Abstract:
A method and an apparatus for analyzing a network configuration against a corporate network policy and determining violation(s) against the corporate network policy. A report indicating the violation(s) can be generated indicating instances of the violation(s). An analysis platform reads in a network policy. The analysis platform collects configuration files from the relevant network devices in the network and builds up an internal instance of a network configuration model based on the configuration files and the network topology. The analysis platform analyzes this network configuration model according to the network policy and adds an entry to its final report each time that it detects a violation against the network policy in the network configuration model. The data in the entries pinpoints the cause of the deviation(s) from the network policy.

Description:
RELATED APPLICATIONS 
   This application is related to and claims the benefit of provisional application Ser. No. 60/279,190, filed Mar. 27, 2001, the contents of which are hereby incorporated by reference. 

   FIELD OF THE INVENTION 
   The present invention relates generally to Internet Protocol (IP) network devices, such as firewalls, routers, switches, servers, and more particularly, to a method and apparatus for policy-based analysis of the configurations of the network devices. 
   BACKGROUND OF THE INVENTION 
   A computer network&#39;s basic functionality is determined by the configuration of the network devices present in the network. Network devices include routers, network switches, servers, firewalls, and virtual private networks. 
   A router is a network gateway that joins two or more IP networks and switches packets between the networks. A network administrator can implement a high-level corporate routing policy by configuring the settings of each router in the network, including setting static routes, one or more dynamic routing protocols, suppressing dynamic routing updates on a per-interface basis, and setting routing preferences. 
   A network switch is a multi-port network bridge, which is generally capable of supporting multiple media types. A network bridge forwards datagrams (packets) according to media type and address (e.g., Ethernet). A network administrator can configure a network switch in much the same way as configuring a router. 
   A server is a host (computer) that offers one or more services used by all the other networked computers to simplify the operations of the network, such as DNS (domain name service), mail (electronic mail), and web services. A network administrator can configure the software for the particular service and can also configure the host itself (e.g., access control to the computer via TCP wrapper configuration). 
   A firewall is a network gateway that filters packets and separates a proprietary corporate network, such as an intranet, from a public network, such as the Internet. Most of today&#39;s firewalls are configured by means of a rule-base. A network administrator can implement a high-level corporate security policy by creating a low-level rule-base for each firewall interface in the corporate network. 
   A virtual private network (VPN) is a network device that secures the privacy of corporate data outside the perimeter of the corporate network. A network administrator can configure VPN devices so that corporate data sent over the public Internet (e.g., from the corporate headquarters to a remote company site) is adequately secured. This typically involves configuring settings for cryptographic key exchanges, choosing the appropriate encryption for sending data (e.g., IP packets) according to the destination, etc. 
   A network topology is a formal description (including IP-addresses, device description, etc.) of the network devices interconnecting the sub-networks and hosts in the network. 
   A network policy is a formal description of the intended capabilities and properties of the network hosts in the network. 
   A configuration file contains configuration data for a single network device, such as a router, firewall, or server. 
   A network configuration model is a data model for representing a global configuration of the network, which uses the configuration files as building blocks. A network configuration store is a device for storage of network configuration models. 
   A network administrator, or a group of administrators in a larger enterprise, is typically responsible for configuring all the network devices in a network, in such a way that the network devices can cooperatively enforce a corporate network policy. Any error in the configuration file of a single network device can invalidate the enforcement of the network policy. Furthermore, errors in the configuration files can go undetected for a long time. For example, a router configuration error can cause IP traffic from the Internet, which is destined for a number of hosts (computers) within the corporate network (enterprise), to be lost. Traditional network management software will not generate any alerts. Since all the routers are up and running, the routers will not generate an event to which the management software would react. 
   As apparent from the above-described deficiencies associated with the manual configuration of network devices, a need exists for a method and apparatus for analyzing a configuration file of each network device in a corporate (enterprise) network, matching the results against a corporate network policy, and generating reports for network administrators indicating any violations in the collective network configuration against the corporate network policy. 
   SUMMARY OF THE INVENTION 
