of the zone example.com) is used within the allow-update substatement of thezone statement as follows:zone “example.com” {type master;
file “zonedb.example.com”;
allow-update { “DU_Allowed_List”; };};Dynamic update requests generally originate from hosts such as DHCP servers that assign IP addresses
dynamically to hosts. Once they assign an IP address to a new host, they need to store the FQDN-to-IP
address mapping (by creating an A RR) and address-to-FQDN mapping (by creating a PTR RR)
information in the primary authoritative name servers for the zones. Creation of this information occurs
through dynamic updates.
As of the time of writing, NSD does not support dynamic update so there are no comparable configuration
options for NSD. All dynamic update messages sent to a DNS sever running NSD will be rejected.
Updates to a zone must be done offline and then the server signaled to reload the new modified zone.
For details on how to secure dynamic update on Windows DNS servers, administrators should consult theMicrosoft TechNet for details 7.
8.1.6

Restricting BIND DNS NOTIFY Transaction Entities

Once zone transfers have been set up between servers, it is a good idea to make sure that secondary nameservers are informed about changes to zone file data through a notification message. By default, a
notification message is sent whenever a primary name server detects a change in the zone file. It sends a
DNS NOTIFY message to every name server listed in the NS RRSet in the zone because they are therecognized secondary name servers of the zone. DNS administrators should keep notification on because
this configuration will allow updates to be propagated quickly to secondary name servers. If the DNS
administrator wants to turn off the functionality for a specific zone, however, the notify substatement
should be used in the zone statement of that zone:
zone “example.com” {type master;
notify no;
file “zonedb.example.com”;};
If there are any additional servers to which the zone administrator wants the DNS NOTIFY message to besent (e.g., a stealth slave server), the “also-notify” substatement should be added to the zone statement,
and the IP addresses of the additional servers should be specified as its parameter values, as shown below:
7

http://technet.microsoft.com/en-us/library/cc771255.aspx

8-9

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

zone “example.com” {type master;also-notify { 192.168.25.2; };file “zonedb.example.com”;};
The receiver of the DNS NOTIFY message, the secondary name server, allows notify messages only fromthe primary name server by default. (Recall that the secondary name server is made aware of its primaryname server through the master substatement in the zone statement.) If the secondary name server wants
to receive notify messages from additional servers, the “allow-notify” substatement in the zone statement
must be added, and then the IP addresses of those servers must be specified in that substatement, as
follows:zone “example.com” {type slave;allow-notify { 193.168.25.4; };file “zonebak.example.com”;
masters { 192.168.25.1; };
};
8.1.7

Restricting NSD DNS NOTIFY Transaction Entities

There are two statements (both placed in the zone: statement block) that a DNS administrator can use to
send DNS NOTIFY messages or restrict listening for DNS NOTIFY messages to a particular IP address(a master server in the case of NSD acting as a slave server).
To configure NSD to send DNS NOTIFY messages to a particular IP address (either a slave secondary ora stealth secondary) and a particular TSIG key or the option “NOKEY” if no TSIG is used, the following
is added to the zone: statement block in the NSD configuration file:zone:
notify: 10.0.0.10

NOKEY

For a slave server the configuration statement in the zone: block to indicate which IP address to accept
DNS NOTIFY messages from:zone:
allow-notify: 10.11.12.13

8.2

NOKEY

Transaction Protection Through Hash-Based Message Authentication Codes (TSIG)

The process of authenticating the source of a message and its integrity through hash-based message
authentication codes (HMAC) is specified through a set of DNS specifications known collectively as
TSIG. The term HMAC is used to denote both the message authentication code generated by using akeyed hash function and the hash function itself. HMAC is specified in RFC 2104 [RFC2104] andgeneralized in the NIST document FIPS 198-1 [FIPS198].
An HMAC function uses two parameters—a message input and a secret key—and produces an output
called a message authentication code (MAC) or hash. The sender of the message uses the HMAC
function to generate a MAC and sends this MAC along with the message to the receiver. The receiver,

8-10

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

who shares the same secret key, uses the key and HMAC function used by the sender to compute theMAC on the received message. The receiver then compares the computed MAC with the received MAC;
if the two values match, it provides assurance that the message has been received correctly and that thesender belongs to the community of users sharing the same secret key. Thus, message source
authentication and integrity verification are performed in a single process.
The hash algorithm (which forms the primitive for the hash function) generates a fixed-size MAC or hashfrom a message of arbitrary size. The HMAC function for TSIG specified in RFC 2845 [RFC2845] andextended in RFC 4635 [RFC4635] to support more hash algorithms (SHA-1 and SHA-2 family ofalgorithms).
Transaction protection through HMAC using a shared secret is not a scalable solution. This is the reason
the TSIG specification is largely used only for zone transfer and dynamic update transactions. These DNS
transactions are either between servers in the same administrative domain or between servers in domains
with previous established interactions.
The MAC or hash value generated by the sender of the DNS message is placed in a new RR called a
TSIG record that is added to the DNS message. The TSIG record, in addition to the generated hash,contains the following:•

Name of hash algorithm•

Key name•

Time the hash was generated (timestamp)•“Fudge factor” –time in seconds (usually 5 minutes) to use as delta on either side of the time
generated for which the TSIG signature should be considered valid; used to account for possible
clock skew between hosts.

The timestamp field specifies the time at which the MAC was generated. The purpose of this field is toprotect against replay attacks. In a replay attack, the attacker could capture the packet containing the
MAC and send it after a period of time. To ensure that this does not happen, the recipient reads the MAC
generation time and the current clock time and verifies whether the MAC was generated within an“allowable expiry time,” which is computed using the “Fudge Factor”.
The “Fudge factor” field specifies the duration of time after the MAC generation time, the message can beconsidered valid. It is computed by applying a “fudge factor” on the MAC generation time (adding or
subtracting a small number of seconds) to allow for clock skew (mismatch) between the MAC generator
and verifier hosts.
To have a secure transaction based on TSIG, a sender computes the hash of the entire DNS message andsecret key and encodes the result in a TSIG RR at the end of the message. At the recipient end, the TSIG
record is stripped from the DNS message and processed. The process whereby the recipient uses the TSIG
record to verify the integrity of the received DNS message is called verification. The verification processuses the hash algorithm name to identify the hash function and the key name to identify the key to be used
to validate the TSIG record. The number in the fudge factor is used to add to or subtract from the signing
time to allow for the possible mismatch of clocks of the signer and verifier. Thus, the fudge factor
provides the tolerance limit for the MAC validity period computed, based on time of generation.

8-11

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEThe purpose of sending the key name in the TSIG record is to enable the verifier (recipient) of the DNS
message to use the right key to verify it. It also enables the recipient to verify that the key name is indeed
one of the keys shared with the sender. The purpose of the “Time Signed” or timestamp field in the TSIG
record is to inform the message recipient about the time of MAC generation. The recipient compares thisvalue with the current clock time at the recipient system to ensure that the MAC was generated within theallowable time specified as part of the TSIG record itself. The purpose of using a timestamp is to prevent
replay attacks. For correct verification of generation time against current time, it is essential that thesystem clocks of the transaction participants be synchronized. Protocols such as the Network Time
Protocol (NTP) are available for this purpose.
The verification process consists of the recipient retrieving the appropriate secret key, generating its own
hash of the received DNS message, and comparing it with the received hash (in the TSIG record). In thisverification process, the receiving name server has performed the following validations:••

The message has been verified as coming from an authenticated source (with whom it shares thesecret key).
The message has not been altered in transit (verified by matching of hash values).

Source authentication counters identify spoofing, and data integrity checking helps to counter corruption
and modification of data in transit.
BIND version 8.2 was the first version to introduce TSIG features, and is present in every later version.
BIND 9.x’s support for TSIG includes features to secure zone transfer and dynamic update transactions8.
The following operations are needed to set up the environment for enabling DNS transactions to use
TSIG:

•

•
•

The system clocks of the name servers (primary and secondary) participating in DNS transactions
must be synchronized (e.g., through NTP). If the primary and secondary servers' clocks differ
more than the "fudge factor" described above, the TSIG will fail to authenticate and the zonetransfer will fail.
There should be a secret key generation utility that can generate keys of the required length with
sufficient entropy. The key file (the file containing the secret key string) must be securely
communicated to the two servers participating in the transaction.
The key information should be specified in the configuration file through appropriate statements(e.g., key statement and server statement in named.conf configuration file of BIND 9.x).

The key generation process is described in Section 8.2.1. The commands needed to define the keys andinstruct the name server to use those keys for all DNS transactions are outlined in Sections 8.2.2 and8.2.4, respectively. The set of checklists for key file creation and key definition within the name servers isgiven in Section 8.2.5. Protection of zone transfer transactions and dynamic update transactions using
HMAC as specified in TSIG are covered in sections 8.2.7 and 8.2.8 respectively.

8

Some server software such as Microsoft Windows Server 2008 does not implement TSIG, but use lower level transaction
security (such as IPSec). To set this up, see http://technet.microsoft.com/en-us/library/ee649243(WS.10).aspx

8-12

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

8.2.1

Key Generation

To enable zone transfer (requests and responses) through authenticated messages, it is necessary to
generate a key for every pair of name servers. The key also can be used for securing other transactions,such as dynamic updates, DNS queries, and responses. The binary key string that is generated by most
key generation utilities used with DNSSEC is base 64 encoded. The program that generates the key in
BIND 9.x is dnssec-keygen. An example of a command that generates a secret key (as opposed to othertypes of keys, such as public keys, which this program also can generate) by invoking the dnssec-keygenprogram is as follows:
dnssec-keygen -a HMAC-SHA256 –b 112 -n HOST ns1-ns2.example.com.
where the various command options (parameters) denote the following:-a option: the name of the hashing algorithm that will use the key (HMAC-SHA256 is preferred,
but may not be available in older implementations. Use of HMAC-SHA1 is allowed, but
migration to HMAC-SHA256 should be done when available)-b option: the length of the key (here – a minimum of 112 bits)-n option: the type of key (in this case, the HOST)
last parameter: the name of the key (ns1-ns2.example.com)
The dnssec-keygen program generates the following files, each containing the key string:Kns1-ns2.example.com.+157+34567.key
Kns1-ns2.example.com.+157.34567.private
When the program is generating a pair of keys (one public and the other private), the file with theextension key will contain the public key string and the file with extension private will contain the private
key. Because in this case only the secret key is being generated, the key strings in both files will be thesame for the TSIG implementation. The key string from any of these files is then copied to a file called
the key file. This file is then referenced using an include statement within the key statement.
8.2.2

Defining the Keys in the Communicating Name Servers

The key generated by using the dnssec-keygen utility has to be defined within the named.conf
configuration file of the two communicating servers (generally one primary name server and one
secondary name server). This is accomplished by using the key statement of BIND:
key “ns1-ns2.example.com.” {algorithm hmac-sha256;
include “/var/named/keys/secretkey.conf”;};
where the file secretkey.conf will contain the keyword secret and the actual key string (in this example):secret “MhZQKc4TwAPkURM==“;

8-13

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

8.2.3

Defining the Keys in a NSD Configuration FileIn NSD, declaring a TSIG key is very similar to the example above, with some minor syntax changes:
key:
name: ns1-ns2.example.com.
algorithm: hmac-sha256
secret: “MhZQKc4TwAPkURM==“
8.2.4

Instructing Name Servers to Use Keys in All Transactions

The command to instruct the server to use the key in all transactions (DNS query/response, zone transfer,
dynamic update, etc.) is as follows:
server 192.249.249.1 {keys { ns1-ns2.example.com.; };The same statement can be used as an entry in an acl statement as well:acl key_acl {ns1-ns2.example.com.; };8.2.5

Checklists for Key File Creation and Key Configuration Process

Checklist item 8: The TSIG key should be a minimum of 112 bits in length if the generator utility has
been proven to generate sufficiently random strings [800-57P1]. 128 bits is recommended.
Checklist item 9: A unique TSIG key should be generated for each pair of communicating hosts (i.e. a
separate key for each secondary name server to authenticate transactions with the primary name server,etc.).
Checklist item 10: After the key string is copied to the key file in the name server, the two files
generated by the dnssec-keygen program should either be made accessible only to the server administrator
account (e.g., root in Unix) or, better still, deleted. The paper copy of these files also should be destroyed.
Checklist item 11: The key file should be securely transmitted across the network to name servers thatwill be communicating with the name server that generated the key.
Checklist item 12: The statement in the configuration file (usually found at /etc/named.conf for BIND
running on Unix) that describes a TSIG key (key name (ID), signing algorithm, and key string) should not
directly contain the key string. When the key string is found in the configuration file, the risk of key
compromise is increased in some environments where there is a need to make the configuration filereadable by people other than the zone administrator. Instead, the key string should be defined in a
separate key file and referenced through an include directive in the key statement of the configuration file.
Every TSIG key should have a separate key file.
Checklist item 13: The key file should be owned by the account under which the name server software is
run. The permission bits should be set so that the key file can be read or modified only by the account that

8-14

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

runs the name server software.
Checklist item 14: The TSIG key used to sign messages between a pair of servers should be specified in
the server statement of both transacting servers to point to each other. This is necessary to ensure thatboth the request message and the transaction message of a particular transaction are signed and hence
secured.

In each of the configuration files of the pair of servers that share a secret key in a zone, the name of the
key to be used for all communication between them must be specified (via the server statement in BIND
configuration files).
8.2.6

Securing Zone Transfers using TSIG

The pair of servers participating in zone transfer transactions must be instructed to use the key definedusing the key statement (Section 8.2.2). This pair generally consists of a primary name server and asecondary name server. The primary name server is configured to accept zone transfer requests only fromsecondary name servers that send MACs using the named key along with a zone transfer request message.
The configuration is accomplished by using the allow-transfer substatement of the zone statement. A
sample allow-transfer substatement that specifies that the primary name server should only allow zonetransfer requests for the example.com zone from name servers that use the ns1-ns2.example.com key is asfollows:zone “example.com” {type master;
file “zonedb.example.com”;
allow-transfer { key {ns1-ns2.example.com.}; };};The secondary name server is instructed to use the key ns1-ns2.example.com in the zone transfer request
to the primary name server (with IP address 192.249.249.1) using the server statement shown in Section
8.2.3.
In NSD, the syntax is similar, but there is a special requirement if no TSIG key is to be used. The zone
option “provide-xfer” is used to indicate which IP addresses can request a zone transfer for this zone on
this server:
zone:
type: master;
file “zonedb.example.come”;
provide-xfer: 192.68.0.1 ns1-ns2.example.com.
provide-xfer: 192.68.0.2 NOKEY
8.2.7

Securing Dynamic Updates Using TSIG or SIG(0)Dynamic update restrictions based on TSIG keys can be specified in BIND 8.2 and later versions 9 byusing the allow-update substatement of the zone statement. The arguments to this statement are the9

As mentioned earlier, Micosoft Server does not use TSIG for dynamic update, but uses IPSec instead. See:
http://technet.microsoft.com/en-us/library/cc753751.aspx

8-15

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

keyword key followed by the name of the TSIG key. (See Section 8.2.2 for details on how to enter in the
key statement in a BIND name server configuration file.) Once the key statement has been entered, thefollowing substatement can be added to the zone statement to make use of the secret key for dynamic
updates:zone “example.com” {type master;
file “zonedb.example.com”;
allow-update { key dhcp-server.example.com.; };};Note that although the string dhcp-server.example.com. looks like a FQDN, it actually denotes the nameof the TSIG key. The implication of the configuration statement example is that any host that possess the
key named dhcp-server.example.com. can submit dynamic update requests (adding, deleting, or
modifying RRs) to the zone file (for the zone example.com) that resides in the primary authoritative nameserver.
To use SIG(0) to authenticate dynamic update messages, the key used must first have its public
component stored in the DNS, so a validating client can obtain it 10. See section 9.5 on publishing keys.
The previous steps above need to be performed to control access (if desired). After that is done, theupdating name server (i.e., primary authoritative name server) should be able to obtain the key andprocess the dynamic update request (if the name server supports SIG(0) with dynamic update).
8.2.8

Configuring Dynamic Update Forwarding Restrictions Using TSIG Keys

Dynamic updates are allowed on the copy of the zone file in the primary authoritative name server only
because that is the only “writable” copy. This does not automatically imply that the primary authoritative
name server is the only one allowed to accept dynamic update requests. In fact, BIND 9.1.0 and later
versions allow secondary name servers to accept dynamic update requests and forward them to theprimary authoritative name server. In this scenario, if there are no restrictions on the basis of the identityof hosts from whom the secondary name server can forward such dynamic update requests, it isequivalent to circumventing the dynamic update restrictions specified in the primary name server because
the request can literally originate from any host to the secondary name server and be forwarded to theprimary name server. To counter this problem, a new substatement, allow-update-forwarding, is now
available in BIND versions that have the dynamic update forwarding feature. An example of this allowupdate-forwarding statement using TSIG keys is given below:
zone “example.com” {type slave;
file “backupdb.example.com”;
allow-update-forwarding { key dhcpserver.example.com.; };
};
8.2.9

Configuring Fine-Grained Dynamic Update Restrictions Using TSIG/SIG(0) Keys

The allow-update substatement specifies dynamic update restrictions based on the originators of dynamic
update requests (a specific set of hosts identified by IP address or holding a TSIG key) but not thecontents of the zone records. To specify dynamic update access (grant or deny) restrictions based on a10

For more information, see the note from the dynamic update howto: http://ops.ietf.org/dns/dynupd/secure-ddns-howto.html

8-16

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

combination of domain/subdomain names and RR types (A, MX, NS, etc.), BIND 9 and later versions
provide the update-policy substatement within the zone statement. The update-policy substatement bases
these restrictions on the TSIG key. In other words, the update-policy statement specifies which TSIG keys
(or holders of keys) are allowed to perform dynamic updates on which domains/subdomains and RR types
within that domain/subdomain.
The general form of the update-policy statement is as follows:
update-policy {(grant | deny) TSIGkey nametype name [type]};where the semantics of each of the statement components are as follows:
grant/deny—allow/disallow dynamic update for the combination that follows
TSIGkey—the name of the TSIG key used to authenticate the update request
nametype—can be one the following with the associated semantics:
name—restriction applies to the domain name specified in the following name field
subdomain—restriction applies to subdomains of the domain specified in the following
name fieldwildcard—restriction applies to the set of domains specified using the wildcard syntax(i.e., *) in the following name fieldself—restriction applies to the domain whose name is the same as that in the TSIGkey
field (i.e., the domain name whose records are to be updated has the same name as the
key used to authenticate the dynamic update request). In this usage, the contents of thename field become redundant but still should be used in the statement (i.e., the name fieldcannot be left blank)
name—used to specify the name of the domain. The syntax used and the domains it covers are
based on the value used in the nametype field (e.g., if subdomain is the value of the nametype
field, then all subdomains of the domain name used are being covered under this statement).
type—an optional field that can contain any valid RRType (except the NSEC type) or the
wildcard type ‘ANY’ (ANY stands for all RR types except the NSEC type). If it is missing, it
denotes all RRTypes except SOA, NS, RRSIG, and NSEC. It also is possible to put in multiple
RRTypes separated by a space (e.g., A NS).
Examples of update-policy statements and their associated semantics are given below.
Suppose there is a domain sales.example.com within example.com and that name server uses a TSIG key
that has the same name as its own domain name (i.e., sales.example.com). All dynamic updates fromsales.example.com could be restricted to all resource records of that domain within the zone file asfollows:zone “example.com” {type master;

8-17

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

file “zonedb.example.com”;
update-policy { grant sales.example.com. self
sales.example.com.; };};All dynamic updates from sales.example.com could be restricted to only A and MX RR types of thatdomain as follows:zone “example.com” {type master;
file “zonedb.example.com”;
update-policy {grant sales.example.com. self sales.example.com. A MX; };};To allow clients with the TSIG key sales.example.com to update all records pertaining to subdomains ofNEsales.example.com except the name server records (RR Type NS):
zone “example.com” {type master;
file “zonedb.example.com”;
update-policy {deny sales.example.com. subdomain NE
sales.example.com. NS;
grant sales.example.com. subdomain NE
sales.example.com. ANY; };};For Microsoft Windows, authentication is provided using GSS-TSIG. System administrators of Windows
Servers should consult their implementation’s documentation on how to integrate secure dynamic updateusing GSS-TSIG.
8.3

Recommendations Summary

The following items provide a summary of the major recommendations from this section:•••

Checklist item 7: It is recommended that the administrator create a named list of trusted hosts (orblacklisted hosts) for each of the different types of DNS transactions. In general, the role of thefollowing categories of hosts should be considered for inclusion in the appropriate ACL:–DMZ hosts defined in any of the zones in the enterprise–All secondary name servers allowed to initiate zone transfers–

Internal hosts allowed to perform recursive queries.

