Original text:
Requirements for Adaptive DNS Discovery <del> draft-add-requirements-latest </del>  Abstract Adaptive DNS Discovery is chartered to define mechanisms that allow clients to discover and select encrypted DNS resolvers.  This <del> document seeks to provide a set of use cases and requirements that proposed mechanisms should address. </del>  1.  The Adaptive DNS Discovery (ADD) Working Group is chartered to define <del> mechanisms that allow clients to discover and select encrypted DNS resolvers in a wide variety of network environments.  This document seeks to provide a set of use cases and requirements that proposed mechanisms should address.  They can do this either by providing a solution, or by explicitly stating why it is not in scope. Use cases are described between first-usecase and last-usecase.  Each use case contains a narrative and a set of requirements that apply in that case.  Each is identified as "Ra.b" where a is the requirement group number and b is the number within that group.  Both a and b are integers starting with 1.  A summary of all requirements in listed in requirements-summary. </del>  <del> 2. </del>  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 14 RFC2119 RFC8174 when, and only when, they appear in all capitals, as shown here.  3. <del> A client may begin with information from the attached networks, and/ or resolvers known from configuration.  This information may be used to then discover one or more associated or equivalent encrypted resolvers.  These are expected to be operated by the same entity that provides the resolver the client started with, but may not have identical behaviour.  For example one entity may offer a set of encrypted resolvers with different levels of filtering or proximity. </del>  3.1. <del> During network attach, networks frequently announce a resolver that they recommend clients should use.  This could be through DHCP, RA, PPP, PCO, or another mechanism.  Historically this is usually a Do53 resolver, declared simply by its IP address.  However it could also be a richer set of information. </del>  3.1.1. benefits of a local caching forwarder with knowledge of the local topology.  3.2. <del> A client with manual configuration directing it to use a particular resolver will generally use that resolver.  However if that communication is unencrypted, it is beneficial for the client to learn which associated resolvers are offered by the same entity and provide the same semantics. </del>  3.3. <del> Narrative required. </del>  4. <del> Narrative required. </del>  4.1. 4.1.1. <del> Narrative required. </del>  4.1.2. 4.1.3. <del> Narrative required. </del>  4.2. <del> Narrative required. </del>  5.  This sections lists the complete set of requirements described above, for ease of reference. <del> 6. </del>  All security considerations relevant to a particular use case are <del> described under that section.  Additional consideration common to all of them are described here. </del>  <del> 7. </del>  This document has no IANA actions.

Reviewer comments:
Authors, please comment.
I’m not sure adding authenticated here is something the group has consensus on as yet. I know this is still in early draft mode, so while I don’t object to including it, we should mark is as an item to be discussed. If we do include it, then some words should be added somewhere in the draft about what “authenticated” in such a context is being proposed to mean as that’s what will be a key discussion item in determining if it get consensus.
Glenn I merged "authenticated" before I read this comment. I added it because it made sense to me; without authentication any encryption is highly weakened. But I take your point that it needs explaining.
Ok, perhaps it can be added in the next rev. Basically I’m suggesting that the aspect of “authenticated” be expanded, discussed, and explained as it does potentially have different notions in different readers perspectives. For example some may say doing a DNSSEC validation of some sort is what is needed, or perhaps some form of PKI, or is it a TLS certificate validation, or mutual exchange? It might even be something as basic as asking special DNS queries and validating the answers received against expectations. Another slice would be authenticating the actual resolver chosen or is it authenticated at the more abstract “service” provided by the operator of the resolver? There is also the consideration of does the authentication means/level vary depending on different environmental threat levels? The bottom line is there is a lot of meanings behind “authentication” and what is being authenticated to declare and discuss as a requirement.
Several possible ways of authenticating the encrypted DNS server are already discussed in our spec URL I don't think the requirements draft should discuss the possible modes of authenticating the server.
How does the client learn the encrypted and unencrypted DNS servers are operated by the same administrative domain ? The client can authenticate the encrypted DNS servers but cannot authenticate the unencrypted DNS server ! Originally posted by NAME in URL
If it is genuinely impossible to prove association between two resolvers when one resolver is unencrypted, then perhaps we should not have the concept at all. But I'm hoping it is not impossible.
draft-pauly-add-deer-00 does this (at least in some circumstances), so it's not impossible.