   One aspect of the present invention is directed to a method and an apparatus for analyzing a network configuration against a corporate network policy and determining violation(s) against the corporate network policy. A report indicating the violation(s) can be generated indicating instances of the violation(s). An analysis platform (e.g., an Ontura server) reads in a network policy. A Policy Modeling Language (PML), for example, can be used to define an instance of the network policy. The analysis platform collects configuration files from the relevant network devices and builds up an internal instance of a network configuration model based on the configuration files and the network topology. The analysis platform analyzes this network configuration model according to the network policy and adds an entry to its final report each time that it detects a violation against the network policy in the network configuration model. The data in the entries pinpoints the cause of the deviation(s) from the network policy. 
   According to another aspect of the present invention, the network policy describes capabilities for particular hosts in the network, such as “mail server,” “DNS server,” etc. The analysis platform receives the network policy as an input and then analyzes the network configuration model to verify that the IP traffic from and to these hosts are limited according to the type of service, and to ensure that the right type of IP traffic get from/to a host, which includes the configuration of relevant routers for switching traffic, firewalls for passing through or dropping traffic, and local access control mechanisms on the host (e.g., TCP wrappers) for making the services accessible. Thus, the network administrator (and his/her management, e.g., Chief Information Officer (CIO)) can determine that relevant IP traffic, and only relevant IP traffic, is able to reach the hosts. 
   According to yet another aspect of the present invention, the network policy describes routes (e.g., sequences of IP addresses of gateways and routers) that the IP traffic should take between different sites of the same enterprise. The analysis platform receives the network policy as an input and then analyzes the configuration of the relevant routers and network switches to verify that the routes taken by the IP traffic within the enterprise, among the different corporate sites, adhere to the network policy. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete understanding of the present invention and its advantages will be readily apparent from the following Detailed Description of the Preferred Embodiments taken in conjunction with the accompanying drawings. Throughout the accompanying drawings, like parts are designated by like reference numbers and in which: 
       FIG. 1  is a schematic illustration of a computer network in accordance with the present invention; 
       FIG. 2  is a flow diagram illustrating a method for determining violation(s) of a network policy in accordance with the present invention; 
       FIG. 3  is a block diagram illustrating a portion of the operating modules of an analysis platform in accordance with the present invention; 
       FIG. 4  is a block diagram illustrating a module structure of an analysis platform in accordance with the present invention; 
       FIG. 5  is an entity-relationship model representing a network topology in accordance with the present invention; 
       FIG. 6  is an entity-relationship model representing a network policy in accordance with the present invention; 
       FIG. 7  is a block diagram illustrating a more detailed module structure of an analysis platform in accordance with the present invention; 
       FIG. 8  is a flow diagram illustrating the operation of a query generator in accordance with the present invention; 
       FIG. 9  is a block diagram illustrating a portion of a module structure of an embodiment of an analysis platform; and 
       FIG. 10  is a flow diagram illustrating an operation of the analysis platform in analyzing changes to the configuration files of the network devices. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1  schematically illustrates a hardware environment of an embodiment of the present invention. A corporate network  100  is connected to a public network  110  (e.g., the Internet) via a router  120 . The corporate network  100  contains a plurality of sub-networks, including a sub-network dmz  130  and a second sub-network  140 . The sub-network dmz  130  is connected to the router  120  and contains a host  150  (e.g., a hardened mail server) for providing one or more services to the corporate network  100 . The second sub-network  140  contains a plurality of networked computers  160 . A firewall  170  filters packets between the second sub-network  140  and the public network  110  to provide security for the networked computers  160  in the corporate network  100 . 
     FIG. 2  is a flow diagram illustrating the operation of an analysis platform (e.g., an Ontura server) in accordance with the present invention. The process starts at step  200 . In step  210 , the analysis platform receives a network policy, which may be defined by a network administrator. The network policy may, for example, be stored in a network policy store on the analysis platform. 
   Then, in step  215 , the analysis platform receives information regarding a topology of the network devices (e.g., router  120 , server  150 , firewall  170 ) in the corporate network  100 . In step  220 , the analysis platform determines which of the network devices in the corporate network  100  are relevant to the network policy. The determination of relevancy is based on the network topology and/or the network policy. Then, in step  225 , the analysis platform receives configuration data from one of the relevant network devices. The configuration data for a particular network device may be determined by reading the configuration file of the network device. 
   Then, in a decisional step  230 , the analysis platform determines whether there are other relevant network devices remaining. If so (Yes in step  230 ), the process repeats step  225  and the analysis platform receives configuration data from another of the relevant network devices. Otherwise (No in step  230 ), the process continues to step  235 , wherein the analysis platform builds an internal network configuration model. 
   Then, in step  240 , the analysis platform analyzes the network configuration model against the network policy to determine whether the network configuration model violates the network policy (i.e., whether a violation exists). In step  245 , the analysis platform determines whether a violation of the network policy has been detected. If so (Yes in step  245 ), the violation is recorded in step  250  and the process continues to step  255 . Otherwise (No in step  245 ), the process continues to step  255 . 