Checklist item 8: The TSIG key (secret string) should be a minimum of 112 bits in length if thegenerator utility has been proven to generate sufficiently random strings [800-57P1]. 128 bits
recommended.
Checklist item 9: A unique TSIG key should be generated for each set of hosts (i.e. a unique key
between a primary name server and every secondary server for authenticating zone transfers).

8-18

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

•

•
•

•
•

Checklist item 10: After the key string is copied to the key file in the name server, the two files
generated by the dnssec-keygen program should either be made accessible only to the server
administrator account (e.g., root in Unix) or, better still, deleted. The paper copy of these files
also should be destroyed.
Checklist item 11: The key file should be securely transmitted across the network to nameservers that will be communicating with the name server that generated the key.
Checklist item 12: The statement in the configuration file (usually found at /etc/named.conf for
BIND running on Unix) that describes a TSIG key (key name (ID), signing algorithm, and key
string) should not directly contain the key string. When the key string is found in theconfiguration file, the risk of key compromise is increased in some environments where there is aneed to make the configuration file readable by people other than the zone administrator. Instead,the key string should be defined in a separate key file and referenced through an include directivein the key statement of the configuration file. Every TSIG key should have a separate key file.
Checklist item 13: The key file should be owned by the account under which the name server
software is run. The permission bits should be set so that the key file can be read or modified onlyby the account that runs the name server software.
Checklist item 14: The TSIG key used to sign messages between a pair of servers should bespecified in the server statement of both transacting servers to point to each other. This isnecessary to ensure that both the request message and the transaction message of a particulartransaction are signed and hence secured.

8-19

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

9.

Guidelines for Securing DNS Query/Response

Section 6.1.4 describes the DNSSEC approach for protection of DNS query/response transactions throughdata origin authentication, data integrity verification, and authenticated denial of existence capabilities.
This section describes the mechanisms involved in the DNSSEC approach, the operations thosemechanisms involve, and a secure way of performing those operations by using checklists. In otherwords, this section provides guidelines for secure deployment of DNSSEC features through DNSSEC
operations that are supported in name server software. These guidelines are covered in Sections 9.3
through 9.7.
To ensure end-to-end protection of DNS query/response transactions, additional protection measures(apart from what DNSSEC specification provides)—such as securing the communication path between
local DNSSEC-aware caching/resolving name servers and stub resolvers—are needed. These measures
are discussed in Section 9.8.
Zone administrators need to have an understanding of the logic of dynamic updates in the presence ofadditional RRsets (especially the NSEC RRSet) introduced into the zone file by DNSSEC specification.
This logic is explained in Section 9.9.
9.1

Enabling DNSSEC processing in BIND & NSD

Before a DNSSEC signed zone can be deployed, a name server must be configured to enable DNSSEC
processing. In BIND, it is done by adding the following line to the options statement in the named
configuration file (named.conf)
options {dnssec-enable yes;};
After restart, the name server will now perform DNSSEC processing for DNS query/responsetransactions.
In NSD and Windows Server, this option statement is not necessary. If the zone file contains DNSSECResource Records, they are loaded automatically and the server correctly sends DNSSEC enabled
responses when signaled by a client query.

Checklist item 15: Name servers that deploy DNSSEC signed zones or query signed zones should be
configured to perform DNSSEC processing.

9.2

DNSSEC Mechanisms and Operations

DNSSEC mechanisms involve two main processes: sign and serve, and verify signature. These processes
are described below.
9.2.1

Sign and Serve

The first task in this process is to generate digital signatures associated with every RR in the zone file.
Instead of generating a signature for every RR, DNSSEC specifies generation of a signature for an RRSet
9-1

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE(a set of RRs with the same owner name, class, and RRType). The digital signature and its associatedinformation (ID of the key used, start and expiry dates for the signature, etc.) are encapsulated in a special
RR of RRType RRSIG (Resource Record Signature). The actual key string and associated information
about the public key that is to be used to verify the signature (in RRSIG) are given in a special RRType
DNSKEY. Another RRType, NSEC (Next Secure), is used to list RRTypes (in canonical order[RFC4034]) available for a given domain (owner name), and a signature (RRSIG RR) for that RRType isgenerated to provide authenticated proof of nonexistence to queries for any nonexistent RRType in thatdomain. In addition, there is an optional RRType DS (Delegation Signer) in case a zone wants tovouchsafe the authenticity of the public key of its child zone. In other words, the DS RR carries thesignature for the RR that contains the (hash) public key of the child zone. The detailed syntax for each ofthese additional RRTypes introduced by DNSSEC specification is specified in RFC 4034 [RFC4034].
The most important of these is the RRSIG RR because it contains the actual signature string.
The RRSIG RR, like any other RR, contains the owner name, TTL, class, RRType, and RDATA fields.
The digital signature and all its associated information are contained within the RDATA field. The layoutof the RDATA field in the RRSIG RR, with all subfields, is shown in Figure 9-1. A brief description ofeach subfield follows.
RRType Covered

Algorithm CodeLabelsOriginal TTL
Signature Expiration
Signature Inception
Key Tag

Signer’s Name
Encoded Signature

Figure 9-1. RRSIG RR’s RDATA Field Layout

The “RRType Covered” field is the type of RRSet for which this RRSIG holds the signature. The“Algorithm Code” field is a code integer assigned to represent a given cryptographic algorithm used togenerate the signature. The “Labels” and “Original TTL” fields are the number of labels (number oflabels in the FQDN) and the TTL value of the RRSet this signature covers. The “Signature Expiration”
and “Signature Inception” values are absolute time values that span the signature validity period—thetime period for which this RRSIG is considered valid for the zone. The “Key Tag” and “Signer’s Name”
fields are the hash and FQDN of the DNSKEY RR that the client needs to validate the signature, and thefinal field is the encoded signature itself.
A zone that contains the additional RRs along with the regular RRs is called a signed zone. A name serverthat hosts these signed zones and includes the appropriate signatures (i.e., the corresponding RRSIGs)
along with requested RRs in its response is called a DNSSEC-aware name server.
9.2.2

Verify Signature

The response coming from a signed zone is called a signed response. A resolver that has the capability toverify signatures in a signed response is called a DNSSEC-aware validating resolver. Before a resolver
can verify the signature associated with RRsets (in the response) of a zone using the public key of thezone (sent along with the response), it has to establish trust in that public key. In DNSSEC thisrequirement is addressed by having the resolver go through a subprocess called building a trusted chain.

9-2

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEIn this subprocess, the resolver starts from a known list of trusted public keys (called trust anchors) andestablishes trust in the public key of the zone in a given response by traversing through a chain of publickeys, building the chain by using the DNS name space hierarchy. The trust anchors in a resolver ideallyconsist of the root public keys (if root servers are DNSSEC-aware) or public keys of zones lower in thehierarchy. The trust anchor list in a resolver is not built through a DNS transaction; it uses an out-of-bandmechanism.
The DNSSEC processes described above involve several name server operations and a few resolver
operations. The name server operations are as follows:•

DNSSEC-OP1: Generation of public key-private key pair•DNSSEC-OP2: Secure storage of private keys•

DNSSEC-OP3: Public key distribution•

DNSSEC-OP4: Zone signing•

DNSSEC-OP5: Key rollover (changing of keys)•

DNSSEC-OP6: Zone re-signing.

The resolver operations are as follows:•

DNSSEC-OP7: Trust anchors’ configuration•

DNSSEC-OP8: Establishing trusted chain and signature verification.

Name server operations DNSSEC-OP1 through DNSSEC-OP4 and resolver operations DNSSEC-OP7
and DNSSEC-OP8 are performed either prior to DNSSEC deployment or for secure operation (serving
signed responses and verification of signatures) and hence are covered in this section. The remaining
name server operations (key rollovers and zone re-signing – DNSSEC-OP5 and DNSSEC-OP6
respectively) are performed on a periodic basis after a fully operational DNSSEC deployment and hence
are covered in Section 11.
9.3

Generation of Public Key-Private Key Pair (DNSSEC-OP1)

DNSSEC specifies generation and verification of digital signatures using asymmetric keys. This requires
generation of a public key-private key pair. Although the DNSSEC specification does not call fordifferent keys (just one key pair), experience from pilot implementations suggests that for easier routine
security administration operations such as key rollover (changing of keys) and zone re-signing, at least
two different types of keys are needed. One set is called Key Signing Key (KSK). This key (specifically,
the private part of the key pair, called KSK-private) will be used only for signing the key set (i.e.,
DNSKEY RRSet) in the zone file. The other key type is called the Zone Signing Key (ZSK) (whose
private part is called ZSK-private) and will be used to sign all RRsets in the zone (including DNSKEY
RRSet). An administrative distinction is made between the KSK and ZSK keys by setting the Secure
Entry Point (SEP) flag bit in the DNSKEY RR that represents the public part of those keys (in this case, itwould be called KSK-public).
The logic behind creation of two types of key pairs is to provide separate set of functions for each key
type and thus reduce the overall complexity of tasks involved in key rollovers and zone re-signing.

9-3

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

Accordingly, the KSK (KSK-private) is used to sign the key set (i.e., DNSKEY RRSet) and is the key
type (public component – KSK-public) that is sent to the parent to be used for authenticated delegation.
This is done by generating a DS RR, using the hash of the child’s KSK-public key and generating acorresponding signature (RRSIG RR) using the parent’s own ZSK. The KSK (KSK-public) may also be
used as a trust anchor (sometimes called the SEP keys) in validating resolvers to establish trust chains forverification of signatures.
The ZSK (ZSK-private) is to be used for signing the entire zone file (all RRsets). The public portion ofthis key (ZSK-public) will not be sent to the parent and will always remain in the zone.
The decision parameters involved in KSK and ZSK key pair generation are as follows:•

Choice of digital signature algorithm•

Choice of key sizes•

Choice of crypto period (duration for which the key will be used).

The choice of digital signature algorithm will be based on recommended algorithms in well-known
standards. NIST’s Digital Signature Standard (DSS) [FIPS186] provides three algorithm choices:•

Digital Signature Algorithm (DSA)

•

RSA

•

Elliptic Curve DSA (ECDSA).

Of these three algorithms, RSA and DSA are more widely available and hence are considered candidatesof choice for DNSSEC. In terms of performance, both RSA and DSA have comparable signature
generation speeds, but DSA is much slower for signature verification. Hence, RSA is the recommended
algorithm as far as this guideline is concerned. RSA with SHA-1 is currently the only cryptographic
algorithm mandated to be implemented with DNSSEC although other algorithm suites (i.e. RSA/SHA256, ECDSA) are also specified. It can be expected that name servers and clients will be able to use the
RSA algorithm at the minimum. It is suggested that at least one ZSK for a zone use the RSA algorithm.
NIST’s Secure Hash Standard (SHS) (FIPS 180-3) specifies SHA-1, SHA-224, SHA-256, SHA-384, andSHA-512 as approved hash algorithms to be used as part of the algorithm suite for generating digital
signatures using the digital signature algorithms in the NIST’s DSS[FIPS186]. It is expected that there
will be support for Elliptic Curve Cryptography in the DNSSEC. The migration path for USG DNSSEC
operation will be to ECDSA (or similar) from RSA/SHA-1 and RSA/SHA-256 before September 30th,2015.
The choice of key size is a tradeoff between the risk of key compromise and performance. The
performance variables are signature generation and verification times. The size of the DNS response
packet also is a factor because DNSKEY RRs may be sent in the additional section of the DNS response.
Because the KSK is used only for signing the key set (DNSKEY RRSet), performance is not much of anissue. Compromise of a KSK could have a great impact, however, because the KSK is the entry point key
for a zone. Rollover of a KSK in the event of a compromise involves potential update of trust anchors inmany validating resolvers.

9-4

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEAs far as the choice of key size for the ZSK is concerned, performance certainly will be a factor because
the ZSK is used for signing all RRsets in the zone. In terms of impact, however, it is restricted to just asingle zone because the ZSK’s usage is limited to signing RRsets only for that zone. This is thejustification for allowing 1024 bit RSA keys for use with DNSSEC beyond the USG stop date of 2010.
Some network components have been shown to have problems handling large DNS responses. The use of1024 bit RSA keys is still considered acceptable to compensate for this as long as other rigorous key
management practices are in place.
The choice of crypto period (rollover period) is dictated by the amount of work required to compromise
the given key. The large size of the KSK implies that the crypto period for that key can be long (usually ayear or two). This aids in DNS operations as well, as KSK rollover is more disruptive and requires thezone administrator to interact with their parent zone to update the KSK’s DS RR in the parent delegation
information. In the case of ZSK, the risk of key guessing is higher of its smaller size. This implies that
ZSKs must be rolled over more frequently than KSKs (usually between 1-3 months). Since the ZSK is
local to the zone, rolling the ZSK is not as disruptive as rolling the KSK.
In the case of ZSK, the risk of compromise may be greater due to more frequent exposure. If the zone allows
dynamic update, the ZSK is often stored on the same server as the zone. This factor, combined with therelatively smaller size of the key, implies that ZSKs must be rolled over more frequently than KSKs (usually
between 1-3 months).

In terms of the number of keys of each type (KSK and ZSK) to be generated, a good practice is togenerate an extra ZSK in addition to the one that will be used for signing. Hence, the zone administrator
should use the key generation program to generate one KSK and two ZSKs during initial deployment of
DNSSEC. One ZSK is treated as the active key, and its private part (ZSK-private) will be used forsignature generation. The other ZSK (ZSK-public) will be made part of DNSKEY RRSet, but its
associated private part (ZSK-private) will not be used for signing RRsets. This additional ZSK will
provide a readily available ZSK for immediate rollover in emergency situations such as key compromise
and a form of advance notification to validating resolvers that this key is to be the one into which the zone
is going to roll over after the current crypto period expires. The mere presence of the key in the DNSKEY
RRSet enables validating resolvers to cache and establish trust in the new key so that they can
immediately use the key for signature verification as soon as rollover occurs.
The recommended digital signature algorithm suite, key sizes, and crypto periods for the KSK and ZSKkeys are given in Table 9-1 [800-57P1]. As with all data authentication keys, this table assumes approved
components 11 (hardware or software) and management operations are in place within the organization.

11

For example, for US Federal Government use, all cryptographic components must be FIPS 140 approved. See
http://csrc.nist.gov/groups/STM/cmvp/documents/140-1/140val-all.htm for a current list of approved components.

9-5

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDETable 9-1. Digital Signature Algorithms, Min. Key Sizes, and Crypto Periods
Key Type
Key-Signing Key (KSK)

Digital Signature
Algorithm Suite

Key Size

RSA-SHA1 (RSASHA-256) until 2015

2048 bits

12-24 months (1-2
years)ECDSA with Curvef = 224-255 bits

12-24 months (1-2
years)RSA-SHA1 (RSASHA-256) until 2015

1024 bits

1-3 months (30-90 days)

ECDSA with Curvef = 224-255 bits

12-24 months (1-2
years)

P-256 or with
Curve P-384
Zone-Signing Key (ZSK)

Crypto Period(Rollover Period)

P-256 or with
Curve P-384

In the above table, the digital signature algorithm suite is given as both RSA-SHA1 and RSA-SHA256.
This is because as of the time of writing, RSA-SHA1 is the only algorithm that is both Mandatory for
implementations and Approved for use in the Federal Government. However, RSA-SHA1 will be phasedout and replaced by RSA-SHA256 within the Federal Government. It is expected that not all software
will be updated – especially outside the Federal Government. Because of this, DNS administrators may
wish to deploy and use both algorithms for a period of time so DNSSEC client software that does notunderstand RSA-SHA256 can still get some protection from DNSSEC. The DNS root zone uses
RSA/SHA-256 for signing, so deployment of RSA/SHA-256 enabled DNS validators has quickened. It is
recommended that new DNSSEC deployments (i.e. initial signing) consider using RSA/SHA-256 or ECDSA,
rather than going through the complicated process of algorithm rollover.
The use of RSA in DNSSEC is approved until the year 2015. By this time, it is expected that Elliptic
Curve Cryptography (ECC) will be specified in the DNSSEC. USG DNS administrators should plan tomigrate to the use of ECDSA (or similar) when it becomes available in DNSSEC components. ECC has
an advantage of having the same precieved strength as RSA with a smaller key size. This means that theZSK can be the same size as the KSK and have a longer cryptoperiod than a 1024 bit RSA ZSK.
9.3.1

Key Pair Generation—Illustrative Example

Every DNSSEC-aware name server implementation should provide a utility program for generating
asymmetric key pairs (a public-private key pair). The use of one such program, dnssec-keygen (provided
by BIND 9.X), is illustrated below:
dnssec-keygen –a algorithm – b bits –n type [options] namewhere algorithm (under –a parameter) can be one of the following:•

RSASHA1•

RSASHA1-NSEC3-SHA1•

DSASHA1•

RSASHA256

9-6

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE•RSASHA512•ECDSAP256SHA256

bits (key size) for the –b parameter has the following ranges:•[512..4096] for RSASHA1 and RSASHA256, etc.