Revised text:
Requirements for Adaptive DNS Discovery  <ins> draft-box-add-requirements-00 </ins> Abstract Adaptive DNS Discovery is chartered to define mechanisms that allow clients to discover and select encrypted DNS resolvers.  This  <ins> document describes several use cases for discovering DNS resolvers that support encrypted transports, and lists requirements that any proposed discovery and selection mechanisms should address. </ins> 1. <ins> Several protocols for protecting DNS traffic with encrypted transports have been defined, such as DNS-over-TLS (DoT) RFC7858 and DNS-over-HTTPS (DoH) RFC8484.  Encrypted DNS can provide many security and privacy benefits for network clients. While it is possible for clients to hard-code encrypted DNS resolvers to use, dynamic discovery and provisioning of encrypted resolvers can expand the usefulness and applicability of encrypted DNS to many more use cases. </ins> The Adaptive DNS Discovery (ADD) Working Group is chartered to define  <ins> mechanisms that allow clients to automatically discover and select encrypted DNS resolvers in a wide variety of network environments. This document describes several use cases for discovering DNS resolvers that support encrypted transports, and lists requirements that any proposed discovery and selection mechanisms should address. They can do this either by providing a solution, or by explicitly stating why it is not in scope. </ins>  <ins> Use cases are described between associated and cdn-content.  Each use case contains a narrative and a set of requirements that apply in that case.  There are additional common requirements in priv-sec. Each requirement is identified as "Ra.b" where a is the group number and b is the number within that group.  Both a and b are integers starting with 1. A summary of all requirements in listed in requirements-summary. 1.1. </ins> The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and 14 RFC2119 RFC8174 when, and only when, they appear in all capitals, as shown here. <ins> 2. This document makes use of the following terms. Encrypted DNS: DNS-over-HTTPS RFC8484, DNS-over-TLS RFC7858, or any other encrypted DNS technology that the IETF may publish, such as DNS-over-QUIC I-D.ietf-dprive-dnsoquic. Associated resolver: A resolver operated by the same entity that provides the resolver the client started with.  See associated. Equivalent associated resolver: An associated resolver that provides DNS responses that are identical to the ones served by the original unencrypted resolver. Alternative associated resolver: An associated resolver that serves different responses to some queries; see associated for examples. </ins> 3.  <ins> A client may begin with information about unencrypted resolvers from the attached networks (local-network), and/or unencrypted resolvers known from configuration (client-selected).  This information may be used to then discover one or more associated encrypted resolvers. Associated resolvers are defined as resolvers operated by the same entity that provides the resolver the client started with.  Such associated resolvers may come in two forms: Equivalent - these provide DNS responses that are identical to the ones served by the unencrypted resolver. Alternative - these serve different responses to some queries. For example one entity may offer a set of encrypted resolvers with different levels of filtering (none, just malware, or malware & adult content), or different proximity (local or central). The client may wish to select an equivalent associated resolver, or select one of the alternatives. Designs for resolver upgrade mechanisms can either add new parameters to existing provisioning mechanisms (for example, adding necessary information to use DoT or DoH to options in DHCP, RAs, or IKEv2) or else provide a way to communicate with a provisioned unencrypted DNS resolver and discover the associated encrypted DNS resolvers. </ins> 3.1.  <ins> DNS servers are often provisioned by a network as part of DHCP options RFC2132, IPv6 Router Advertisement (RA) options RFC8106, Point-to-Point Protocol (PPP) RFC1877, 3GPP Protocol Configuration Options, or another mechanism.  Historically this is usually one or more DNS resolver IP addresses, to be used for traditional unencrypted DNS.  However it could also be a richer set of information. Using an encrypted and authenticated resolver that is associated to the one provisioned by the network can provide several benefits that are not possible if only unencrypted DNS is used: Prevent other devices on the network from observing client DNS messages Verify that answers come from the selected DNS resolver Authenticate that the DNS resolver is the one provisioned by the network Frequently, network-provisioned resolvers are forwarders running on a local router.  The discovered encrypted resolvers in these cases may either be local forwarders themselves, or an associated resolver that is in the network (thus bypassing the router's DNS forwarder). </ins> 3.1.1. benefits of a local caching forwarder with knowledge of the local topology. <ins> Another benefit occurs with IoT devices.  A common usage pattern for such devices is for it to "call home" to a service that resides on the public Internet, where that service is referenced through a domain name (A or AAAA record).  