   In step  255 , the analysis platform determines whether the analysis has been completed. If not (No in step  255 ), the process returns to step  245  and continues to detect for violations of the network policy. If the analysis is complete (Yes in step  255 ), the process continues to step  260  wherein the analysis platform provides a report indicating the violation(s), if any, of the network policy. The report includes specific instance(s) where a device configuration causes a violation in the network policy. The process then ends in step  265 . 
     FIG. 3  is a block diagram illustrating a portion of the operating modules of an embodiment of an analysis platform (e.g., an Ontura server)  300 . The analysis platform  300  includes a Policy Wizard Module  310  for assisting a network administrator with the creation of a network policy for the corporate network. Once the network policy has been created, it can then be stored in a network policy store  315  in the analysis platform  300 . 
   The analysis platform  300  further includes a Network Discovery Wizard Module  320  for collecting data regarding the basic network connectivity (e.g., the network topology). The Network Discovery Wizard Module  320  can guide the network administrator through the process of defining the locations of the configuration files of the network devices in the corporate network that are to be analyzed by the analysis platform  300 . The configuration files are typically basic text (ASCII) files such as, a configuration file  325  for a Cisco router using IOS (Internet Operating System) commands, a configuration file  330  for a Nortel switch, and a configuration file  335  for a Checkpoint firewall. Once the locations of the configuration files have been defined, the analysis platform  300  can retrieve the configuration files from the relevant network devices as required. 
     FIG. 4  is a block diagram illustrating a top-level module structure of an embodiment of an analysis platform (e.g., an Ontura server)  400 . The analysis platform  400  includes a Policy Modeler Module  410  having a Policy Wizard Module  420  and a Policy Modeling Language Parser  430 . The Policy Wizard Module  420  is capable of assisting a network administrator in generating an instance of a Network Policy, which can be expressed as a PML program. The network administrator can also directly write a PML program that defines the network policy for input to the analysis platform  400 . The Policy Modeling Language Parser  430  then transforms the PML program into an internal data model. 
   The Policy Modeling Language is a simple descriptive language, which can capture the intended capabilities of the network hosts. These capabilities define, among other things, the allowed access to a host, the allowed traffic interactions among the hosts, and the required security protection on each of the hosts. 
   The analysis platform  400  further includes a Topology Modeler Module  440 . The Topology Modeler Module  440  can execute a network discovery phase (e.g., by employing the Network Discovery Wizard Module  320 ) for discovering the topology of the corporate network, including the locations of the network devices and the interconnections between the network devices. Once the network discovery phase has been performed, the Topology Modeler Module  440  can transform the newly acquired knowledge of the network topology into an internal data model. The Topology Modeler Module  440  can also prompt the network administrator for the location(s) and/or the access authorization information (e.g., passwords) of the respective configuration file(s) of the newly discovered network devices. 
   The analysis platform  400  also includes a Configuration File Parser Module  450  for reading the configuration files of the relevant network devices. The Configuration File Parser Module  450  is capable of understanding the syntax and semantics of the different configuration files that may be found in the relevant network devices. The Configuration File Parser Module  450  then transforms the contents of each of the configuration files and forms an internal data model, which is independent of the make of the particular network device. For example, there is an internal data model for the configuration of a router, another internal data model for a firewall, VPN, etc. 
   After the Policy Modeler Module  410 , the Topology Modeler Module  440 , and the Configuration File Parser Module  450  have processed all the required information, the internal data models contain both the Network Policy and the actual Network Configuration Model (including the network topology). An Analyzer Module  460  of the analysis platform  400  can now start its work. For each defined capability of each host (in the Network Policy), the Analyzer Module  460  generates one or more queries regarding the Network Configuration Model. Answers to the queries can either confirm or deny that the defined capability has been correctly realized. For example, with respect to a network host having the capability of a DNS server, queries regarding the type of traffic that can reach the server, the type of traffic that can leave the server, and what security and performance settings are on the server may be part of the set of queries. The Analyzer Module  460  then executes the collected set(s) of queries. This involves executing various algorithms on the data structures representing the Network Configuration Model. Finally, the Analyzer Module  460  collects the answers to the queries and issues a report with appropriate entries for each detected violation. 