•

[512..1024] for DSA (must be divisible by 64)•[] for ECDSA

Note: Even though the key generation utility allows for key sizes smaller than the approved size, US
Federal DNS administrators shall only generate keys with approved lengths for use to sign their DNS
zone [800-57P3].
type for the –n parameter can be one of ZONE | HOST
name is the owner of the key (usually the domain name in the zone apex).
This command generates two files—one containing the public key and the other the associated private
key. The generic names for these files are as follows:
K<domain_name>+algorithm_id+Key_id.key
K<domain_name>+algorithm_id+Key_id.private
The domain_name is the value of the name parameter specified on the command line. Example algorithm
IDs are:003 – DSA
005 – RSASHA1
008 – RSASHA256
The key_id is the unique identifier for the key generated by the program.
For example, to generate a ZSK keyset of length 1024 bits that uses the RSASHA256 algorithm suite forsigning the zone example.com, the following command would be issued:dnssec-keygen –a RSASHA256 –b 1024 –n ZONE example.com
For this command, the following files containing the private and public keys, respectively, are generated:
Kexample.com.+008+28345.private
Kexample.com.+008+28345.key
In these file names, 005 stands for the algorithm_id and 28345 is the unique key ID.
In the *.key file, the public key information is expressed in the same syntax as that of a zone file RR. The
content of the file Kexample.com.+005+28345.key will be:
9-7

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

example.com
key string)IN

DNSKEY256

3

8

BQFG+KGJ7………. (Base64 encoded

Hence, the contents of the file containing the public key can be readily added to the zone file(zonedb.example.com) contents by using the following command:cat *.key >> /var/named/zonedb.example.com
After the DNSKEY RR containing the public key is added to the zone file, the zone’s serial number in the
SOA RR must be increased prior to actually signing the zone.
9.4

Secure Storage of Private Keys (DNSSEC-OP2)

The private keys in the KSK and ZSK key pairs should be protected from unauthorized access. If
possible, the private keys should be stored offline (with respect to the Internet facing DNSSEC-aware
name server) in a physically secure, non-network-accessible machine along with the zone file mastercopy. The signatures generated by using the private keys should be transferred to the primaryauthoritative name servers through a load process, using a dynamically established network connection(rather than a permanent network link).
This strategy is not feasible in situations in which the DNSSEC-aware name server has to support
dynamic updates. To support dynamic update transactions, the DNSSEC-aware name server (whichusually is a primary authoritative name server) has to have both the zone file master copy and the private
key corresponding to the zone-signing key (ZSK-private) online to immediately update the signatures forthe updated RRsets. The private key corresponding to the key-signing key (KSK-private) can still be kept
offline. In this scenario, the following measures must be taken to protect ZSK-private:

•
•

•

•

The shell from which the key generation program/utility is invoked should not be accessible
except by authorized zone administrators/operators.
The directory in which the private key files are stored (usually a subdirectory under a directorywith the same name as the zone, with the file names in that directory having the name structure
K<zonename>.+AlgorithmID+<keytag>.private in BIND) should not be accessible/visible except
to authorized zone administrators/operators.
Sufficient fault tolerance should be provided for the contents of the directory containing theprivate key files, either by having a mirrored disk or through periodic backups on tape,
CD/DVDs, or optical media. The backup media also should be protected against disclosure,tampering, or theft.
Another strategy is to store the private keys in an encrypted file system.

Checklist item 16: The private keys corresponding to both the ZSK and the KSK should not be kept on
the DNSSEC-aware primary authoritative name server when the name server does not support dynamic
updates. If dynamic update is supported, the private key corresponding to the ZSK alone should be kept
on the name server, with appropriate directory/file-level access control list-based or cryptography-based
protections.

9-8

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

9.5

Publishing the Public Key (DNSSEC-OP3) and Setting Up Trust Anchors (DNSSECOP7)

The operation of verifying the zone data of a particular zone by a DNSSEC-aware resolver begins withthat resolver knowing and trusting the public key of that zone (the one whose data is to be verified) or anyof its ancestors (parent and zones above the parent in the DNS tree). If the zone to be verified (e.g.,
example.com) is secure and its parent (.com) is not, the point of trust begins with the zone itself. If theparent (i.e., the .com zone) is secure (i.e., DNSSEC-aware) and the parent of the parent (i.e., a root zone)
is not secure, the starting point of trust is the parent (i.e., .com zone). If a root zone is secure, of coursethat becomes the origin of trust.
Whatever the starting point of the chain of trust (enterprise level, TLD, or root zone), the public key ofthat starting point should be made known by the associated name server to the DNSSEC-aware resolvers
of interest. These public keys known to DNSSEC-aware resolvers are called trust anchors. Because there
is no feature within DNS for third-party authentication of public keys (like X.509 Public Key
Infrastructure), the public key has to be distributed through an out-of-band (relative to DNS) process. This
distribution may be accomplished through channels such as Web sites or e-mails.
The entries in the trust anchor list of a DNSSEC-aware resolver determine whether a signed response
from a zone will be categorized as secure or insecure. As mentioned in section 9.2.2, the resolver has toestablish trust in the public key of the zone that has sent the response before it performs signature
verification. If that trust cannot be established due to not being able to build a trusted chain using any ofthe entries in trust anchor list, the response will be tagged as insecure. The reason for not being able tobuild a trusted chain may be one of several potential reasons: There maybe an intermediate zone signed
using an algorithm the end validators does not understand or it may be due to the fact that the DNS
namespace contains islands of signed zones as opposed to unbroken hierarchical sequence of signed
zones. The effect of each trust anchor in labeling responses for a given mapping of islands of signed
zones (Figure 9-2) is given in Table 9-2.

root “.”

gov

org
com

nist.gov
example.com

Figure 9-2. A Mapping of Islands of Security

9-9

example.org

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDETable 9-2. Impact of Trust Anchor in Labeling Responses
Trust anchor installedon DNSSEC-aware
resolverDNSSEC Response Status for Queries Addressed towww.nist.gov

www.example.com

www.example.org

none

insecure

insecure

insecureroot

secureinsecure

insecure.com

insecure

secureinsecure

example.org

insecure

insecure

secure9.6

Zone Signing (DNSSEC-OP4)

The following sequence of actions takes place when a zone file is signed:•

The zone file is sorted in canonical order of domain names.

•

An NSEC RR (or NSEC3) is generated for every owner name in the zone.

••

The KSK (i.e., KSK-private) is used to sign the DNSKEY RRSet. The KSK is identified by using
the SEP flag in the RDATA portion of the DNSKEY RRType.
The ZSK (i.e., ZSK-private) is used to sign all RRsets in the zone (including DNSKEY RRsets
and NSEC RRsets).

Checklist item 17: Signature generation using the KSK should be done offline, using the KSK-private
stored offline or using a secure, protected module; then the DNSKEY RRSet, along with its RRSIG RR,
can be loaded into the primary authoritative name server.

9.6.1

Zone Signing—Illustrative Example

Use of a zone signer program (dnssec-signzone provided by BIND 9.5.x) to sign the zone is illustrated
below:
dnssec-signzone -o <name of the zone> -k <name of the filecontaining KSK> <location of zone file> <name of file containing
ZSK>
To sign the data for the zone example.com with the KSK located in the file
Kexample.com.+005+76425.key and the ZSK generated earlier the command would be as follows:
dnssec-signzone -o example.com -k Kexample.com.+005+76425.key \/var/named/zonedb.example.com Kexample.com.+005+28345.key
The process of signing a zone file consists of generating a hash for each RRSet (RRs with the same ownername, class, and RRType), generating a signature for the RRSet (using the private key ZSK-private), andthen capturing this signature information in a new RR of type RRSIG RR. The KSK signs just the
DNSKEY RRSet, whereas a ZSK signs all RRsets in the zone file. The entity whose private key is used tosign the zone data is called the signer or signing authority. In most cases, the signer is the domain
9-10

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

associated with the zone. In response to a DNS query, a DNSSEC-aware authoritative name server serves
the relevant zone data along with the digital signature of the data. The recipient validates the digital
signature associated with the received zone data, using the public key of the signer (after establishing trustin that public key). This decryption provides the hash of the original stored zone data. The recipient also
generates a hash (using the same hash function algorithm used by the signer) of the received zone data.
This computed hash is then compared with the hash retrieved from the digital signature. If the hashes
match, then the digital signature in the response is considered validated.
9.7

Establishing Trust Chain and Signature Verification (DNSSEC-OP8)

Until all zones become signed zones, there could be a situation in which a zone is signed but its parent is
not signed. The only point of trust (assurance) for the DNSSEC-aware resolver in this scenario is thepreconfigured public key of the zone data signer. In the absence of any other source vouching for theauthenticity of the public key of the signer, the only source the resolver can trust is the zone signer itself,
assuming that the public key is configured to be trusted. On the other hand, if there is a DNS zone X thatwill vouch for the authenticity of the public key of zone signer child-of-X, the resolver can establish trustin zone signer child-of-X through X. The sequence of zones in the DNS tree through which a resolver
establishes trust in the public key of the signer serving signed zone data is called the chain of trust.
In the example, the chain of trust starts (originates) with DNS site X and ends with the zone data from site
Y. Usually DNS site X will be the zone that is the immediate parent of zone Y (called the parent zone) in
the global DNS zone tree. The parent zone vouches for the authenticity of the child zone by digitallysigning the hash of the public key of the child zone. This hash is stored in a DNS RR called theDelegation Signer (DS) RR. The parent zone also must be a signed zone because a nonsigned zone will
not possess the private key (of a private key/public key pair) to sign the hash of the public key of the childzone. Depending on the absence or presence of the chain of trust, a signed zone can be one of thefollowing:••

Island of security—a zone that is self-signed. The reason it is self-signed is that the zone’s
parent is not a signed zone or is unwilling to set up secure delegations and hence cannot vouch forthe authenticity of the public key of the child zone. In this instance, there really is no chain oftrust.
Chained secure zone—the zone’s parent and possibly one or more of the ancestors up the DNS
zone tree that are signed. In this hierarchy of signed zones and the associated hierarchy of
configured keys, the resolver usually will choose the key at the zone that is highest in thishierarchy of signed zones as the origin of the chain of trust. The public key of the zone at theorigin of the chain of trust is called the trust anchor. In fact, there may be more than one key thatis a trust anchor. In other words, the trust anchors refers to the group of keys that are initiallytrusted by a resolver, which can then be used to build the chain of trust that leads all the way tothe public key of the zone whose signatures the resolver wants to verify. If the chain of trust
leads all the way up the DNS tree to the root zone, the zone in question is said to be “globally
secure”.

Zone data protection through digital signature service for an island-of-security zone consists of thefollowing basic tasks:•

Public key/private key pair generation•

Secure storage (if necessary, offline from the name server) of the private key

9-11

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE•

Publication of the public key by incorporation of the DNSKEY RR in the zone file•

Generation of digital signatures for zone data (zone signing).

Additional tasks are involved for a chained secure zone. To be part of the chain of trust, the zone has toinform its parent of its public key (KSK-public) securely (through out-of-DNS channel means). The
parent then creates a hash of the public key of its child and stores it in the parent zone in a new RR calleda DS RR [RFC4034]. Or the child zone can generate its own DS RRset and upload that to its parent zone
for inclusion in the parent zone. It also signs this DS RR by generating a RRSIG RR. Practical conditions
dictate, however, that the keys periodically have to be changed because any key can be broken with
sufficient computing power. In a chained secure zone, whenever a zone changes its KSK, its parent has tobe notified of the new key. The parent then has to add or generate a new DS RR and sign it again. To
reduce the administrative burden involved, a common strategy is to use another key pair, the KSK. The
KSK is used for signing only the DNSKEY RRSet; all of the other authoritative RRsets in the zone file
are signed with the ZSK. The KSK is the key that is published to the parent. The parent will generate (orsimply add) the DS RR and a RRSIG RR using the parent’s own ZSK. The KSK is used less frequently to
only sign the DNSKEY RRset and hence needs to be changed less frequently. There may be situations,however, in which either because of the manageable frequency of key rollovers (key change) or thecriticality of DNS information served by the zone, administrators may not use two distinct key pairs for
the ZSK and KSK.
In summary, the additional tasks that take place in a chained secure zone are as follows:••

Secure transmission of the KSK or DS by a secure zone to its parent. This transmission isaccomplished out of band and may not involve any DNS transactions.
The parent stores the child’s KSK in a special keyset file directory, generates a hash of the key,and stores the hash in a DS RR. The parent also generates the digital signature (RRSIG RR) for
this DS RR and includes it with the other delegation information. OR The parent adds the DS to
its zone database after the child uploads the DS RRset.

The zone whose signed response needs to be verified (i.e., the target zone) is the leaf node of the chain oftrust. The pre-requisite for this operation is to establish trust in the ZSK of the zone. The trust in a zone’s
ZSK is established through the following operations:•

Authenticated referral from the parent•

Authenticating the KSK of the child zone.

To understand authenticated referral from the parent, it is necessary to look at the normal DNS referral
process. In the normal DNS query, a zone that does not have authoritative information pertaining to aquery for a domain name in a child zone provides a hint or referral by providing the NS RRSet andassociated additional information (RRs that contain the IP addresses for servers provided in the NS
RRSet). In normal DNS query processing, following this referral is an acceptable processing step. This
process is insufficient, however, from the point of view of establishing a chain of trust; the information
about the NS RRSet and associated glue RRs (or any other additional information) cannot be consideredauthentic because they are not signed with the public key of the authoritative source (i.e., the child zone).
To enable authentication of this referral information, the parent provides the cryptographic hint throughthe DS RR.

9-12

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

Consider an example of how a DNSSEC-aware resolver validates a DNS response from the signed zone
example.com. The resolver, following its chain of trust starting from its trust anchor, has authenticated the
public key for .com zone and trusts it. Hence, it trusts the NS RRs and DS RR served by the .com zone.
From the NS RR and its associated glue RRs, the resolver has determined that the authoritative nameserver for example.com is ns.example.com and also knows its IP address. Using this information, it goesto ns.example.com and retrieves the KSK of example.com from its DNSKEY RRSet. It hashes this key
and compares it with the hash in the DS RR of its parent zone .com. A match of these two hashes
authenticates the referral from the .com zone to the example.com zone, as well as the KSK of theexample.com zone. Because the ZSK is signed by the KSK of example.com, the resolver can obtain the
ZSK of example.com in an authenticated way. Because the ZSK has signed all the RRsets inexample.com, any signed response from that zone can be validated by using the now-trusted ZSK.
As the description of the process indicates, the delegation information from a parent in the case of achained secured zone consists of the following:••

The NS RRs for the child zone. Because the authoritative source for these NS records is thechild zone, the NS RRs provided are called the hints or referral.
The glue RRs. They provide the location of the servers specified in the NS RRs (IP addresses ofreferred name servers).

•

The DS RR. It provides the hash of the KSK or ZSK of the child zone.

•

The RRSIG RR. It covers the DS RR (signature of the DS RR).

9.7.1

Recording and Communicating Results of Signature Verification

Some name servers can be authoritative for some zones and not authoritative for others. For zones forwhich they are not authoritative, they perform the function of caching RRsets from previous queries
pertaining to those zones. A DNSSEC-aware caching/resolving name server has additional tasks in its
caching function. These additional tasks pertain to classifying the security state of the response RRsets
and caching them accordingly [RFC4035]. The three possible states of RRsets are as follows:

•

•

•

Secure. A DNSSEC-aware caching name server (or resolver) will categorize an RRSet as secure
if verification of its associated RRSIG was successful. This means a valid chain of trust can beformed from the RRSet to a trust anchor. The RRSet would then be placed in the cache and be
purged when the data is deemed no longer up to date (according to the TTL) or no longer
verifiable (according to the RRSIG validity period).
Insecure. A DNSSEC-aware cache categorizes an RRSet as insecure if it is provably insecure.
That means that no DNSSEC security RRs were in the response, and the recipient knew not to
expect them. This happens when a delegation to an unsigned zone (a delegation that does nothave a DS RR found in the referral response) is received. Insecure RRsets are handled the same
way as secure RRsets, but local policy on the end system may dictate not to trust insecuredelegations or data in a response.
Bogus. The DNSSEC-aware cache categorizes a response as bogus when verification of thesignatures (RRSIG RRs) fails or contains incorrect fields (e.g., RRSIG has expired). The cachepolicy determines how to deal with these RRsets. They can either be dropped or put into a special
BAD cache containing only RRsets that are found to be bogus.