As discussed in Manufacturer Usage Description Specification RFC8520, because these devices tend to require access to very few sites, all other access should be considered suspect.  However, if the query is not accessible for inspection, it becomes quite difficult for the infrastructure to suspect anything. </ins> 3.2.  <ins> Client devices often allow the device administrator to select a specific DNS resolver to use on certain networks, or on all networks. Historically, this selection was specified only with an IP address. Discovering which equivalent encrypted resolvers are offered by the same entity allows the client to "upgrade" connections to use encrypted DNS.  This can provide several benefits: Prevent devices along the network path to the selected resolver from observing client DNS messages Verify that answers come from the selected DNS resolver Authenticate that the DNS resolver is the one selected by the client In doing so it is critical that the new resolver is an equivalent resolver.  Switching to a non-equivalent alternative resolver would break the expectation of the user who previously selected that resolver. </ins> 3.3.  <ins> Virtual Private Networks (VPNs) also can provision DNS resolvers.  In addition to being able to use DHCP or RAs, VPNs can provision DNS information in an explicit configuration message.  For example, IKEv2 can provision DNS servers using Configuration Attributes RFC7296. VPNs can also configure Split DNS rules to limit the use of the configured resolvers to specific domain names RFC8598. Discovering an encrypted resolver that is provisioned by a VPN can provide the same benefits as doing so for a local network, but applied to the private network.  When using Split DNS, it becomes possible to use one encrypted resolver for private domains, and another for other domains. </ins> 4.  <ins> Similar to how VPN DNS configurations can use Split DNS for private names, other network environments can support resolution of names that are specific to the local environment.  For example, an enterprise-managed Wi-Fi network might be able to access both the Internet and a private intranet.  In such a scenario, the private domains managed by the enterprise might only be resolvable using a specific DNS resolver. Discovering an encrypted resolver for a subset of names allows a client to perform Split DNS while maintaining the benefits of encrypted DNS.  For example, a client could use a client-selected encrypted resolver for public domains, but use a different encrypted resolver for enterprise-private domains. Such domain-specific resolver discovery mechanisms additionally need to provide some information about the applicability and capabilities of encrypted resolvers.  This information can either be provisioned or can be discovered based on clients actively trying to access content. </ins> 4.1. 4.1.1.  <ins> Accessing locally-hosted content can require the use of a specific resolver.  For example, captive networks or networks with walled- garden content like media on airplane Wi-Fi networks can rely on using a resolver hosted on the local network. In cases where a client is using an encrypted resolver provisioned by a network, and that encrypted resolver is able to resolve names of local content, this can fall into the use case described in local- network.  However, it might be necessary to discover a local encrypted resolver along with specific domains if: the network-provisioned encrypted resolver is not able to resolve local-only names, or the client has a more-preferred encrypted resolver for generic traffic, and would otherwise not be able to access local content The first point can occur in a hybrid deployment, e.g. when the local resolver is unencrypted but a central one is encrypted.  Clients choosing the encrypted resolver for most queries will need to be advised to refer to the local one for some names. This case also include accessing content specific to a home network. </ins> 4.1.2. 4.1.3.  <ins> As stated above, an enterprise-managed Wi-Fi network might be able to access both the Internet and a private intranet.  The private domains managed by the enterprise might only be resolvable using a specific DNS resolver, hence use of that resolver is essential for such domains.  However it does not necessarily mean that all queries for all domains have to flow through that resolver. Only sending the necessary queries through the enterprise resolver, and not generic Internet queries, has the privacy benefit of only exposing traffic to the enterprise that fall within a limited set of domains. Using encrypted DNS for private names also opens up the possibility of doing private name resolution outside of the content of a VPN or managed network.  If the DNS resolver authenticates clients, it can offer its resolver for private names on a publicly accessible server, while still limiting the visibility of the DNS traffic. </ins> 4.2.  <ins> Content Delivery Networks (CDNs), and content-providers more broadly, can also provide encrypted DNS resolvers that can be used by clients over the public Internet.  These resolvers can either allow resolution of all public names (like normal recursive resolvers), or be designed to serve a subset of names managed by the content provider (like an authoritative resolver).  