     FIG. 5  illustrates a portion of an embodiment of an Entity-Relationship (ER) model representing a network topology. In the illustration, a single arrow denotes a one-to-one relationship while a double arrow denotes a one-to-many relationship. An analysis platform (e.g., an Ontura server) uses the ER model to capture and model the topology of the corporate network. The relevant data concerns sub-networks (SubNet) of the network under consideration, and the gateway interfaces (GW-Interface) connecting the SubNet. Each SubNet consists of a plurality of HostGroups. Each HostGroup consists of a plurality of Hosts and has a range of IP addresses (IP_range). Each Host has an IP address (IP_address). The Gateways switch packets between the SubNets and can typically include routers, firewalls, or network switches (e.g., an ATM switch). The ER model also includes pointers to the configuration models of the gateways and the relevant hosts (servers). The ER model includes a vendor-independent configuration model for each type of Gateway (firewall, router, switch), which allows the Analyzer Module  460  to simulate the actions of the gateway when it receives a given type of IP traffic. 
     FIG. 6  illustrates a portion of an embodiment of an Entity-Relationship (ER) model representing a Network Policy. In the illustration, a single arrow denotes a one-to-one relationship while a double arrow denotes a one-to-many relationship. 
   A group of hosts (HostGroup) can have capabilities associated therewith. In the illustrated embodiment, the ER model distinguishes two kinds of capabilities: IP-Capabilities and Host-Capabilities. An IP-Capability describes IP-based traffic, possibly including its source (defined as another capability), security (encryption) requirements, routing requirements, protocol requirements and more. Hosts associated with such IP-Capability are allowed to be the recipient of the described IP-based traffic. Of course, the description of routing and security properties will require further modeling. Optional features include integration with emerging standards for routing and encryption policies, such as the Routing Policy Specification Language (RPSL, a proposed standard for a Routing Policy Format), and encryption policies currently under study by the Internet Engineering Task Force (IETF) Security Policy Working Group. 
   A Host-Capability models a host&#39;s functionality and configuration (e.g., as a server for DNS, Mail, Web or other server functionalities). The ER model provides a specific configuration model for each of the functionalities. The capabilities can be defined and written in the Policy Modeling Language (PML) in the form of a PML program. 
   The ER models form a data repository, which enables the Analyzer Module  460  to verify that the capabilities defined in the policy are indeed realized in the network configuration, and, equally important, that no other capabilities are allowed. The analysis performed by the Analyzer Module  460  includes simulation of relevant network devices and the nature of the interconnections between the network devices to determine how certain IP-based traffic flows through the network under consideration. The analysis also includes simulation of the servers&#39; actual configurations by responding to TCP-based incoming client requests (e.g., DNS, mail). 
   The analysis platform of the present invention uses a scripting language, such as the Policy Modeling Language (PML), to enable the network administrator to expressly define the capabilities of each of the network devices in the PML program. As part of the analysis process, the PML program is translated into the ER model. The capabilities can then be used in the ER models. 
   A capability, such as the Host-Capability, can be pre-defined. In this case, the PML parser recognizes the name of the capability. For example, the PML parser understands that the capability “dns_server” refers to a host being able to receive and send name-server related traffic from just about any source. 
   A capability can also be custom-defined in PML by defining the corresponding services and host groups. For example, consider the capabilities of a “hardened mail server” and “vulnerable mail server”. The “hardened mail server” is typically a host that is easily accessible to a public network (e.g., the Internet). The “vulnerable mail server” is typically a host on a trusted network for delivering mail, for example, to the employees of a corporation. The network administrator defines the “hardened mail server” (in the appropriate syntax of PML) as receiving mail (SMTP, which is TCP on port  25 ) from any machine on the Internet, and defines the “vulnerable mail server” as receiving SMTP only from machines which have been assigned the capability of “hardened mail server”. Thus, the “vulnerable mail server” is not accessible to the public network. 
   PML language constructs also allow for expressing routing policies and encryption policies as the policies relate to defined services and host groups, and to server policies for dns, mail, and other services. A PML program defining the corporate network policy is simply a text (ASCII) file that can be stored on the analysis platform. 
     FIG. 7  is a block diagram illustrating a more detailed module structure of an embodiment of an analysis platform  700 . The analysis platform  700  includes a Policy Wizard Module  710 , a software tool for allowing the network administrator to define capabilities without actually programming in PML, but rather, by filling out forms and templates (e.g., Web-based forms). The Policy Wizard Module  710  then transforms the information obtained from the network administrator into a PML program  715 . The following is a fragment of a PML program, in a possible embodiment of syntax, for defining the capabilities of the two types of mail servers discussed above:
         vuln_mail_server_cap &lt;-&gt; hardened_mail_server_cap: (TCP,  25 )   hardened_mail_server_cap &lt;-&gt; Internet_cap: (TCP,  25 )       
   In another example, the network administrator wants to set a policy such that the internal routers, using either the Routing Information Protocol (RIP) or the Open Shortest Path First (OSPF) routing protocol, can only accept route updates from routers that are trusted by the network administrator. The following is an embodiment of the corresponding PML syntax:
         internal_router_cap &lt;-trusted_router_cap: (RIP_update, OSPF_update)       