9-13

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

When a DNSSEC-aware caching name server receives a query from a non-security-aware resolver, it
serves either secure or insecure data from its cache. Because this query is non-security aware, DNSSEC
RR types are not included in the response; so only normal DNS processing applies.
DNSSEC-aware resolvers receive either secure or insecure data from the DNSSEC-aware caching serverwith a bit set in the header of the DNS response. This header bit is the Authenticated Data (AD) bit,which signals that the RRsets in the response passed all security checks performed by the caching nameserver. Depending on that server’s security posture, the client may choose to accept this response or
perform its own set of security checks.
In some cases, a client may want data that has been deemed bogus by a caching name server. In this case,the client sends a query with the Checking Disabled (CD) bit set in the DNS message header [RFC4035].
This tells the DNSSEC-aware caching name server to respond with bogus data from the BAD cache, notan error message. The client system must then perform its own security checks on the response RRsets. In
these types of responses, the server does not set the AD bit, indicating that the response has not passed all
security checks performed by the server.
9.8

Additional Protection Measures for DNS Query/Response

DNSSEC specifications for protection of DNS query/response transactions cover the following
communications:••

DNS responses from remote authoritative name servers to local (enterprise) resolving nameservers
DNS responses from remote caching name servers to local resolving name servers.

Because most of the queries in DNS originate from stub resolvers (on behalf of client software requiring
Internet resource access) protection for the DNS response message must be extended to the stub resolver
to the resolving name server path as well. The protection approach for this path (also called the DNS last
hop or last mile) is determined by the nature of the stub resolver and how the network is set up.
Stub resolvers can be non-DNSSEC aware, DNSSEC-aware nonvalidating, and DNSSEC-aware
validating. Most of the stub resolvers deployed today are non-DNSSEC aware. In other words, not only
do they not have the capability to verify signatures associated with returned RRSets, they also cannot
make a distinction between an authenticated (signature verified) response and a nonauthenticatedresponse (passed through by their local resolving name servers). To have complete end-to-end protection
for DNS query/response, the minimal requirement for these types of stub resolvers is that they shouldhave the capability to perform origin authentication and data integrity for responses coming down thechannel connecting them to the resolving name server providing DNS name resolution service for them.
This capability can be installed in these types of stub resolvers with the HMAC approach specified in
TSIG (as HMACs are implemented for protecting zone transfer and dynamic update transactions).
Alternatively, the enterprise can provide this capability through other network security mechanisms, suchas IP Security (IPsec). Guidance on how to set up IPsec protected connections between clients and arecursive caching server is beyond the scope of this document. Whatever mechanism is used for channel
security of the last hop (resolving name server to stub resolver), this capability should be present in
DNSSEC-aware nonvalidating stub resolvers as well, in addition to non-DNSSEC aware stub resolvers. A
DNSSEC-aware nonvalidating stub resolver can leverage this trusted path to examine the setting of theAD bit in the message header of a response message it receives. These types of stub resolvers can thenuse this flag bit as a hint to find out whether the resolving name server was able to successfully validate
the signatures for all of the data in the Answer and Authority sections of the response.

9-14

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEIn some situations, establishing a trusted path between stub resolvers and the resolving name servers thatprovide DNS service for them is not feasible. An example is where the resolving name server is not under
the administrative domain of the enterprise but is run by an ISP. In this situation, the end-to-end
protection for DNS query/response can be ensured only by having a DNSSEC-aware stub resolver or astub resolver that can use message authentication (TSIG or SIG(0)) to communicate with a trusted
caching server. A DNSSEC-aware stub resolver can indicate to the local resolving name server that it
wants to perform its own signature validation by setting the checking disabled (CD) bit in its query
messages. Or a stub resolver must be manually configured to use a specific resolving name server anduse TSIG/SIG(0) to provide message authentication.
9.9

Dynamic Updates in a DNSSEC-aware Zone

Section 4.3 lists the various logical operations on a zone file during the course of a dynamic update. The
four logical operations can be regarded as consisting of two basic operations: addition of RRs anddeletion of RRs. Updating an RR can be regarded as a combination of the two basic operations of additionand deletion. Addition and deletion of RRs involve no further operation on the rest of the RRs in the zone
file in nonsecure zones. In a secure zone, however, there is one NSEC RR (and a corresponding RRSIG
RR) to cover every gap in the namespace.
There is one NSEC RR for every unique owner name in the zone. This NSEC RR points to the next
owner name in the canonical order (ordering obtained by lexicographically sorting the domain names
within a zone). The NSEC RR for the last owner name in canonical order points to the zone apex name(in other words, the zone name). Hence, conceptually NSEC RRs form a circular link list that traverses
the unique domains names in a zone.
Consider the organization and contents of NSEC RRs for the zone example.com. Suppose the following is
the canonical order of the unique domain names in the zone:
example.com.
IN SOA ns.example.com. admin.example.com. (12985 3600 2700 8000 3600 )IN RRSIG ( SOA )IN NS ns.example.com.
IN RRSIG ( NS )IN MX mail.example.com.
IN RRSIG ( MX )ns.example.com.

IN

mail.example.com.

AIN

192.253.101.7
A

192.253.101.8

marketing.example.com.
IN A 192.253.101.9
IN RRSIG ( A )IN MX mail.example.com.
IN RRSIG ( MX )sales.example.com.
IN NS
IN RRSIG ( NS )ns.example.com.

www.example.com.
IN A 192.253.101.10
IN RRSIG ( A )9-15

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEThe pseudo format (containing only the important fields) of NSEC RRs covering the gaps in thenamespace relating to domain names and RR types found at each name in our zone is as follows:
example.com.
NSEC)

IN

NSEC

marketing.example.com. (NS SOA MX RRSIG

marketing.example.com.
RRSIG NSEC)
sales.example.com.
www.example.com.

IN

IN

IN

NSEC

NSEC

NSEC

sales.example.com. (A MX

www.example.com. (NS RRSIG NSEC)

example.com. (A RRSIG NSEC)

Note that there are as many NSEC RRs as the number of unique domain names in the zone. The NSECRR whose owner name is example.com points to the next domain in the canonical order (i.e.,marketing.example.com). The same is true for NSEC RRs pertaining to the domains
marketing.example.com and sales.example.com. The NSEC RR pertaining to the last domain name (i.e.,
www.example.com) points to the first domain name in the zone (i.e., example.com).
When a query for “package.example.com IN A” arrives (which does not exist in the zone), theauthoritative server replies with the NSEC RRSet that proves that the name does not exist in the zone. In
this case, the response from the server will consist of the normal DNS reply indicating that the name does
not exist and:•••marketing.example.com. NSEC RR indicating there are no authoritative names between“marketing.example.com.” and “sales.example.com”
www.example.com. NSEC RR (the last domain in the zone) proving that there are no wildcard
names in the zone that could have been expanded to match the query [RFC4035].
Accompanying RRSIG RRs for each of the foregoing NSEC records for authentication.

The modifications to the NSEC RRs that are required for the following two operations are as follows:•

Adding a new RR type to an existing domain•

Deleting an RR type from an existing domain•

Adding a new domain name to the zone•

Deleting a domain name from the zone.

Adding a new RR type to an existing domain:
Suppose a new mail host is added to www.example.com. This change will require addition of an MX RR
to this domain name. Hence, the NSEC RR for www.example.com must be modified to read as follows:
www.example.com. IN

NSEC

example.com. (A MX RRSIG NSEC)

Deleting an RR type from an existing domain:

9-16

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

Suppose the enterprise example.com decides that it no longer needs a separate set of e-mails for people inthe marketing department. This change will require removal of the mailserver (RRType = “MX”) fromthe domain marketing.example.com. The modified NSEC RR will now read as follows:marketing.example.com.
NSEC)

IN

NSEC

sales.example.com. (A RRSIG

Adding a new domain name to the zone:
To enable customers to order goods online, the enterprise decides to add a separate domain
websales.example.com. A separate name server and a set of new hosts also will be added. This new
domain requires the following changes to the zone file:•

A new NSEC RR (or NSEC3 RR) for the newly added domain. In this case, a NSEC RR for thedomain name websales.example.com should be added. Its location in the canonical order must be
determined. This NSEC RR should be set to point to the next domain name in the new canonicalorder. The presence of the name server and hosts should be reflected in this new RR (through the
NS and A codes in the RR types list field) as shown below, and the associated RRSIG RR mustbe generated. Also note that the new delegation is unsecure, since there is no DS RR indicated in
the RR list. Any client looking in the domain should not expect DNSSEC information in replies.
websales.example.com.
NSEC)

•

IN

NSEC

www.example.com. (A NS RRSIGThe NSEC RR (or NSEC3 RR) pertaining to the domain name just preceding the added domain(in the canonical order) has to be modified to point to the newly added domain. The NSEC RR
should now point to the domain websales.example.com. A new RRSIG RR must be generated(signed by the ZSK) covering the newly generated NSEC RR.
sales.example.com.
NSEC)

IN

NSEC

websales.example.com. (A RRSIG

Deleting a domain name from the zone:
Suppose the enterprise has decided that all sales will be made through the Web only. Because hosts havebeen created to handle this function in the new domain websales.example.com, the hosts in the domain
sales.example.com are no longer needed because usage of the applications that reside on them has been
discontinued. This implies that all RRs belonging to a domain should be removed from the zone. Thischange involves the following operations:••

The NSEC RR (or NSEC3 RR) corresponding to the deleted domain should be removed, so the
NSEC RR pertaining to domain sales.example.com has to be deleted.
The NSEC RR (or NSEC3 RR) pertaining to the domain name just preceding the deleted domain
has to be modified to point to the domain name immediately following the deleted domain (i.e.,
pointed to by the deleted NSEC RR). In this case, the NSEC RR for marketing.example.com
should point to the domain websales.example.com as follows:marketing.example.com.
RRSIG NSEC)INNSEC websales.example.com. (A MX

9-17

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

9.10 Recommendations Summary
The following items provide a summary of the major recommendations from this section:•••

Checklist item 15: Name servers that deploy DNSSEC signed zones or query signed zones
should be configured to perform DNSSEC processing.
Checklist item 16: The private keys corresponding to both the ZSK and the KSK should not be
kept on the DNSSEC-aware primary authoritative name server when the name server does notsupport dynamic updates. If dynamic update is supported, the private key corresponding to the
ZSK alone should be kept on the name server, with appropriate directory/file-level access control
list-based or cryptography-based protections.
Checklist item 17: Signature generation using the KSK should be done offline, using the KSKprivate stored offline; then the DNSKEY RRSet, along with its RRSIG RR, can be loaded into theprimary authoritative name server.

9-18

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

10.

Guidelines for Minimizing Information Exposure Through DNS Data Content
Control

The DNS security extensions provide only origin authentication and data integrity protection. This
protection does not provide confidentiality because of the design decision to keep DNS data public.
Features such as split DNS provide some means of keeping internal network information from appearing
on the global Internet, but they are not official parts of the DNS protocol specification.
It is possible that an attacker may learn valuable information about an organization’s network throughDNS and use that information to launch an attack. For some elements, such as IP addresses of public
servers, this possibility is unavoidable. There are steps a DNS administrator can take in generating a zone
file to keep network exposure to a minimum, however; these steps are given below. This process should
be done prior to signing a zone. Network information that should be kept absolutely private should not be
published in DNS at all.
10.1 Choosing Parameter Values in SOA RR
The first action a DNS administrator should undertake is to ensure that the values in the SOA resource
record data are correct. The values in this RR regulate the communication between primary and secondary
servers for the zone, such as when secondary servers should periodically perform zone transfers from theprimary server. This data also contains the minimum TTL value field, which gives client resolvers anindication of how long to cache non-DNSSEC negative responses. Some suggested values for these fields
are as follows [RFC1912]:

•
•

•

•

Serial Number. The serial number in the SOA RDATA is used to indicate to secondary nameservers that a change to the zone has occurred and a zone transfer should be performed. It should
always be increased whenever a change is made to the zone data.
Refresh Value. The refresh value tells the secondary servers how many seconds to wait between
zone transfers. For zones that are updated frequently, this number should be small (20 minutes to
2 hours). For zones that are updated infrequently, a larger number can be substituted (2 to 12hours). For a signed zone, this value should not be more than the length of the RRSIG validity
period, to ensure that secondary zones do not contain zones with expired RRSIGs. This value also
may depend on bandwidth constraints on the primary server side. Note that if a primary serverissues a NOTIFY message when it is updated, a secondary server will immediately perform azone transfer and not wait until the refresh value has timed out.
Retry Value. The retry value is the period a secondary server should wait before attempting toperform a zone transfer if the previous attempt failed. This value should be a fraction of therefresh value. If using a Refresh value in the range given above, the possible range of values forthis field would be 5 minutes to 1 hour.
Expire Value. The expire value is the length of time a secondary server should consider the zone
information valid if it can no longer reach the primary server to refresh. This field allows
secondary servers to continue to operate until network disruptions are resolved. This value
depends on the frequency of changes to the zone and the reliability of the connection between
name servers. It should be a multiple of the refresh value and possibly set to as long as 2 to 4
weeks.

10-1

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE•

Minimum TTL. The minimum TTL value is the default value used as the TTL of zone RRsets aswell as in negative caching. The minTTL value depends on how often the information changes inthe zone. If a zone is static, the value could be large; if the zone is dynamic, the value should besmall. A minimum value of 5 minutes is advised, with a recommended range of 30 minutes to 5
days.

Checklist item 18: The refresh value in the zone SOA RR should be chosen with the frequency of
updates in mind. If the zone is signed, the refresh value should be less than the RRSIG validity period.
Checklist item 19: The retry value in a zone SOA RR should be 1/10th of the refresh value.
Checklist item 20: The expire value in the zone SOA RR should be 2 to 4 weeks.
Checklist item 21: The minimum TTL value should be between 30 minutes and 5 days.

10.2 Information Leakage from Informational RRTypes
There are several types of RRs in the DNS that are meant to convey information to humans andapplications about the network, hosts, or services. These RRs include the Responsible Person (RP) record,
the Host Information (HINFO) record, the Location (LOC) record, and the catch-all text string resource
record (TXT) [RFC1035]. Although these record types are meant to provide information to users in goodfaith, they also allow attackers to gain knowledge about network hosts before attempting to exploit them.
For example, an attacker may query for HINFO records, looking for hosts that list an OS or platform
known to have exploits. Therefore, great care should be taken before including these record types in azone. In fact, they are best left out altogether.
More careful consideration should be taken with the TXT resource record type. A DNS administrator will
have to decide if the data contained in a TXT RR constitutes an information leak or is a necessary piece of
information. For example, several authenticated email technologies use TXT RR's to store email senderpolicy information such as valid email senders for a domain. These judgments will have to be made on acase-by-case basis.

Checklist item 22: A DNS administrator should take care when including HINFO, RP, LOC, or other RR
types that could divulge information that would be useful to an attacker, or the external view of a zone if
using split DNS. These RR types should be avoided if possible and only used if necessary to support
operational policy.
Checklist item 23: A DNS administrator should review the data contained in any TXT RR for possible
information leakage before adding it to the zone file.

10.3 Using RRSIG Validity Periods to Minimize Key Compromise
The best way for a zone administrator to minimize the impact of a key compromise is by limiting thevalidity period of RRSIGs in the zone and in the parent zone. This strategy limits the time during whichan attacker can take advantage of a compromised key to forge responses. An attacker that has

10-2

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

compromised a ZSK can use that key only during the KSK’s signature validity interval. An attacker thathas compromised a KSK can use that key for only as long as the signature interval of the RRSIG covering
the DS RR in the delegating parent. Therefore, it makes sense to keep these validity periods short, which
will require frequent re-signing.
To minimize the impact of a compromised ZSK, a zone administrator should set a signature validity
period of 1 week for RRSIGs covering the DNSKEY RRSet in the zone (the RRSet that contains the ZSK
and KSK for the zone). The DNSKEY RRSet can be re-signed without performing a ZSK rollover, butscheduled ZSK rollover should still be performed at regular intervals.
To prevent the impact of a compromised KSK, a delegating parent should set the signature validity period
for RRSIGs covering DS RRs in the range of a few days to 1 week. This re-signing does not require
frequent rollover of the parent’s ZSK, but scheduled ZSK rollover should still be performed at regularintervals.

Checklist item 24: The validity period for the RRSIGs covering a zone’s DNSKEY RRSet should be in
the range of 2 days to 1 week. This value helps reduce the vulnerability period resulting from a key
compromise.
Checklist item 25: A zone with delegated children should have a validity period of a few days to 1 week
for RRSIGs covering the DS RR for a delegated child. This value helps reduce the child zone’s
vulnerability period resulting from a KSK compromise and scheduled key rollovers.

Following checklist items 24 and 25 in a particular organization depends on the content management and
DNSSEC tools in place and local policy regarding content management. A DNS administrator may beforced to choose one signature validity period for all of the content in a zone if the available tools do notallow for fine grain control in zone signing.
In these cases it may be necessary for a DNS administrator to choose a single signature valididity period
for the entire zone. In these cases, the DNS administrator should choose a value that minimizes the risk
should a key compromise or emergency update be necessary.
10.4 Hashed Authenticated Denial of Existance
One of the side effects of the DNSSEC security extensions as they were first specified is the ability for auser to “walk” a zone by sending a series of queries for NSEC RRs (See Section 9). In that section the
NSEC RR and its use in authenticated denial of existence is introduced. A client could send a query for
an NSEC RR in the zone, then “walk” the zone by sending a follow-up query for the NSEC RR at thename indicated as the next name in the received NSEC RR. This would result in a client being able toenumerate the entire contents of a zone. While this is not an attack by itself (all DNS data is consideredpublic), it would most likely be a prelude to an attack. An attacker would enumerate a zone to discover
the IP addresses of servers to attack directly.
There are several operational means to minimize the impact of zone enumeration and a protocol option in
DNSSEC [IEEE]. The operational options are to use split DNS or some other means of segregating
public servers from internal hosts. One way of doing that is described in Section 7.2.8 and Section 7.2.9.

10-3

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEThe protocol means to minimize information leakage through zone enumeration is to deploy the NSEC3
variant of the NSEC RR [RFC5155]. The NSEC3 RR is similar in format to the NSEC RR, but uses
hashed domain names for the owner name and next name (e.g. using SHA-1 or another one-way hashalgorithm). Like RRSIG and NSEC RRs, NSEC3 RRs are generated during the zone signing process,however both servers and clients may need to perform hash calculations during run time to constructresponses (on the server side) and validate responses (on the client side). Because of this, NSEC3
deployment requires more computation than DNSSEC signed zones containing NSEC RRs.