Using these resolvers can allow the content provider to directly control how DNS answers are used for load balancing and address selection, which could improve performance of connections to the content provider. Using a content-provider's encrypted resolver can also provide several privacy and security benefits: Prevent devices along the network path to the content-provider's resolver from observing client DNS messages Verify that answers come from the entity that manages the domains being resolved Reduce the number of entities able to monitor the specific names accessed by a client to only the client and the content provider, assuming that the content provider would already see the names upon a secure connection later being made based on the DNS answers (e.g., in the TLS SNI extension) </ins> 5. <ins> Encrypted (and authenticated) DNS improves the privacy and security of DNS queries and answers in the presence of malicious attackers. Such attackers are assumed to interfere with or otherwise impede DNS traffic and corresponding discovery mechanisms.  They may be on-path or off-path between the client and entities with which the client communicates RFC3552.  These attackers can inject, tamper, or otherwise interfere with traffic as needed.  Given these capabilities, an attacker may have a variety of goals, including, though not limited to: Monitor and profile clients by observing unencrypted DNS traffic Modify unencrypted DNS traffic to filter or augment the user experience Block encrypted DNS Clients cannot assume that their network does not have such an attacker unless given some means of authenticating or otherwise trusting the communication with their DNS resolver. Given this type of attacker, resolver discovery mechanisms must be designed carefully to not worsen a client's security or privacy posture.  In particular, attackers must not be able to: Redirect secure DNS traffic to themselves when they would not otherwise handle DNS traffic. Override or interfere with the resolver preferences of a user or administrator. Cause clients to use a discovered resolver which has no authenticated delegation from a client-known entity. Influence automatic discovery mechanisms such that a client uses one or more resolvers that are not otherwise involved with providing service to the client, such as: a network provider, a VPN server, a content provider being accessed, or a server that the client has manually configured. Beyond these requirements, standards describing resolver discovery mechanisms must not place any requirements on clients to select particular resolvers over others. When discovering DNS resolvers on a local network, clients have no mechanism to distinguish between cases where an active attacker with the above capabilities is interfering with discovery, and situations wherein the network has no encrypted resolver.  Absent such a mechanism, an attacker can always succeed in these goals.  Therefore, in such circumstances, viable solutions for local DNS resolver discovery should consider weaker attackers, such as those with only passive eavesdropping capabilities.  It is unknown whether such relaxations represent a realistic attacker in practice.  Thus, local discovery solutions designed around this threat model may have limited value. 5.1. Opportunistic encrypted DNS, when the client cannot authenticate the entity that provides encrypted DNS, does not meet the requirements laid out here for resolver discovery.  While opportunistic encryption can provide some benefits, specifically in reducing the ability for other entities to observe traffic, it is not a viable solution against an on-path attacker. Performing opportunistic encrypted DNS does not require specific discovery mechanisms.  Section 4.1 of RFC7858 already describes how to use DNS-over-TLS opportunistically. 5.2. Even with encrypted DNS resolver discovery in place, clients must be prepared to handle certain scenarios where encrypted DNS cannot be used.  In these scenarios, clients must consider if it is appropriate to fail open by sending the DNS queries without encryption, fail closed by not doing so, or presenting a choice to a user or administrator.  The exact behavior is a local client policy decision. Some networks that use Captive Portals will not allow any Internet connectivity until a client has interacted with the portal I-D.ietf- capport-architecture.  If these networks do not use encrypted DNS for their own resolution, a client will need to perform unencrypted DNS queries in order to get out of captivity.  Many operating systems have specific client code responsible for detecting and interacting with Captive Portals; these system components may be good candidates for failing open, since they do not generally represent user traffic. Other networks may not allow any use of encrypted DNS, or any use of encrypted DNS to resolvers other than a network-provisioned resolver. Clients should not silently fail open in these cases, but if these networks are trusted by or administered by the user, the user may want to specifically follow the network's DNS policy instead of what the client would do on an unknown or untrusted network. 6. </ins> This sections lists the complete set of requirements described above, for ease of reference.  <ins> 7. </ins> All security considerations relevant to a particular use case are  <ins> described under that section.  Additional considerations common to all of them are described in priv-sec. </ins>  <ins> 8. </ins> This document has no IANA actions.