   In a further example, the network administrator wants to define a policy component for VPN gateways to encrypt and decrypt all traffic with 128-bit key strength between them. The following is an embodiment of the corresponding PML syntax:
         vpn_gtw_cap &lt;-&gt; vpn_gtw_cap: enc — 128 k       

   Information regarding the basic interconnection of gateways and sub-networks in a network, along with the corresponding IP-address ranges of the gateways and sub-networks, can be obtained using commercially available network discovery tools. For example, HP Openview&#39;s Network Node Manager collects this data and displays it as a network map. 
   The analysis platform  700  further includes a Network Discovery Module  720  for collecting network topology information. In one embodiment, the Network Discovery Module  720  may incorporate one of the commercially available network discovery tools (e.g., HP Openview&#39;s Network Node Manager). The Network Discovery Module  720  employs an Interface Module  730  (e.g., an HP Openview interface if the HP Openview tool is used) for extracting the collected information underlying the displayed network map and for translating the information into a TML program  735 . The TML program  735  uses a scripting language, such as a Topology Modeling Language (TML), to define the topology of the network. The network administrator can then add to this basic TML program  735  by defining additional host groups, with their associated IP ranges or set of IP addresses. The network administrator can further add to the TML program  735  by defining the IP addresses corresponding to network devices to be analyzed (e.g., firewalls, routers, etc). More importantly, the network administrator can attach capabilities, which are defined in the corresponding PML program  715 , to the host groups. Analogous to the PML program  715 , the resulting TML program  735  is a simple text (ASCII) file, which can be stored on the analysis platform  700 . 
   The analysis platform  700  further includes a Network Definition Wizard Module  740 , a software tool for allowing the network administrator to define host groups and network devices in the network, and to attach capabilities to the host groups and network devices without actually programming in TML, but rather, by filling out forms and templates (e.g., Web-based forms). The Network Definition Wizard Module  740  then transforms the obtained information into a TML program. The Network Definition Wizard Module  740  can also allow the network administrator to edit an existing TML program, such as the TML program  735  created by the Interface Module  730 . The following is a fragment of the TML program  735 , in a possible embodiment of syntax, for defining a Cisco IOS router with the name “internal_router_nyc” as having two interfaces (if1, if2), along with the respective IP addresses. A sub-network “dmz” is connected to the rest of the network by two routers, one of which is the router “internal_router_nyc” at its “if1” interface. A host “mail_server” is on the “dmz” sub-network since the IP address of the host falls within the IP address range of the “dmz” sub-network. 
                                                                         GATEWAYS {           internal_router_nyc = {if1: {IP=111.222.1.1,                if2: {IP=111.222.2.1}                MAKE cisco_ios WITH internal_router_cap                }           SUB-NET {           dmz  = [111.222.1.0/24]: {if1, if3} WITH server_cap           }           HOST {           mail_server = [111.222.1.17] WITH hardened_mail_server_cap           }                        
The network administrator can attach the capability “internal_router_cap” to the router “internal_router_nyc.” The network policy and the network topology can thus be joined on the analysis platform  700 . The TML program  735  further defines a sub-network “dmz” connected, at one end, to an external router and a host with the name “mail_server” in the sub-network “dmz.” The host has been assigned the capability “hardened_mail_server_cap.”
 