Figure 10-1. NSEC3 Hash Name SpaceNSEC3 RRs contain other options than just the (hashed) next name and RRType bitmap. There are also 2
values associated with the NSEC3 RR: the iterations (number of times each name is hashed) and the salt
(string appended to each name before hashing). These values are configurable during signing and areused to increase the work necessary by an attacker. Both values should be changed on a regular basis tomaintain protection against zone enumeration.
The salt value should be changed every time the entire zone is resigned. The salt value should be arandom string with a length small enough to ensure that appending the salt value to the domain name does
not result in a FQDN considered too long for the DNS protocol (a single label in the DNS protocol can be
256 octets). A value between 1 - 15 octets would be acceptable for the majority of cases. Note that forzones that are dynamically resigned as needed may not be able to change the salt for NSEC3 RR's as an
atomic process. In these cases, the salt rollover procedure is similar to the key algorithm rollover
procedure in that the NSEC3 RR chain with the new salt is generated first (ending with the
NSEC3PARAM RR) before removing the old (outgoing) NSEC3 chain.
The iterations value depends on factors that are largely beyond the control of DNS administrators. The
iterations value is included to increase the amount of work an attacker must complete when conducting abrute force attack offline against a known hashed name from the zone. The goal is to set this value so theamount of work is equal to the time needed to conduct a brute force attack against the server itself(including network lag, etc.). So choosing a value for the iterations requires a good estimate of thecomputational power available to an attacker and the time required to perform a brute force dictionary
attack against an online DNS server. However, setting the value too high would result more work being
done by the server and valid clients that may result in slow response time for end users and may be apossible Denial of Service (DoS) vector for servers and clients.

10-4

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEThe iterations value should be reviewed annually and changed if conditions warrant. Since the average
DNS query response time is in the range of 25 to 200 ms, the iterations value should be chose to reflect
the number of hash calculations the average processor could do in that time period. However, in practicethe value should be lower to account for client processing power and to speed response for valid clients.
Basic empirical work has shown that the ideal iteration value when using SHA-1 should be between 1 and
500, with values 1-200 recommended for average zones. Anything over 500 iterations, it becomes more
expensive than validating a signature generated by a 2048 bit RSA key for valid clients. If SHA-256 isused, the value should be between 1 and 300, again with values between 1 and 100 for the majority ofzones in practice, as it takes more time to perform a SHA-256 calculation than a SHA-1 calculation.
The OPT-OUT option is designed to aid deployment in very large zones with a very large number ofdelegations (signed and unsigned). With the OPT-OUT option, the NSEC3 span only links signed
delegations, so there are no NSEC3 RRs for names of unsigned delegation. This means there would be ashorter NSEC3 chain, as not every name has an NSEC3 RR associated with it, and the zone signingshould take less time. Also, if there are frequent changes, the NSEC3 chain may not have to be changed
if unsigned delegations are added or deleted. The drawback is there may be an NSEC3 span an attacker
can use to spoof the existence of new, unsigned zones. However, NSEC signed zones can also have
spoofed unsigned delegations, but the attacker is limited to re-directing existing names in the zone, while
an attacker could spoof completely new unsigned delegations in a zone signed with NSEC3 and using
OPT-OUT.

Checklist Item 26: If the zone is signed using NSEC3 RRs, the salt value should be changed every time
the zone is completely resigned. The value of the salt should be random, and the length should be shortenough to prevent a FQDN to be too long for the DNS protocol (1 to 15 octets should be sufficient).
Checklist Item 27: If the zone is signed using NSEC3 RRs, the iterations value should be based on
available computing power available to clients and attackers. The value should be reviewed annually andincreased if the evaluation conditions change. Initial values should be between 1 and 200 iterations whenusing SHA-1 and 1 and 100 if using SHA-256.

10.5 Resource Record TTL Value Recommendations
As mentioned in Section 3.1, each RRset in a zone has its own Time-to-Live value (TTL). This TTL tells
clients how long (in seconds) it should be stored in its cache upon receipt. When a client receives a DNS
response to a query and performs all relevant checks, it stores the resulting RRsets in its cache. The client
cache decrements the TTL value of each RRset in its cache, and when the TTL for any RRset reaches
zero, it is purged form the cache. This prevents caches from growing too large, as well as getting old,
possibly incorrect, DNS data out of caches and prevents them from being returned to future queries.
The zone administrator assigns the TTL value for each RRset individually or for the entire zone anddifferent RRsets in the same zone can have different values. The zone administrator should set the TTL
value long enough to insure that the RRset will be useful for caches, but short enough that any changes to
the RRset will be propagated quickly through the DNS (and old information purged). DNSSEC (Section
X.X) signature validity periods should also be taken into consideration as well. TTL values should be afraction of the validity period of the RRSIG that covers the RRset. DNSSEC aware clients will
decrement the TTL value of an RRset it its cache to the signature expiration date if that date is before theprojected Time-to-Live. That way, the RRset will be purged before it the signature expires (and will be

10-5

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

seen as BOGUS to other DNSSEC validators). However, DNSSEC-unaware clients will not know to do
this comparison, so there is the risk that invalid DNSSEC RRsets will be stored in DNSSEC unawarecaches.
TTL values should be on the order of hours, with a recommended range of 1800 (30 minutes) to 86400 (1day). If a zone administrator knows that the DNS data is likely to change frequently, the TTL value
should be set low, to insure that old, stale data is purged from client caches. If the zone administrator
believes the DNS data will not change frequently, then the TTL value can be set higher, to gain optimal
benefit of caching in client systems. Note that some specialized load-balancing scenarios rely on much
shorter time periods (60 seconds or less), but for the majority of DNS data, 30 minutes to 24 hours is
sufficient. If the data is signed using DNSSEC, the value should always be long enough to insure that thedata will not be purged from client caches before those clients have a chance to validate it. Experiencehas shown that very low TTL values (e.g. 30 seconds and under) can cause problems with DNSSEC
validating caches and these values should be avoided for DNSSEC signed RRsets.
Checklist item 28: TTL values for DNS data should be set between 30 minutes (1800 seconds) and 24
hours (86,400 seconds).
Checklist item 29: TTL values for RRsets should be set to be a fraction of the DNSSEC signaturevalidity period of the RRSIG that covers the RRset.

10.6 Recommendations Summary
The following items provide a summary of the major recommendations from this section:•

Checklist item 18: The refresh value in the zone SOA RR should be chosen with the frequency
of updates in mind. If the zone is signed, the refresh value should be less than the RRSIG validity
period.

•Checklist item 19: The retry value in a zone SOA RR should be 1/10th of the refresh value.

•

Checklist item 20: The expire value in the zone SOA RR should be 2 to 4 weeks.

•

Checklist item 21: The minimum TTL value should be between 30 minutes and 5 days.

•

Checklist item 22: A DNS administrator should take care when including HINFO, RP, LOC, orother RR types that could divulge information that would be useful to an attacker, or the externalview of a zone if using split DNS. These RR types should be avoided if possible and only used if
necessary to support operational policy.

•

Checklist item 23: A DNS administrator should review the data contained in any TXT RR for
possible information leakage before adding it to the zone file.

•

Checklist item 24: The validity period for the RRSIGs covering a zone’s DNSKEY RRSet
should be in the range of 2 days to 1 week. This value helps reduce the vulnerability period
resulting from a key compromise.

•

Checklist item 25: A zone with delegated children should have a validity period of a few days to
1 week for RRSIGs covering the DS RR for a delegated child. This value helps reduce the childzone’s vulnerability period resulting from a KSK compromise and scheduled key rollovers.

10-6

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE•Checklist item 26: If the zone is signed using NSEC3 RRs, the salt value should be changed
every time the zone is completely resigned. The value of the salt should be random, and thelength should be short enough to prevent a FQDN to be too long for the DNS protocol (i.e. under
256 octets).

•

Checklist item 27: If the zone is signed using NSEC3 RRs, the iterations value should be basedon available computing power available to clients and attackers. The value should be reviewed
annually and increased if the evaluation conditions change.

•

Checklist item 28: TTL values for DNS data should be set between 30 minutes (1800 seconds)
and 24 hours (86400 seconds).

•

Checklist item 29: TTL values for RRsets should be set to be a fraction of the DNSSEC
signature validity period of the RRSIG that covers the RRset.

10-7

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

11.

Guidelines for DNS Security Administration Operations

Section 9 deals with operations for deployment and usage of DNSSEC features for protection of DNS
query/response transactions. This section deals with periodic security administration operations (andassociated checklists) in a DNSSEC-aware enterprise-level zone and how to perform those operations
securely.
11.1 Organizational Key Management Practices
The DNS is an intrical part of an enterprise’s infrastructure and DNSSEC operations dealing with key
management should be in line with policies covering other data authentication keys maintained by theenterprise. These policies would address such where keys are stored, designated personnel tasked withkey management operations, etc. Such an overall security policy is out of scope for this document anddepends on the enterprise’s current infrastructure and any pre-existing security policies already in place.
NIST Special Publication 800-57 Part 1 and Part 3 [800-57P1], [800-57P3] has general guidance on key
management and more specific guidance for specific applications. In these two Special Publications,DNSSEC keys fall under “data authentication” keys because they are used to provide authentication andintegrity protection to DNS data.
11.2 Scheduled Key Rollovers (Key Lifetimes)
The keys used for zone signing (ZSK) and key signing (KSK) have to be changed because the keys
become vulnerable (liable to be cracked) with enough effort. The compromise of a private key means thatany site can spoof the zone by signing a bogus RRSet with the private key, thus defeating the purpose ofsigning the zone file. Key rollover can take place as a scheduled event (scheduled rollover), or it may takeplace as a result of an emergency (emergency rollover). Emergency rollover occurs for one of thefollowing reasons:••

The private key of the zone has been compromised.
The private key of the zone has been lost, and the zone is to be updated before the RRSIGs
expire.

In scheduled key rollover, the time period (or frequency of change) after which the keys must be changed
is determined by several factors.

•

The amount of effort needed to rollover the key, and the potential disruption to the zone or current
operation.

•

Information security policy within the organization.

•

The smaller the size of the private key, the easier it is to crack.

Based on these factors, each zone arrives at a desired frequency for key rollovers for ZSKs and KSK.
Recall that the KSK (KSK-private) is used for signing only the DNSKEY RRSet, whereas the ZSK (ZSKprivate) is used for signing the entire zone file. Apart from the volume of data, the ZSK also is used much
more frequently, as in the following situations:

11-1

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE•••

When a new RR is added (e.g., a new mail server is added and hence a new MX RR is added thezone file)
When an existing RR’s RDATA has changed (e.g., the IP address of a server has changed andhence the corresponding A RR has to be replaced)
When the signature has expired for an RRSIG RR.

Because of the volume of data handled and the frequency of usage, the size of the ZSK-private key
becomes a factor in overall CPU cycles consumed by the digital signature generation operations. Hence,
the ZSK used is often relatively small.

Checklist item 30: The (often longer) KSK needs to be rolled over less frequently than the ZSK. The
recommended rollover frequency for the KSK is once every 1 to 2 years, whereas the ZSK should berolled over every 1 to 3 months for operational consistency but may be used longer if necessary forstability or if the key is of the appropriate length. Both keys should have an Approved length according
to NIST SP 800-57 Part 1 [800-57P1], [800-57P3].

The impact of a key rollover on the rest of DNS depends on whether the secure zone is locally secure or
globally secure (part of a chain of trust).
For a more detailed discussion of the operational steps involved in a key rollover, see the IETF document
on DNSSEC operations [RFC4641]. Note that the two processes described for key rollovers (prepublished and dual-signature) can be used in rolling over the ZSK or the KSK. The recommendations
below are based on common practice and minimizing the impact of larger responses on clients.
11.2.1 Key Rollover in a Locally Secure Zone
A zone that is only locally secure will have a ZSK, and possibly a KSK that is configured in client
resolvers, as a trusted key. Certain challenges arise when either key is rolled over, although having a KSK
even for a locally signed zone makes rolling over the ZSK easier. When a zone changes its ZSK(s) andhas a KSK that remains unchanged, the only problem that must be addressed is introducing the new key
when the old key may be in some distant resolver’s or name server’s cache.
The solution is to pre-publish the new public key before the rollover. The DNS administrator needs topublish the new key as a DNSKEY RR in the zone file before it is used to generate signatures. The
process is as follows:•

Generate a new key pair.

•

Add the public key of the new key pair to the zone file (DNSKEY record).

•

Sign the zone using the private key of the currently active key pair and the KSK (if present).

•

Wait for a period equal to the TTL of the DNSKEY RRSet or the MinTTL of the zone SOA
record (whichever is greater).

•

Delete the RRSIG RRs generated by the outgoing key, but retain the DNSKEY RR. Resign the zone
using the new ZSK (and current KSK, if used).

11-2

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE•

Wait the TTL of the zone’s DNSKEY RRset.

•

Remove the old, outgoing ZSK from the DNSKEY RRset to reduce the size of the DNSKEY RRset in
responses.

•

Re-sign the DNSKEY RRSet with the new ZSK.

It might be in the DNS administrator’s best interest to perform a ZSK rollover continuously. The
administrator can perform the first three steps and wait indefinitely before deleting the old DNSKEY
from the key set, even continuing to sign the zone with the old DNSKEY when the RRSIGs in the zone
expire. This procedure allows the administrator to perform an emergency key rollover more efficiently(see below).

Zones that pre-publish the new public key should observe the following:
Checklist item 31: The secure zone that pre-publishes its public key should do so at least one TTL period
before the time of the key rollover.
Checklist item 32: After removing the old public key, the zone should generate a new signature (RRSIGRR), based on the remaining keys (DNSKEY RRs) in the zone file.

In rolling over the KSK, the secure zone may not know which resolvers have stored the public key as atrust anchor. If the network administrator has an out-of-band method of contacting resolver administratorsthat have stored the public key as a trust anchor (such as e-mail), the network administrator should sendout appropriate warnings and set up a trusted means of disseminating the new trust anchor. Otherwise, theDNS administrator can do nothing except pre-publish the new KSK with ample time to give resolver
administrators enough time to learn the new KSK.
11.2.2 Key Rollover in a Chained Secure Zone
A globally secure zone uses two sets of keys: the ZSK and the KSK.
11.2.3 ZSK Key Rollover in a Chained Secure Zone
The operations involved in a ZSK rollover in a globally secure zone are no different from those in a ZSK
key rollover in a locally secure zone. (See Section 11.2.)
11.2.4 KSK Key Rollover in a Chained Secure Zone (Manually without Revoke Bit)
The KSK (KSK-public) is the key for which the secure parent of a secure zone provides trust. It provides
this trust by using a new RR type called the DS RR, which contains the hash of the child’s KSK. The
delegating parent signs this DS RR with its own ZSK. When a zone changes its KSK, the trust
relationship between itself and its parent will be broken. To maintain this trust relationship, the zone
making the KSK rollover must:•

Generate a new KSK and add it to the zone key set•

Sign the zone key set with the new KSK as well as the old KSK (the key about to expire)
11-3

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE••

Communicate its new KSK to its parent in a way that the parent can authenticate it
In addition, the parent has to generate a new DS RR (which will replace the old DS RR) that
contains the hash of the new KSK and then sign the newly generated DS RR.

To enable the parent to authenticate the new KSK (referred to as KSK2) based on the existing chain oftrust, the zone performing the key rollover (i.e., the child zone) generates a DNSKEY RRSet using the
DNSKEY RRs of the existing key together with a new DNSKEY RR for the KSK2. It then generates two
signatures (two RRSIG RRs)—one using the private key of the existing KSK and the other using theprivate key of the new KSK. It then sends the newly generated DNSKEY RRSet together with the RRSIG
RRs (in plural because there are two signatures, corresponding to KSK and KSK2, respectively) to theparent. The set of RRs sent to the parent is given below in a pseudo format:
example.com
DNSKEY <key-id: 43543> /* the new KSK */example.com
DNSKEY <key-id: 78546> /* the existing KSK */example.com
DNSKEY <key-id: 98342> /* ZSK */example.com
RRSIG(DNSKEY) <signer:example.com signing-key:78546> /* the entire DNSKEY RRSet signed with existing KSK */example.com
RRSIG(DNSKEY) <signer:example.com signing-key:43543> /* the entire DNSKEY RRSet signed with new KSK */On receiving this information, the parent performs the following functions:••

It verifies whether the KSK it trusts (i.e., 78456) has indeed signed the newly generated
DNSKEY RRSet that includes the new KSK (KSK2). It does this using the newly generated
DNSKEY RRSet, the first of the RRSIG RRs (the one that says 78456 as the signing key,) and its
own DS RR.
It also verifies that KSK2 is authentic (the child zone holds the private key of KSK2) by verifying
the second RRSIG RR (the one that says 43543 as the signing key) against the DNSKEY RRSet.

•It then generates a new DS RR containing the hash of the new KSK (KSK2).

•

The parent must then generate a RRSIG covering the new DS RR of the KSK2.

With these tasks performed by the parent, the KSK rollover process from the child’s perspective isessentially complete. The parent zone must then update its zone file with the new DS RR. This is similar
to updating any other RRSet, which includes generating any new RRSIGs, as required.

•

The parent zone adds the new DS RR (DS2) to the DS RRset in the delegation•

The parent waits for the new DS RR to propagate (min of the TTL of the DS RRset)•

The parent can then remove the expiring DS RR from the DS RRset, triggering a resigning of thezone

The delegated child zone should keep the expiring KSK in its key set until it has confirmed that the parent
has performed the appropriate DS update to ensure that the chain of authentication remains intact during
the KSK rollover. After the child zone confirms that the delegating parent has updated the delegation
information for the child, the child zone administrator should remove the old KSK from the keyset and resign the keyset with the ZSK and the new KSK. The administrator should remove the old (now expired)

11-4

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEKSK to keep the DNSKEY RRset response size from growing too large for a UDP packet. This triggers
the use of TCP, which is requires more network resources for the server to respond to clients.
11.2.5 KSK Key Rollover in a Chained Secure Zone (using the Revoke Bit)
There is a standardized method developed in the IETF to provide a stable means to updating a KSK that
may or may not be installed in clients as a trust anchor [RFC5011]. Since it is impossible to know if aserver’s KSK is a trust anchor for a client, it is recommended to use this method to insure that clients
update their trust anchors.
This KSK rollover method involves a series of timed steps and changing of flag bits in the KSK
DNSKEY RR, which may be difficult to perform manually. There have been several tools developed toautomate this process, but the steps are presented below to help administrators understand the process:1. The administrator adds the new KSK but does not use it to generate any signatures (similar topre-publishing during a ZSK rollover.
2. Wait a pre-calculated time (recommended – 30 days)
3. Set the Revoke bit (flag) in the outgoing KSK and sign the DNSKEY RRset with both old and
new KSK’s. The old key is now revoked and will not be used by clients who understand therevoked bit and the process described in RFC 5011. Have the delegating parent add a new DS
with the hash of the new KSK.
4. Wait a pre-calculated time (recommended – 30 days)
5. Remove the old KSK and resign using the new KSK only. Contact the delegating parent (if theparent is signed) to delete the corresponding DS RR for the old KSK.

11.3 Emergency Key Rollovers
An emergency key rollover occurs when one or more of the keys in the zone (ZSK or KSK) have become
compromised or the private component has been lost and a re-signing needs to occur. These types ofrollovers are not planned, so there is a greater chance of a break in the chain of authentication if the zone
administrator needs to perform this type of rollover.
11.3.1 Emergency ZSK Rollover
A DNS administrator needs to perform multiple steps to perform an emergency ZSK rollover in the eventone (or more) ZSK’s are compromised. This multi-step recovery process is another reason for a zone
administrator to perform a ZSK rollover continuously. If a zone administrator has the new DNSKEY RR
already published in the zone as part of the zone key set (the first three steps in the process listed in
Section 11.1.1), the next step depends on the key that has been compromised.
If the ZSK currently in use has been compromised, the zone administrators can immediately rollover tothe new key. There is no need to wait for the TTL value to expire because the new key has already been
published for a period of time. The administrator can simply strip the old RRSIGs from the zone and resign with the new ZSK earlier than the planned rollover. DNS operation should continue normally after
this re-signing, with no breaks in the authentication chain.

11-5

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

If the new ZSK (the next ZSK to be used) has been compromised, it should be replaced immediately inthe zone key set. This replacement also can be done atomically because the new key was not used to
produce any RRSIGs; only the zone key set needs to be re-signed. It also is possible simply to remove thecompromised key and replace it with a new ZSK in one update.
There still is a danger of attackers using the compromised ZSK to forge responses coming from the zone.
This danger exists as long as the current KSK is still active, with the option of initiating a KSK rollover.
Once a ZSK has been compromised, a zone administrator should initiate a rollover of the KSK as soon as
possible.
11.3.2 Emergency KSK Rollover
When a zone’s KSK has been compromised, the only response is to initiate a KSK rollover with theparent. This rollover, however, is not the same process as a scheduled KSK rollover. There must be a way
to alert the parent zone administrator that the old KSK has been compromised and not to accept any KSK
rollover messages using that key. The replacement key must be transmitted and verified using some othersecure channel to ensure the child zone’s identity and may involve one or more non-DNSSEC
authentication methods. This process could be the same as that used to establish the original KSK of thechild zone.

Checklist item 33: A DNS administrator should have the emergency contact information for theimmediate parent zone to use when an emergency KSK rollover must be performed.
Checklist item 34: A parent zone must have an emergency contact method made available to itsdelegated child subzones in case of emergency child subzone KSK rollover. There also should be a securemeans of obtaining the subzone’s new KSK.

The only way to minimize the exposure period to a minimum, it is a good idea for a zone administrator tokeep the signature validity period (time between signature inception and expiration fields in the RRSIG
record RDATA) as short as possible. Since a compromised KSK can only be used by an attacker for as
long as the non-compromised parent zone vouches for its security. The downside to shorter signaturevalidity periods is that it leads to frequent re-signing (see below). A zone administrator must take theresources required in re-signing into consideration when choosing a signature validity period fordelegation DS RRset RRSIG(s).
11.4 Re-Signing a Zone
The zone file has to be re-signed (new RRSIG RRs generated) in the following situations:•

The signatures have expired or are about to expire.

•

The zone file content has changed (as a result of dynamic updates for example).

•

One of the signing keys has been compromised or is scheduled for replacement.

There are two strategies for re-signing zone data:

11-6

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE••

Complete Re-Signing. All existing signature records (RRSIG RRs) are deleted, the zone file issorted again, all of the NSEC RRs are regenerated, and finally the new signature records are
generated. Complete re-signing is performed under the following scenarios:–

The zone file is generated from a back-end relational database.

–The zone administrator has set up a batch job to run at a specific date/time (called a cron job
in Unix), based on the expiration time set in the signer.

Incremental Re-Signing. NSEC RRs are modified because an existing RRSet has been dropped,
NSEC RRs are added because a new RRSet set has been added to the zone file, and signatures are
generated only for RRsets that have changed. Incremental re-signing is done when changes tozone file contents have been minimal since the last time of signature generation, as is usually thecase after a dynamic update.

Checklist item 35: Periodic re-signing should be scheduled before the expiration field of the RRSIG RRs
found in the zone. This is to reduce the risk of a signed zone being rendered bogus because of expired
signatures.
Checklist item 36: The serial number in the SOA RR must be incremented before re-signing the zone
file. If this operation is not done, secondary name servers may not pick up the new signatures because
they are refreshed purely on the basis of the SOA serial number mismatch. The consequence is that somesecurity-aware resolvers will be able to verify the signatures (and thus have a secure response) but others
cannot.

11.5 DNSSEC Algorithm Migration
Eventually, it may be necessary to migrate to a new DNSSEC signing algorithm. This may be because ofa discovered weakness in the currently used algorithm, or due to overriding policy decisions. The
migration from the current DNSSEC algorithm to a new algorithm requires a set of steps and delays. Thisis due to the need to wait for old data to be removed from caches before the next step.
A proposed process can be found in RFC 4641bis 12, which outlines the basic steps. In order to reduce therisk of a validator thinking it is under a downgrade attack. To reduce this risk, the signatures for the newalgorithm are added before the DNSKEY RR with the new algorithm public key. Likewise, whenremoving the retiring algorithm, the public key DNSKEY RR is removed first, followed by thesignatures. The steps outlined in the document are repeated below:
1. The zone starts with public keys of algorithm A and RRSIG RRs generated by algorithm A. Thezone administrator then generates a new KSK and ZSK for the new algorithm B.
2. The zone administrator signs the zone with the new algorithm B and adds the RRSIGs generated
by algorithm B. There is no need to resign the entire zone (unless algorithm A RRSIGS are due
to expire). Only the SOA RR needs to be resigned by algorithm A’s ZSK as the serial number
has changed (indicating a zone update).
12

As of the time of writing, RFC 4641 is currently undergoing a revision. The working version is draft-ietf-dnsops-rfc4641bis01.txt

11-7

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

3. Zone administrator waits the TTL of the zone, then adds the new DNSKEY RRs of algorithm B
and resigns the SOA RR and DNSKEY RRset.
4. The Zone administrator sends the algorithm B key material (either DNSKEY RR or DS RR) to
their parent zone if applicable. If the zone administrator believes they will sign with bothalgorithm A and B for some time, there should be an agreement that the parent zone maintains
both DS RRs (for both algorithms) during this period.
At this point the zone can be maintained as is. When the zone administrator wishes to retire algorithm A
and solely sign using algorithm B, the following steps should be taken:
5. The zone administrator asks its parent zone to remove the DS RR generated using the KSK ofalgorithm A.
6. The zone administrator waits for the TTL of the (now removed) DS. Next, the zone administrator
removes the KSK and ZSK of algorithm A from the zone, resigns the SOA RR and DNSKEY
RRset (with both algorithm A and B), but maintains the RRSIGs generated by algorithm A overthe zone data.
7. The zone administrator waits the TTL of the zone, and then strips the RRSIGs of algorithm A
from the zone. Only the SOA RR needs to be resigned at this step, as the rest of the zone data has
not changed. The zone is now signed solely with algorithm B at this point.
There are two main concerns an administrator needs to keep in mind when transitioning to a new
DNSSEC algorithm. First, during this transition period, response messages from the zone will be larger
due to the presence of multiple RRSIGs for every RRset in the zone. During steps 4-6, the DNSKEY
RRset for the zone would contain four DNSKEY RRs and four RRSIGs (one from each key in the set).
It is not recommended that any other key maintenance operation be done in conjunction with thistransition, such as pre-publishing a new ZSK. If the administrator chooses to operate using two signingalgorithms for an extended period of time, it is recommended to stagger key maintenance operators so
only one key (out of the minimum two KSK’s and two ZSK’s) should be rolled at a given time.
Secondly, when the transition is complete, the zone may become provably unsecure for some validators.
End user validators that do not understand the new algorithm will act as if the zone was provably
insecure, as valiadators will not be able to validate RRSIGs generated by the unknown algorithm.
11.5.1 Special Considerations in Key Rollovers with Multiple Signing Algorithms in Use
There is an issue that arises when deploying a new digital signature algorithm with a signed zone. Thezone administrators now have to maintain two different key life cycles. There can be a problem when theadministrator wishes to pre-publish new ZSK's or KSK's in a zone that is signed by two or morealgorithms.
One common rollover regime is to pre-publish the next ZSK the same time as the current ZSK. So there
would normally be three DNSKEY RRs in a zone's keyset: one active KSK, one active ZSK, and one
pre-published ZSK. So if two algorithms are used, there would be twice that number, or six DNSKEY
RRs in the zone's keyset. Once a year (or once every 1 to 2 years), the KSK is also rolled over so there
will be a period when a pre-published KSK will also appear in the zone for a total of four DNSKEYs in
the zone for each cryptographic suite in use (eight total if two different suites are used).
The solution would be to stagger key rollovers to minimize the number of pre-published keys in the zone(see Figure 11-X using RSA/SHA-1 and RSA/SHA-256 as the old and new algorithms). One way of
doing this is to extend the active key lifetime of each ZSK to two months, then alternating the rollover of
either cryptographic suite's ZSK each month. At least one key is rolled over every month, but which
11-8

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

cryptographic suite is rolled alternates each month. Extending the active key life time to two months
from one (more traditional) is considered a larger security risk, since the key is in use longer (andtherefore subject to a longer window of attack), but the risk is still not severe enough for the majority ofzones.

Figure 11-1: Key Lifecycles Using Multiple Algorithms

KSK rollovers can be handled the same way, but since KSK's are rolled over less frequently this is not a
big contributor compared to ZSK rollover schedules.
Also note that this process assumes that the zone’s KSK is not being used as a trust anchor, and that there
is a signed parent for the zone. If that is not case then the zone administrator must follow a variation of
the steps in RFC 5011 Section 6.3 [RFC 5011] when finishing the rollover of the KSK. This would
involve setting the revoke bit on the algorithm A KSK then waiting the hold-down time before removing
it
11.6 Special Consideration When Transitioning from NSEC Signed Zones to NSEC3
Signed Zones
For whatever policy decision, an organization using a DNSSEC signed zone may wish to transition theirzone from using NSEC for authenticated denial of existence to using NSEC3 RRs. Due to the nature ofcaching in the DNS and DNSSEC, this cannot be done in an atomic fashion; i.e., the NSEC chain
replaced by an NSEC3 chain in a single operation. This transition is done by moving the DNSKEY
RRset from the algorithm code (see Section 9.3) associated with RSA/SHA-1 (code 5) to a special
NSEC3 signaling code for RSA/SHA-1 (code 7).
Transitioning is similar to rolling over to a new algorithm since (sometimes) the algorithm code changes
as well. The only real difference is at the end when an NSEC3 chain in the signed zone replaces the
NSEC chain.

11-9

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEThe process is similar to the process described in Section 11.4 above with the following changes:
1. The zone administrator follows the steps 1-5 in Section 11.4 above. The administrator does nothave to wait after the NSEC3RSASHA1 KSK appears in the parent zone.
2. The zone administrator waits the TTL of the parent zone and then removes the DNSKEY RRs of
the RSA/SHA-1 algorithm. At this point, it is safe to strip the NSEC RRs and replace them with
NSEC3 RRs. The DS in the parent zone signals that the zone uses NSEC3 rather than NSEC. As
with the algorithm rollover process above, the RSA/SHA-1 RRSIGs over zone data RRsets
remain in the zone.
3. The zone administrator waits for the TTL of the zone and then removes the RRSIGs generatedusing RSA/SHA-1.
Note that the last two steps can be done in a single operation if the zone is small enough. If the zone is
too large to sign in a single operation, the resigning procedure defined in RFC 5155 [RFC 5155] in adding
NSEC3 RRs using dynamic updates and resigning with NSEC3.
Secondly, when the transition is complete, the zone may become provably unsecure for some validators.
This is part of the design consideration of NSEC3 to allow for non-NSEC3 aware validators to revert to aprovably unsecure state when encountering NSEC3 RRs rather than generating validation errors. Enduser validators that do not understand NSEC3 will not recognized the NSEC3 RSA/SHA-1 DNSKEY
algorithm code (code 7) and will treat the zone as signed by an unknown algorithm and no validation will
be done.
11.7 DNSSEC in a Split-Zone Deployments
11.7.1 Ideal Solution: Internal Delegation
If deploying split horizon DNS for the first time, or if there is a means to re-design an enterprises’ DNS, it
is recommended that the administrator choose a new internal-only delegation from the primary zone. For
example, if the zone were named “example.net”, the internal zone would be “internal.example.net”.
It may even been that the internal-only zone has an entire separate zone name invisible from the externalInternet. For example, an enterprise with the zone “example.net” may choose to have their internal zone
be renamed “internal.example.” Since the internal network is only accessible by internal hosts, and it issimple to have primary recursive servers act as stealth secondaries for this zone, there would not be anissue of having internal hosts unreachable from inside.
In this case, the network topology would fall along these lines. In the external network:•

Zone file: contains only external (i.e., globally accessible) servers (Web, email, etc.).

•

Server configuration: an authoritative-only server with the external zone.

Internal network:•

Zone file: contains internal hosts and any internal interfaces to external servers.

•

Server configuration: an authoritative server for internal zone. Or a recursive caching server that
also acts as a slave to the internal zone.

11-10

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

11.7.2 Deploying DNSSEC in the Ideal Solution Scenario
In the ideal case (i.e., separate internal zone), DNSSEC deployment can be done independently on boththe internal and external zones. The external zone can be signed and have a secure delegation from its
parent zone. The internal zone can be signed using a different set of DNSSEC keys (with the name of theinternal zone) but does not necessary need a signed delegation from its parent zone (which may not exist).
It would be sufficient to simply have the internal public key installed as a trust anchor in all the validatorsof the organization. This would be similar to managing any other security credentials for theorganization.
Since the zones are independent, they can be managed independently. This means that the internal andexternal zones can have different key rollover schedules if needed. However, as this can complicate
operations, it is recommended that the internal and external zones maintain the same rollover schedules toease operations.
11.7.3 Usual Solution: Same Zone Name with Different Files
In many already existing deployments, it is not possible to create a new subzone or new zone name forinternal use. There may be pre-established applications or too many users to make such a transition. In
these cases, the DNS administrator for the zone can still deploy DNSSEC on the external and internalzone views, but must take steps in key management to prevent DNSSEC validation failures for internalclients and information leakage to external attackers.
In this scenario, the zone and server configuration should be similar to the following. In the externalzone:•

Zone configuration: Similar to the previous scenario. The external zone file (or view) contains
only externally accessible hosts such as web, mail and other servers.

•

Server configuration: The server either has only the external zone file, or has the zone configured
as a separate view that is accessible from the entire Internet.

Internal zone:•

Zone configuration: Again, similar to the previous scenario. The internal zone file (or view)contains only internal hosts and any internally accessible interfaces to external hosts.

•

Server configuration: The server either has only the internal zone (or view) or has a separate
view with an ACL restricting queries to only internal hosts.

11.7.4 Deploying DNSSEC in the Usual Solution Scenario
DNS administrators need to take some precautions when deploying DNSSEC in this scenario. The
administrator can choose to use the same key DNSSEC signing keys for both the internal and externalzones, or choose to use separate keys for both zone files. Either choice has its advantages and drawbacks
to consider.
If the same key is used for both the internal and external views, the management of they keys are
simplified (there is only one set). However, it is easier for an attacker, or mis-configured recursive server
to send back name error messages to internal client queries for valid internal hosts. If this happens, aclient will receive a valid error message for a host that exists, which will result in a denial of service.

11-11

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

If different keys are used; one set for the internal view and another set for external zones, the zone
administrator needs to keep operations separate to prevent using the wrong set for a zone. If this happens,
it will appear as if there was an incorrect key rollover, which will result in validation failures.
11.7.5 Considerations for Mobile End Users
The main issues with split horizon DNS deployments come from mobile users moving from the internal
LAN to an external network and vice-versa. If the mobile host always relies on an internal recursive
server for resolution and validation (via a Virtual Private Network tunnel or some other method), thenthere is not much that a mobile client needs to be concerned about. The user just needs to be sure that
there is a secure connection back to the internal network and an internal recursive server that can accept
queries.
It is recommended that mobile hosts have a means to securely access internal hosts via the ability to use
an internal recursive DNS server. The client (or administrators) may wish to configure the mobile client
to send all DNS queries back through the local internal recursive server if there is any question as to thesecurity of the remote network the mobile system is currently using.
If the client does its own validation (i.e., does not rely on validation being done at the recursive cachingserver), then it needs to be configured with any internal and external keys as trust anchors. This adds
complexity to the DNS management of the end system and may require the end system to change
configuration when moving across network boundaries.
It is recommend that a mobile host that does its own validation have the internal DNSSEC key configured
as a trust anchor along with any other trust anchors. This would allow the mobile host to perform its own
validation even when the local network may not be fully trustworthy. However, when traveling acrossnetwork boundaries, the mobile client must be able to reconfigure its validator to not use the internal zone
key as a trust anchor when not on the internal network.
11.8 Recommendations Summary
The following items provide a summary of the major recommendations from this section:

•

•

•

Checklist item 30: The (often longer) KSK needs to be rolled over less frequently than the ZSK.
The recommended rollover frequency for the KSK is once every 1 to 2 years, whereas the ZSKshould be rolled over every 1 to 3 months for operational consistency but may be used longer ifnecessary for stability or if the key is of the appropriate length. Both keys should have an
Approved length according to NIST SP 800-57 Part 1 [800-57P1], [800-57P3].
Zones that pre-publish the new public key should observe the following:–

Checklist item 31: The secure zone that pre-publishes its public key should do so at least one
TTL period before the time of the key rollover.