   Each network device has a configuration file associated therewith. The network administrator typically reads from and writes to a configuration file of a network device by opening, for example, a secure (password protected) telnet session (from his/her desktop) to the network device. The network administrator can manually place all configuration files of the relevant network devices (as defined in the TML program above) in a pre-defined directory on the analysis platform  700  and add the corresponding path and access information to the TML program  735 . Alternatively, the network administrator can add, in the TML program  735 , a remote location and password for each configuration file to allow the analysis platform  700  to access the configuration file of a network device or a host and to collect the configuration files automatically. Following is a fragment of the TML program  735  provided above, which has been extended to include the location of the configuration file of the interface if1 of the router “internal_router_nyc”: 
   
     
       
             
             
           
             
             
           
             
             
           
         
             
                 
                 
             
           
           
             
                 
               GATEWAYS { 
             
             
                 
               internal_router_nyc = 
             
           
        
         
             
                 
               {if1: {IP=111.222.1.1, FILE=“/Ontura/conf_files/rules_if1”} 
             
           
        
         
             
                 
               } 
             
             
                 
                 
             
           
        
       
     
   
   The analysis platform  700  further includes a plurality of software modules for building an internal ER model  745 , using the TML program  735  and the PML program  715  as inputs. The ER model  745  is typically a data structure stored in main memory while the analysis platform  700  is performing its analysis of the network.  FIGS. 5 and 6  illustrate portions of an embodiment of an ER model. From this description, a programmer can easily build actual data structures in high-level programming languages, such as C or Java, using arrays and dynamic pointers (for dynamic storage allocation). 
   A PML Parser Module  750  is built using standard compiler technology to parse each capability definition of the PML program  715  and to create an instance of the ER model  745  for the capability in the main memory of the analysis platform  700 . For example, the software tools “lex” (or “flex”) and “yacc” (or “bison”), which are freely available in the UNIX operating system environment under the GNU license, can be used to implement the PML Parser Module  750 . It is standard practice to program these tools with the syntax of the language under consideration (in this case, PML) and the preferred output structure (in this case, the ER model) to thereby obtain a module (the PML Parser Module  750 ) for transforming the PML program  715  into the ER model  745 . 
   A TML Parser Module  760  is built using standard compiler technology to parse each statement in the TML program  735  and to create an instance of the ER model  745  representing the sub-network, host group, or network device defined by the statement, together with its connectivity. A possible way to implement the TML Parser Module  760  is with the lex and yacc software tools. Each time the TML Parser Module  760  encounters, in a TML statement, a network device which needs to be analyzed, the TML Parser Module  760  calls an appropriate Device Parser Module  770  for the particular network device. The TML Parser Module  760  also passes to the Device Parser Module  770  the device type (e.g., router) and make (e.g., Cisco IOS version x.y). 
   The Device Parser Module  770  is a software module for creating a configuration model for the network device. The Device Parser Module  770  obtains the configuration file of the network device (e.g., from the TML program) and parses the configuration file using standard compiler technology (e.g., tools such as lex and yacc). The Device Parser Module  770  is capable of parsing the syntax of the different types of configuration files associated with the network devices. The configuration model is preferably different for each type of network device (e.g., firewall, router), but is preferably the same for a particular type of network device independent of vendor. That is, the configuration model for a network device, such as a firewall, captures all the salient configuration features of a firewall (e.g., rules to determine if an IP packet is passed or dropped) by abstracting from the vendor specific expression of these rules. 
   The configuration model is part of the ER model  745  and describes the actual configuration of a particular network device. For example, if the network device is a firewall, the configuration model captures the filtering rules, such that the analysis platform  700  can simulate the behavior of the firewall when receiving a given IP packet. The description is general, yet detailed enough to capture the different kinds of firewalls (i.e., the “lowest common denominator”). 
   For example, access control lists (ACLs) in Cisco routers filter IP traffic without keeping any internal state (i.e., are “stateless”). Consequently, a configuration file for allowing an incoming telnet session should specify the incoming TCP initiation packets and the outgoing reply packets. In a firewall with “stateful” inspection (e.g., a Checkpoint firewall), the configuration file only needs to allow incoming telnet traffic because the internal state automatically remembers to pass the outgoing reply packets. 