–Checklist item 32: After removing the old public key, the zone should generate a newsignature (RRSIG RR), based on the remaining keys (DNSKEY RRs) in the zone file.

Checklist item 33: A DNS administrator should have the emergency contact information for theimmediate parent zone to use when an emergency KSK rollover must be performed.

11-12

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE•••Checklist item 34: A parent zone must have an emergency contact method made available to itsdelegated child subzones in case of emergency KSK rollover. There also should be a securemeans of obtaining the new KSK.
Checklist item 35: Periodic re-signing should be scheduled before the expiration field of the
RRSIG RRs found in the zone. This is to reduce the risk of a signed zone being rendered bogus
because of expired signatures.
Checklist item 36: The serial number in the SOA RR must be incremented before re-signing thezone file. If this operation is not done, secondary name servers may not pick up the newsignatures because they are refreshed purely on the basis of the SOA serial number mismatch.
The consequence is that some security-aware resolvers will be able to verify the signatures (andthus have a secure response) but others cannot.

11-13

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

12.

Guidelines on Securing Recursive Servers (Resolver) & Stub Resolvers

12.1 Setting up the Recursive Service
As discussed earlier in this document, the DNS consists of two basic components: Authoritative nameservers that publish DNS data, and DNS resolvers that issue queries for DNS data. Resolvers can befurther broken down into two general categories: Stub resolvers (often found on individual hosts) thatissue queries, but cannot follow DNS referrals, and recursive resolvers/servers that can follow referrals.
The typical set up within an enterprise is to have one or two recursive resolvers/servers providing DNS
resolution for all the stub resolvers on the network.
The security threats to an enterprise’s recursive resolver/server infrastructure differ slightly from thesecurity threats to its authoritative name servers. Likewise, the measures taken to secure a recursive
server differ from an authoritative name server.
12.1.1 Threats to Recursive Servers/Resolvers
Recursive and stub resolvers share a set of threats with authoritative name servers. These threats include
the general threats to the host platform (see Section 5.1) and threats to software used to perform DNS
transactions (see Section 5.2). However, since recursive (and stub) resolvers do not have zone files(except the root hints file used by recursive resolvers) the administrator only needs to focus onconfiguration files. The differences would be:
The threats to DNS data contents (Section 5.3) do not directly apply to recursive and stub resolvers as
they do not have control over the DNS information published; they only consume it.
12.1.2 Securing the Host Platform for Recursive Servers/Resolvers
Section 7 gives the basic steps in securing the host platform of any DNS component such as running thelatest version of software, isolating it on a dedicated server, being aware of known vulnerabilities.
Administrators should refer to the relevant Checklist Items from Section 7 with the knowledge thatrecursive servers do not have zone files (except the root hints file) so there is no need to partition the file
as described in Section 7.2.8.
There are additional steps an administrator can take when setting up a recursive server for an enterprise.
Since it is unwise to allow queries from the outside Internet to the recursive server, the recursive server
can be placed behind a firewall that blocks inbound connections from UDP and TCP port 53 (used by
DNS).
Recursive servers should be configured to only accept queries from internal hosts. This is done using theallow-query sub-statement as described in Section 8.1.1.

Checklist item 37: Recursive servers/resolvers should be placed behind an organization’s firewall andconfigured to only accept queries from internal hosts (e.g., Stub Resolver host).

Stub resolvers do not have many configuration options. Often the only configuration necessary for an
administrator is to enter the IP addresses of one or more recursive resolvers that the stub would rely on toservice queries. It is a good idea for administrators to include at least two IP addresses of recursive

12-14

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

servers to increase the availability of the DNS service for end users. This can be done manually, or using
a protocol like DHCP.
There is a known class of malware that attempts to change the settings on a system’s stub resolver todirect queries to another (usually malicious) recursive server. The server may then direct end users to amalicious site where another attack takes place, or the server may simply direct users to a webpage thatserves ads or similar non-intended content. To combat this, administrators should make sure end user
systems have the latest anti-virus/malware software and should consider blocking all outbound DNS
traffic with the exception of known recursive servers.

Checklist item 38: Administrators should configure two or more recursive resolvers for each stub clienton the network.
Checklist item 39: Enterprise firewalls should consider restricting outbound DNS traffic to only theenterprise’s designated recursive resolvers.

12.2 Guidelines for Establishing Servers
Enterprise network administrators should set up their recursive server infrastructure with availability andspeed of service in mind. However, security is a concern as well as enterprises realize the vulnerability of
the DNS to spoofing and importance of monitoring DNS traffic.
12.2.1 Aggregate Caches
Larger enterprises that want to minimize their external network presence may want to consider deploying
aggregate caches. An aggregate cache is a recursive resolve that exists on an enterprises’ DMZ and only
accepts forwarded queries from other recursive resolvers (sometimes called forwarders in this scenario).
This reduces the number of servers that need monitoring on the DMZ, but still allows for local recursive
servers for separate network segments in an enterprise.
The figure below shows how multiple forwarders send queries to the aggregate cache, which thenperforms any recursion necessary to find the final answers. The individual forwarders may have theirown cache, as well as act as secondaries for internal zones. This is to provide faster service for local stub
resolvers.

12-15

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

Figure 12-1: Example of Resolver Architecture Using an Aggregate Cache.

When the local forwarder receives a query it cannot answer out of cache (or authoritative data, if present)
it forwards the query on to the aggregate cache for recursion. The aggregate cache then performs therecursion and returns the final answer to the forwarder, which in turn sends the final answer back to thestub resolver.
DNSSEC validation on an enterprise with aggregate caches is slightly more complicated than anenterprise that does not use aggregate caches. It is considered wise to perform validation as close to thestub resolver as possible, which in this scenario would mean the forwarder. However, if the aggregate
cache does not perform validation, there is the risk of the aggregate cache having DNSSEC invalid data in
its cache that would then be sent back to the forwarders. This BOGUS data would only be purged upon
the TTL expiration, and not be updated with valid data until that time as passed.
One solution is to perform validation at the forwarder and the aggregate cache, and have the forwarder
send all queries to the aggregate cache with the Checking Disable (CD) bit flag set. That signals to theaggregate cache that the forwarder wishes to perform its own validation and should receive the full
response back. The aggregate cache is still free to perform its own validation of the data, and theresulting caches on both sides will remain in sync.
12-16

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEChecklist item 40: Whenever Aggregate Caches are deployed, the forwarders must be configured to be
Validating Resolvers.

Stub resolvers do not have many configuration options. Often the only configuration necessary for an
administrator is to enter the IP addresses of one or more recursive resolvers that the stub would rely on toservice queries. It is a good idea for administrators to include at least two IP addresses of recursive
servers to increase the availability of the DNS service for end users. This can be done manually, or using
a protocol like DHCP.
There is a known class of malware that attempts to change the settings on a system’s stub resolver todirect queries to another (usually malicious) recursive server. The server may then direct end users to amalicious site where another attack takes place, or the server may simply direct users to a webpage thatserves ads or similar non-intended content. To combat this, administrators should make sure end user
systems have the latest anti-virus/malware software and should consider blocking all outbound DNS
traffic with the exception of known recursive servers.

Checklist item 41: Administrators should configure two or more recursive resolvers for each stub clienton the network.
Checklist item 42: Enterprise firewalls should consider restricting outbound DNS traffic to only theenterprise’s designated recursive resolvers.

12.3 Setting up the Root Hints File
Every recursive server needs to know the IP address of one or more root servers. If the recursive server
cannot find the answer to a stub resolver’s query in its cache, it must perform a recursive lookup for thecorrect information to respond to the client. It does this by first sending the query to one of the root zone
servers and following one or more referrals until it reaches one of the authoritative servers of the DNS
zone data. See Section 2.2 for a more detailed discussion of the DNS resolution process.
The IP addresses of the root servers are stored in a file often referred to as the root hints file. It is notconsidered authoritative information, but only hints for the locations of the root zone servers. On startup(and regular intervals), recursive servers send queries to each IP address in the root hints file for the fulllist of root zone servers. It then maintains this information in its cache until it is time to refresh this data.
It is also necessary for a human administrator to also regularly check the contents of the root hints file forany necessary updates. Missing any changes to the set of root zone servers may result in delays at startup(while waiting for queries to time out) or security vulnerabilities (attackers using spoofed root zone IP
addresses). The list of public DNS root zone servers are maintained as part of the IANA functions 13 andadministrators can download the current root hints file as well as the DNSSEC public key of the root
zone.

13

http://www.iana.org/domains/root/files

12-17

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEChecklist item 43 : Each recursive server must have a root hints file containing the IP address of one ormore DNS root servers. The information in the root hints file should be periodically checked forcorrectness.
Checklist item 44 : The root hints file should be owned by the account under which the name server
software is run. The permission bits should be set so that the root hints file can be read or modified onlyby the account that runs the name server software.

12.4 Securing the Stub Resolver
Stub resolvers do not have many configuration options. Often the only configuration necessary for an
administrator is to enter the IP addresses of one or more recursive resolvers that the stub would rely on toservice queries. It is a good idea for administrators to include at least two IP addresses of recursive
servers to increase the availability of the DNS service for end users. This can be done manually, or using
a protocol like DHCP.
There is a known class of malware that attempts to change the settings on a system’s stub resolver todirect queries to another (usually malicious) recursive server. The server may then directs end users to amalicious site where another attack takes place, or the server may simply direct users to a webpage thatserves ads or similar non-intended content. To combat this, administrators should make sure end user
systems have the latest anti-virus/malware software and should consider blocking all outbound DNS
traffic with the exception of known recursive servers.

Checklist item 45: Administrators should configure two or more recursive resolvers for each stub
resolver on the network.
Checklist item 46: Enterprise firewalls should consider restricting outbound DNS traffic from stub
resolvers to only the enterprise’s designated recursive resolvers.

12.5 Recommendations Summary•

Checklist item 37: Recursive servers/resolvers should be placed behind an organization’s
firewall and configured to only accept queries from internal hosts (e.g., Stub Resolver host).

Checklist Item 38: Whenever Aggregate Caches are deployed, the forwarders must be configured to be
Validating Resolvers.
•

Checklist item 39 : Each recursive server must have a root hints file containing the IP address of
one or more DNS root servers. The information in the root hints file should be periodicallychecked for correctness.

•

Checklist item 40 : The root hints file should be owned by the account under which the nameserver software is run. The permission bits should be set so that the root hints file can be read or
modified only by the account that runs the name server software.
12-18

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE•Checklist item 41: Administrators should configure two or more recursive resolvers for each stub
resolver on the network.

•

Checklist item 42: Enterprise firewalls should consider restricting outbound DNS traffic fromstub resolvers to only the enterprise’s designated recursive resolvers.

•

Checklist item 43 : Each recursive server must have a root hints file containing the IP address of
one or more DNS root servers. The information in the root hints file should be periodicallychecked for correctness.

•

Checklist item 44 : The root hints file should be owned by the account under which the nameserver software is run. The permission bits should be set so that the root hints file can be read or
modified only by the account that runs the name server software.

•

Checklist item 45: Administrators should configure two or more recursive resolvers for each stub
resolver on the network.

•

Checklist item 46: Enterprise firewalls should consider restricting outbound DNS traffic fromstub resolvers to only the enterprise’s designated recursive resolvers.

12-19

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

13.

Guidelines on Securing Validating Resolvers

As discussed in Section 6.1.4, DNSSEC provides data origin authentication and integrity protection for
DNS data. This is done using digital signatures stored with the DNS data and validated by DNS clients.
DNS clients that perform this validation are referred to as validators.
DNS validators check responses by building an authentication chain (a.k.a. chain of trust) from theresponse data to a pre-configured trusted key. This may involve the validator sending follow up queries
for public keys or other information (such as DS RR’s) to obtain the full DNSSEC chain. Therefore,
validators are often recursive servers that can perform their own DNS resolution process and cache
results.
13.1 Enabling DNSSEC Validation
Configuring DNSSEC validation requires two tasks: configuring the server to perform validation (notalways required) and configure one or more trusted public keys to act as trust anchors. How this is done
depends on the DNS implementation used, but BIND will be used as an example.
The policy in determining which DNSSEC public keys to configure as trust anchors is beyond the scopeof this guide and would be the part of the organization's security policy. However, given the hierarchy
used in DNS, the higher the public key, the wider range of DNS responses can be validated using thatkey.
13.1.1 On the Stub Client
A stub resolver software, by definition, does not perform full resolution and hence is not DNSSEC-aware.
Even if the stub is DNSSEC aware, it may not be able to perform its own validation, but rely on an
upstream server to perform validation on its behalf.
13.1.2 Configuring DNSSEC on a Windows 7 or Windows 8 System
Microsoft Windows 7 and Windows 8 operating systems have the option of having the stub resolver toexplicitly request DNSSEC validation for a given domain. This is done using the Name Resolution Policy
Table (NRPT). A system administrator can set the domain and whether the system should expect
DNSSEC validation from responses. These policies can be set on an individual or enterprise wide basis.
The details on how to set this option in Windows is available from the Microsoft TechNet website14. Notehowever that the option is set for an entire domain, not a single zone. If the policy is set for a domain, theclient will automatically expect DNSSEC validation for any subzone under the domain as well, and will
reject any non-validated responses.
13.1.3 Using a Validating Recursive Server
Most current stub resolvers cannot perform DNSSEC validation and may not understand DNSSEC at all.
These stub resolvers will have to rely on an upstream validating recursive resolver to perform DNSSEC
validation on its behalf. On an trusted enterprise network, this does not pose a significant risk, as there14

http://technet.microsoft.com/en-us/library/ee649207(v=WS.10).aspx

13-20

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

are several options to protect the link between a validating recursive resolver and a non-DNSSEC aware
stub resolver (see Section 9.8).
If the enterprise network is considered trusted (using one of the last hop mechanisms listed in Section 9.8
or similar), then the stub resolvers can be considered to be using DNSSEC. However, end users will not
see DNSSEC validation (especially errors), and network administrators should be aware that DNSSEC
validation failures might complicate diagnosis of Internet error messages. DNSSEC validation failures
will be seen by the upstream validator and not the stub resolver that initiated the query. The stub resolver
will only see a generic server failure message, which applications interpret differently. Network
administrators should check validator logs when responding to network errors to rule out DNSSEC
validation failures.

Checklist item 47: Non-validating stub resolvers (both DNSSEC-aware and non-DNSSEC-aware) musthave a trusted link with a validating recursive resolver.
Checklist item 48: Validators should routinely log any validation failures to aid in diagnosing network
errors.

13.1.4 Recommendations for Providing Service to Mobile Hosts
Mobile or nomadic hosts present a particular challenge for network administrators. These systems areoften (or always) accessing the network outside of the trusted enterprise; so nomadic hosts must either
perform their own validation or have a trusted tunnel back to the enterprise with which to send DNS
queries.

Checklist item 49: Mobile or nomadic systems should either perform their own validation or have atrusted channel back to a trusted validator.

If the mobile (or nomadic) hosts can perform their own validation, then the same policy in place for theenterprise validators should be applied for the mobile host. That is, the same trust anchors and validation
policy should be set for mobile hosts as for validators on the enterprise networks.

Checklist item 50 : Mobile or nomadic systems that perform its own validation should have the same
DNSSEC policy and trust anchors as validators on the enterprise network.

A mobile hosts that can perform its own validation that frequently migrates between the enterpise
network and external networks may run into the problem that its local validator may not choose thecorrect trust anchor for the enterprise zone. In this scenario, the enterprise has an internal and external
network with the same zone name (e.g., “example.com”) but different zone keys (ZSK and/or KSK).
When the mobile host is on the enterprise network, it uses the internal example.com trust anchor tovalidate DNS responses for internal services. When accessing external facing services, the internal zone

13-21

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

trust anchor will not be able to validate the responses about external services and vice-versa (using theexternal zone key as a trust anchor will result in validation failures for internal DNS zone responses).
In these cases, it may be necessary for a mobile systems to configure its valiadator when migrating fromthe enterprise to an external network and vice-versa. Or, if the enterprise network is trusted, the mobile
host can rely on the enterprise validator when on the enterprise network, and perform its own validation
when on external networks. Ideally, network administrators can avoid this problem by using alternative
names for internal and external zones, thus having different trust anchors.
If the mobile host cannot perform its own validation, it must have a secure tunnel back to the enterprisenetwork and use an enterprise managed validator. Many enterprises already have a means for mobile
hosts to access internal resources (like file servers, etc.); a validating recursive server should be added as
one of the services provided to mobile hosts through a secure channel.
13.2 Establishing Initial Trust Anchors
Each DNSSEC validator must have one or more public keys stored locally to use in building
authentication chains. These public keys are often referred to as trust anchors. There is nothing
particularly special about a trust anchor to anyone other than the client that has configured it as a trusted
public key.
Since DNSSEC validation builds upon the DNS hierarchy for trust, the public key for the DNS root zone
and TLD’s are more valuable because they can (assuming a fully signed DNS domain tree), be used tovalidate the widest number of nodes (thus a larger portion of the domain name space) and thus provide
coverage for a wider range of DNS responses. Validator administrators should identify the set of keys
they believe they would need to provide DNSSEC validation for the segments of the name space deemedmost important. For most administrators, this would be the DNS root zone key, but may possible include
one or more TLD keys (such as the keys for their local ccTLD or similar). Administrators should obtain
the public keys (usually the KSK) in a trusted manner (e.g., a SSL secure webpage) and configure the key(or in some cases, a hash of the key) in the implementation specific manner. In BIND, trusted keys are
configured using the “trusted-keys” statement in the named configuration file. The format for these keys
are:<zone name><flags> <protocol> <algorithm> <base 64 encoded key material>;

Where the <flags>, <protocol> and <algorithm> numbers are taken from the DNSKEY RDATA values.
So that in the named configuration file, an example root key would appear as:trusted-keys {“.”257 3 8 “AwEAAagAIKlVZrpC6Ia7gEzahOR+9W29eux
hJhVVLOyQbSEW0O8gcCjFFVQUTf6v58fLjw
Bd0YI0EzrAcQqBGCzh/RStIoO8g0NfnfL2M
TJRkxoXbfDaUeVPQuYEhg37NZWAJQ9VnMVD
xP/VHL496M/QZxkjf5/Efucp2gaDX6RS6CX
poY68LsvPVjR0ZSwzz1apAzvN9dlzEheX7ICJBBtuA6G3LQpzW5hOA2hzCTMjJPJ8LbqF6
dsV6DoBQzgul0sGIcGOYl7OyQdXfZ57relS
Qageu+ipAdTTJ25AsRTAoub8ONGcLmqrAmR
LKBP1dfwhYB4N7knNnulqQxA+Uk1ihz0=”;
};

13-22

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

Since this is the public key, no new security risks are introduced. However, the basic security
recommendations for configuration files (see Section 8) should be followed. The risk is an attacker may
be able to change the key material to point to a key that the attacker controls. However, this would
involve other successful attacks (such as redirecting queries as well) to be fully effective, so it is notconsidered a high risk.

Checklist item 51 : Validator administrator must configure one or more trust anchors for each validator
in the enterprise.

13.3 Maintaining Trust Anchors
Once the trust anchors are initially configured, the administrator must set up a means to monitor that the
key is still in use for the give zone, and perform updates as required. Often, zone administrators will
perform key rollovers (see Section 11.2) on keys that some clients use as trust anchors. Since there is nomeans in DNS for servers to contact clients to inform them their trust anchors are out of date, clients must
perform their own checks to insure they have an up-to-date list of trust anchors.
The IETF has specified a way for signed zones to signal that a key rollover is underway so clients thathave the current zone key can successfully migrate to the new key. This method is specified in RFC 5011
[RFC5011], and some DNSSEC validators that implement this specification can rollover trust anchors in
an automated fashion.
Validator administrators should be cogent that not every zone uses the rollover method specified in RFC
5011, and not every implementation can perform automated trust anchor rollovers so regular pollingshould be done to insure that the validator has the most current key installed as a trust anchor. This could
be done via a normal query for the DNSKEY RRset and look for a new key, or by regularly checking thesame method the original key was obtained. For example, the DNS root zone key is maintained on
website 15 that can be routinely checked (manually or via an automated script) for updates.
How often these checks need to be performed is difficult to determine, but some hints may be found the
TTL and RRSIG validity period of the DNSKEY RRset that contains the key used as the trust anchor.
Using these values and understanding the timed steps of a key rollover (See section 11.2), a validator
administrator can calculate the minimum time between checks.
If a key rollover is detected as in progress, or a new key is anounced, validator administrators should
update the validator configuration file with the new key material. This process may be similar to theinitial configuration or different, depending on the software used as the validator.

Checklist item 52: The validator administrator regularly checks each trust anchor to ensure that it is stillin use, and updates the trust anchor as necessary.

15

https://www.iana.org/dnssec

13-23

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

13.4 Recommendations Summary•

Checklist item 47: Non-validating stub resolvers (both DNSSEC-aware and non-DNSSECaware) must have a trusted link with a validating recursive resolver.

•

Checklist item 48: Validators should routinely log any validation failures to aid in diagnosing
network errors.

•

Checklist item 49: Mobile or nomadic systems should either perform their own validation or
have a trusted channel back to a trusted validator.

•

Checklist item 50 : Mobile or nomadic systems that perform its own validation should have thesame DNSSEC policy and trust anchors as validators on the enterprise network.

•

Checklist item 51 : Validator administrator must configure one or more trust anchors for eachvalidator in the enterprise.

•

Checklist item 52: The validator administrator regularly checks each trust anchor to ensure that itis still in use, and updates the trust anchor as necessary.

13-24

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEAppendix A—Definitions of Important Terms
Selected terms used in the Secure Domain Name System (DNS) Deployment Guide are defined below. The
terms come from the DNSSEC specifications [RFC4033], [RFC4034], [RFC4035] and [RFC3833] withsome additions for specific terms used in this guide.
Authentication Chain: An alternating sequence of DNS public key (DNSKEY) RRsets and Delegation
Signer (DS) RRsets forms a chain of signed data, with each link in the chain vouching for the next. A
DNSKEY RR is used to verify the signature covering a DS RR and allows the DS RR to be authenticated.
The DS RR contains a hash of another DNSKEY RR, and this new DNSKEY RR is authenticated bymatching the hash in the DS RR. This new DNSKEY RR, in turn, authenticates another DNSKEY RRSet
and, in turn, some DNSKEY RR in this set may be used to authenticate another DS RR, and so forth until
the chain finally ends with a DNSKEY RR whose corresponding private key signs the desired DNS data.
For example, the root DNSKEY RRSet can be used to authenticate the DS RRSet for “example.” The“example.” DS RRSet contains a hash that matches some “example.” DNSKEY, and this DNSKEY’s
corresponding private key signs the “example.” DNSKEY RRSet. Private key counterparts of the“example.” DNSKEY RRSet sign data records such as “www.example.” as well as DS RRs fordelegations such as “subzone.example.”
Authentication Key: A public key that a DNSSEC-aware resolver has verified and can therefore use toauthenticate data. A DNSSEC-aware resolver can obtain authentication keys in three ways. First, theresolver generally is configured to know about at least one public key; this configured data usually iseither the public key itself or a hash of the public key as found in the DS RR (see “trust anchor”). Second,
the resolver may use an authenticated public key to verify a DS RR and the DNSKEY RR to which the
DS RR refers. Third, the resolver may be able to determine that a new public key has been signed by theprivate key corresponding to another public key that the resolver has verified. Note that the resolver must
always be guided by local policy in deciding whether to authenticate a new public key, even if the local
policy is simply to authenticate any new public key for which the resolver is able verify the signature.
Authoritative RRSet: Within the context of a particular zone, an RRSet (RRs with the same name, class,and type) is authoritative if and only if the owner name of the RRSet lies within the subset of the namespace that is at or below the zone apex and at or above the cuts that separate the zone from its children, if
any. RRs of type NSEC, RRSIG and DS are examples of RRSets at a cut that are authoritative at theparent side of the zone cut, and not the delegated child side.
Chain of Trust: See “authentication chain.”
Chained Secure Zone: A DNS zone in which there is an authentication chain from the zone to a trust
anchor.
Delegation Point: The name at the parental side of a zone cut. That is, the delegation point for“foo.example” would be the foo.example node in the “example” zone (as opposed to the zone apex of the“foo.example” zone). See also “zone apex.”
DNS Administrator: Used in this document to cover the person (or persons) tasked with updating zone
data and operating an enterprise’s DNS server. This term may actually cover several official roles, butthese roles are covered by one term here.
DNSSEC-Aware Name Server: An entity acting in the role of a name server that understands the DNS
security extensions defined in this document set. In particular, a DNSSEC-aware name server is an entitythat receives DNS queries, sends DNS responses, supports the EDNS0 [RFC2671] message size

A-1

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

extension and the DO bit [RFC4035], and supports the RR types and message header bits defined in thisdocument set.
DNSSEC-Aware Recursive Name Server: An entity that acts in both the DNSSEC-aware name serverand DNSSEC-aware resolver roles. A more cumbersome equivalent phrase would be “a DNSSEC-aware
name server that offers recursive service.” Also sometimes referred to as a “security-aware caching nameserver.”DNSSEC-Aware Resolver: An entity acting in the role of a resolver (defined in section 2.4 of[RFC4033]) that understands the DNS security extensions. In particular, a DNSSEC-aware resolver is anentity that sends DNS queries, receives DNS responses, and understands the DNSSEC specification, even
if it is incapable of performing validation.
DNSSEC-Aware Stub Resolver: An entity acting in the role of a stub resolver that has an understandingof the DNS security extensions. DNSSEC-aware stub resolvers may be either “validating” or“nonvalidating,” depending on whether the stub resolver attempts to verify DNSSEC signatures on its
own or trusts a friendly DNSSEC-aware name server to do so. See also “validating stub resolver” and“nonvalidating stub resolver.”Island of Security: A signed, delegated zone that does not have an authentication chain from itsdelegating parent. That is, there is no DS RR containing a hash of a DNSKEY RR for the island in itsdelegating parent zone. An island of security is served by DNSSEC-aware name servers and may provide
authentication chains to any delegated child zones. Responses from an island of security or its
descendents can be authenticated only if its authentication keys can be authenticated by some trusted
means out of band from the DNS protocol.
Key Rollover: The process of generating and using a new key (symmetric or asymmetric key pair) toreplace one already in use. Rollover is done because a key has been compromised or is vulnerable tocompromise as a result of use and age.
Key Signing Key (KSK): An authentication key that corresponds to a private key used to sign one ormore other authentication keys for a given zone. Typically, the private key corresponding to a key signing
key will sign a zone signing key, which in turn has a corresponding private key that will sign other zone
data. See also “zone signing key.”
Nonvalidating DNSSEC-Aware Stub Resolver: A DNSSEC-aware stub resolver that trusts one or more
DNSSEC-aware recursive name servers to perform most of the tasks discussed in this document set on its
behalf. In particular, a nonvalidating DNSSEC-aware stub resolver is an entity that sends DNS queries,
receives DNS responses, and is capable of establishing an appropriately secured channel to a DNSSECaware recursive name server that will provide these services on behalf of the DNSSEC-aware stub
resolver. See also “DNSSEC-aware stub resolver” and “validating DNSSEC-aware stub resolver.”
Nonvalidating Stub Resolver: A less tedious term for a nonvalidating DNSSEC-aware stub resolver.
Signed Zone: A zone whose RRsets are signed and which contains properly constructed DNSKEY,
Resource Record Signature (RRSIG), Next Secure (NSEC), and (optionally) DS records.
Trust Anchor: A configured DNSKEY RR or DS RR hash of a DNSKEY RR. A validating DNSSECaware resolver uses this public key or hash as a starting point for building the authentication chain to asigned DNS response. In general, a validating resolver will need to obtain the initial values of its trust
anchors via some secure or trusted means outside the DNS protocol. The presence of a trust anchor also

A-2

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

implies that the resolver should expect the zone to which the trust anchor points to be signed. This issometimes referred to as a “secure entry point.”
TSIG Key: A string used to generate the message authentication hash stored in a TSIG RR and used toauthenticate an entire DNS message. This is not the same as signing a message, which involves acryptographic operation.
Unsigned Zone: A zone that is not signed.
Validator: A component that validates DNSSEC signatures. Usually not a separate component but partof a DNSSEC-aware recursive server (sometimes referred to as a validating resolver or validating
recursive server).
Zone Apex: The name at the child’s side of a zone cut. See also “delegation point.”
Zone Signing Key (ZSK): An authentication key that corresponds to a private key used to sign a zone.
Typically a zone signing key will be part of the same DNSKEY RRSet as the key signing key whose
corresponding private key signs this DNSKEY RRSet, but the zone signing key is used for a slightly
different purpose and may differ from the key signing key in other ways, such as validity lifetime.
Designating an authentication key as a zone signing key is purely an operational issue: DNSSEC
validation does not distinguish between zone signing keys and other DNSSEC authentication keys, and it
is possible to use a single key as both a key signing key and a zone signing key. See also “key signing
key.”

A-3

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEAppendix B—Vendor Specific Steps to Meet Checklist Items
This publication does not attempt to list configuration examples for every DNS component available. The
authors have made the decision to only include major open-source implementations to use in theexamples. Other implementations may have vendor specific features or configuration options available toachieve the same set of recommendations in this guide. In some cases, implementations may not containevery features described in this publication and so those checklist items would not apply when installingand configuring those implementations.
Whenever possible, a shorthand guide on how to meet the checklist items using a specific implementation
will be produced as a separate documents. These documents are not official NIST Special Publications,but supplemental guidance for specific implementations.
The repository for these supplemental documents can be found at http://www.dnsops.biz/vendors

B-4

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEAppendix C—Acronyms
Selected acronyms used in the Secure Domain Name System (DNS) Deployment Guide are defined below.
A
ACL
AD
ANSI
ARP

Address
Access Control List
Authenticated Data
American National Standards Institute
Address Resolution Protocol

CA
ccTLD
CD

Certificate Authority
Country-code Top-level Domain
Checking Disabled

DHCP
DNS
DNSSECDSDSADSS

Dynamic Host Configuration Protocol
Domain Name System
Domain Name System Security Extensions
Delegation Signer
Digital Signature Algorithm
Digital Signature Standard

ECDSA

Elliptic Curve Digital Signature Algorithm

FIPS
FISMA
FQDN

Federal Information Processing Standards
Federal Information Security Management Act
Fully Qualified Domain NamegTLD

Generic Top-level Domain

HINFO
HMAC

Host Information
Hash-based Message Authentication Code

ICANN
IETF
IN
IP
ISP
IT
ITL

Internet Corporation for Assigned Names and Numbers
Internet Engineering Task Force
Internet
Internet Protocol
Internet Service Provider
Information Technology
Information Technology Laboratory

KSK

Key Signing Key

LAN
LOC

Local Area Network
Location

MAC
MD
MX

Message Authentication Code
Message Digest
Mail Exchanger

NIST
NS

National Institute of Standards and Technology
Name Server

C-1

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE

NSEC
NSEC3
NTP

Next Secure
Hashed Next Secure
Network Time Protocol

OMB
OS

Office of Management and Budget
Operating System

PKI
PIR

Public Key Infrastructure
Public Internet Registry

RFC
RP
RR
RRSIG

Request for Comments
Responsible Person
Resource Record
Resource Record Signature

SEP
SHA
SHS
SOA

Secure Entry Point
Secure Hash Algorithm
Secure Hash Standard
Start of Authority

TCP
TLD
TSIG
TTL
TXT

Transmission Control Protocol
Top-level Domain
Transaction Signature
Time to Live
Text

UDP

User Datagram Protocol

ZSK

Zone Signing Key

C-2

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDEAppendix D—Bibliography
I. DNS – Resource Records and Generic Architecture[RFC3833][RFC2181][RFC1912][RFC1034][RFC1035][BCP140][RFC2671][RFC1996][IEEE]

D. Atkins and R. Austein, “Threat Analysis of the Domain Name System (DNS)”, RFC
3833, August 2004.
R. Elz and R. Bush, “Clarifications to the DNS Specification”, RFC 2181, July 1997.
D. Barr, “Common DNS Operational and Configuration Errors”, RFC 1912, Feb. 1996
P. Mockapetris, “Domain Names - Concepts and Facilities”, STD 13, RFC 1034,
November 1987.
P. Mockapetris, “Domain Names - Implementation and Specification”, STD 13, RFC
1035, November 1987.
J. Damas and F. Neves, “Preventing Use of Recursive Nameservers in Reflector
Attacks”, BCP 140, RFC 5358. October 2008.
P. Vixie, “Extension Mechanisms for DNS (EDNS0)” RFC 2671 August 1999
P. Vixie, “A Mechanism for Prompt Notification of Zone Changes (DNS NOTIFY)”
RFC 1996. August 1996.
S. Rose and A. Nakassis. “Minimizing Information Leakage in the DNS” IEEE Network
Magazine vol. 22 no. 2 April 2008.

II. DNSSEC[RFC4033][RFC4034][RFC4035][RFC4641][RFC5011][RFC5155]

R. Arends, R. Austein, M. Larson, D. Massey, and S. Rose, “DNS Security Introduction
and Requirements”, RFC 4033, March 2005.
R. Arends, et al, “Resource Records for DNS Security Extensions”, RFC 4034, March
2005.
R. Arends, et al, “Protocol Modifications for the DNS Security Extensions”, RFC 4035,
March 2005.
O. Kolkman and R. Gieben. “DNSSEC Operational Practices”, RFC 4641. September
2006.
M. StJohns. “Automated Updates for DNS Security (DNSSEC) Trust Anchors”. RFC
5011, September 2007.
B. Laurie, G. Sisson, R. Arends and D. Blacka. “DNS Security (DNSSEC) Hashed
Authenticated Denial of Existance” RFC 5155, March 2008

III. Dynamic Update[RFC3007][RFC2136]

B. Wellington, “Secure Domain Name System (DNS) Dynamic Update”, RFC 3007,
November 2000.
P. Vixie, S. Thomson, Y. Rekhter, and J. Bound. “Dynamic Updates in the Domain Name
System (DNS UPDATE)”. RFC 2136, April 1997.

IV. TSIG[RFC2931][RFC2845]

D. Eastlake 3rd, “DNS Request and Transaction Signatures (SIG(0)s)”, RFC 2931,
September 2000.
P. Vixie, O. Gudmundsson, D. Eastlake 3rd, and B. Wellington, “Secret Key Transaction
Authentication for DNS (TSIG)”, RFC 2845, May 2000.

D-1

SECURE DOMAIN NAME SYSTEM (DNS) DEPLOYMENT GUIDE[RFC3645][RFC4635]

S. Kwan, P. Garg, J. Gilroy, L. Esibov, J. Westhead and R. Hall. “Generic Security
Service Algorithm for Secret Key Transaction Authentication for DNS (GSS-TSIG)”.
RFC 3645 October 2004.
D. Eastlake 3rd, “HMAC SHA TSIG Algorithm Identifiers” RFC 4635, August 2006.

V. Cryptographic Standards and Other USG Guidance Documents[RFC2104][FIPS198][FIPS180][FIPS186][800-57P1][800-57P3]

H. Krawczyk, M. Bellare, and R. Canetti. “HMAC: Keyed-Hashing for Message
Authentication”, RFC 2104, February 1997
“The Keyed-Hash Message Authentication Code (HMAC)”, Federal Information
Processing Standard, National Institute of Science and Technology. FIPS 198-1, July
2008.
FIPS 180-2, “Secure Hash Standard (SHS)”, Federal Information Processing Standard,
National Institute of Science and Technology. FIPS 180-2, August 2002.
“Digital Signature Standard” FIPS 186-3. March 2008.
E. Barker, W. Barker, W. Burr, W. Polk and M. Smid. “Recommendation for Key
Management – Part 1: General” NIST Special Publication 800-57 Part 1 (Revision 3).
July 2012.
A. Clay, E. Barker, W. Burr, W. Polk, S. Rose. “Recommendation for Key Management– Part 3: Application-Specific Key Management Guidance” NIST Special Publication
800-57 Part 3, December 2009

VI. Security Vulnerabilities[CERT][NVD][BINDSEC]

CERT®/CC’s Vulnerability Notes Database at http://www.kb.cert.org/vuls/
NIST NVD metabase at http://nvd.nist.gov/
BIND vulnerabilities page at http://www.isc.org/products/BIND/bind-security.html

D-2