   The configuration model thus needs to capture the lowest level of configuration granularity (e.g., the level of ACL or lower, in the case of firewalls). Therefore, when parsing a configuration file for a Checkpoint firewall, the Device Parser Module  770  needs to generate, for each “stateful” rule, at least two rules in the configuration model, describing the allowed traffic in each direction. Similarly, for routers, VPNs, gateways, etc., the Device Parser Module generates a configuration model that expresses the device configuration data at the lowest level of abstraction. 
     FIG. 8  is a flow diagram illustrating the operation of a query generator in accordance with the present invention. The process starts at step  800 . In step  810 , the query generator traverses (in a memory of the analysis platform) the data structure representing the ER model for the network under consideration. For each host or host-group, the query generator determines if it has an attached capability (step  820 ). If there is an attached capability, the query generator then determines the type of capability (custom or pre-defined) in step  830 . 
   For each attached capability, the query generator generates the appropriate queries. For IP capabilities, for example, typically traffic flow queries are generated. A traffic flow query asks what IP-based services can move trough the network under consideration, either from or to (or both) the fixed host-groups or hosts (source and destination, respectively), and can also include sub-queries about the routes of the traffic under consideration. 
   If the capability is a custom-defined capability (Custom in step  830 ), such as a simple custom-defined IP capability (no routing or security info is included in the service), the queries can be generated in a rather straightforward way (in step  840 ). One or more traffic flow queries are generated, which have the current host-group as destination and represent all the possible sources. For example, if a host H has the capability “trusted_mail_server_cap” as defined earlier, then the query generator can add the following query: “what IP traffic, from any source, can reach host H as its destination?” 
   If the capability is a pre-defined capability (Pre-defined in step  830 ), the queries can be retrieved from a knowledge base (in step  845 ). For simple pre-defined IP capabilities (e.g., “dns_server”), the query generator accesses the knowledge base to retrieve the required queries. 
   For host capabilities, server behavior queries are generated. The nature of the queries is dependent on the capability itself. For example, a “dns-info” capability may specify whether the server is a “primary server”, “secondary server”, or just a “resolver” and may also specify how the server should initialize its cache. Queries directed at the dns configuration model can be generated to verify the capability. The queries are then stored in step  850 . As one exemplary solution, the queries are stored in main memory, possibly in an array data structure. 
   Then, in step  860 , the query generator determines if the host or host group has another attached capability. If so (Yes in step  860 ), the process returns to step  830  to determine the capability type. Otherwise (No in step  860 ), the process continues to step  870  wherein the query generator determines if there is another host or host group in the network to be analyzed. If there is another host or host group (Yes in step  870 ), the process returns to step  810  wherein the host or host group is examined. Otherwise (No in step  870 ), the process ends at step  880 . 
   The knowledge base contains the expert knowledge of the analysis platform, including security knowledge, network administration knowledge, etc. For example, the knowledge base may include queries that need to be asked to ensure that the security delivered by all the filtering devices present is adequate for the pre-defined capabilities, such as “mail-server”, “dns-server”, etc. For example, the knowledge base understands that “dns-server” is a sensitive capability and that no unauthorized host should be able to telnet to a host with the dns-server capability. The knowledge base contains information for each pre-defined capability. The internal structure of the knowledge base may be simple file-based name-value pairs or a small database. The knowledge base has an interface that allows updates to be made by the staff of the analysis platform as part of upgrades to the analysis platform. Additionally, the knowledge base can be updated by a network administrator who wishes to encode some of his/her expert knowledge into the knowledge base. 
     FIG. 9  is a block diagram illustrating a portion of a module structure of an embodiment of an analysis platform  900 . The analysis platform  900  includes a Query Generator  930  that generates the appropriate queries for analyzing a network based on an ER Model  910  of the network and a Knowledge Base  920 , which contains the security expert knowledge of the analysis platform  900 . 
   The analysis platform  900  further includes a Core Analyzer  940  that traverses the main memory data structure containing all the queries. For each query, the Core Analyzer  940  executes the following: If the query is a traffic flow query, then the Core Analyzer  940  executes a flow analysis algorithm on a graph, derived from the connectivity information in the ER Model  910 . The Core Analyzer  940  starts with the source of the flow and computes all the paths the flow can follow to reach the destination. Each node in this graph is a gateway and each edge is a sub-network connecting the two gateways. The Core Analyzer  940  consults the configuration model for the gateway device (which is part of the overall ER model) and uses the configuration model to simulate the gateway&#39;s behavior (under its current configuration) and thereby obtains the gateway&#39;s actions, such as filtering, encrypting/decrypting or forwarding/routing to another sub-network. The Core Analyzer  940  also consults the configuration of the source and/or destination to understand whether these hosts influence the traffic flow. The Core Analyzer  940  writes the resulting flows to a file, where all the query&#39;s answers are accumulated. For example, the query “what IP traffic, with any source can reach host H as its destination?” from above causes the Core Analyzer  940  to execute a traffic flow analysis that returns all the IP traffic which can reach host H as destination, given the current configuration of all the gateway and hosts in the network under consideration. 
   If the query is a server behavior query (e.g., DNS server configuration), then the Core Analyzer  940  retrieves the corresponding values of the configuration model of the host under consideration. The values are then interpreted to simulate the resulting behavior, which is presented in the answer file. 
   Finally the Core Analyzer  940  scans through the answers generated in response to an analyzed capability to determine whether the collection of answers indicates any unwanted traffic or functionality or, at the other extreme, whether there are any missing traffic or functionality (in other words, a policy violation). If so, the Report Generator  950  is invoked. The Core Analyzer  940  passes the corresponding capability and the answer causing the violation. For traffic flow queries and answers, the Core Analyzer  940  might also pass the gateway, which causes the violation. For example, the answer to the above query regarding host H may reveal that telnet traffic from any host in some corporate sub-network is able to reach host H. This traffic is not part of the traffic specified by the attached capabilities for host H. The Core Analyzer  940  consequently invokes the Report Generator  950  and passes to the Report Generator  950  information regarding host H, the telnet traffic and its sources, and possibly the gateway or host configuration that passes the telnet traffic instead of filtering it out. 
   In another example, a server might have a host capability for DNS, which indicates it should be a primary server. The executed query might reveal that the actual configuration makes the server a secondary DNS server. Again, the Core Analyzer  940  invokes the Report Generator  950 , with the information regarding the host, including the part of the host configuration that causes the server not to fulfill the desired capability. 
   The Report Generator  950  creates a formatted file (Report  960 ) with an entry for each time that it was invoked by the Core Analyzer  940 . The Report Generator  950  can format the file in HTML for easy viewing with a browser or can e-mail the file as ASCII text to the network administrator. An example of an entry, generated for the policy violation concerning the trusted mail server host H (see above) might look as follows. The entry includes specifications about which part of the policy (e.g., which capability) is not being enforced, the host(s) that are affected, how the violation manifests itself, and which device needs to re-configured to remove the violation. The Report Generator  950  obtains all the necessary information from the Core Analyzer  940  each time it is invoked to generate another entry. The entry below (e.g., the 5th in the Report  960 ) indicates that the router “internal_router nyc” needs to filter telnet traffic from the corporate sub-net to the mail server.
         POLICY VIOLATION ENTRY #5:   CAPABILITY: trusted_mail_server_cap   HOST: mail_server [111.222.1.17]   VIOLATION: telnet FROM corp_net   CONFIGURATION: internal_router_nyc       

   Alternatively, graphical representations may also be incorporated as part of the Report  960 . The Report  960  may include a map of the network under consideration, highlighting the hosts that are affected by policy violations in one particular fashion and highlighting network devices whose configurations cause the violations in a different fashion. 
   Up to this point, the analysis platform has been used to define a network policy, to collect all the necessary data (configuration files, network topology, etc.) regarding a network, and to obtain reports indicating violations of the configuration of the network against the network policy. A network administrator can establish the network policy and use the report generated to correct the initial problems with the configuration files. The analysis platform can also be used in another mode of operation. After the initial configuration of the network devices has been performed, the network administrator and his/her team will likely have to make changes to the configuration files of the network devices to accommodate changes within the company, such as new business relationships, new internal corporate structures, etc. 
     FIG. 10  is a flow diagram illustrating an operation of the analysis platform in analyzing changes to the configuration files of the network devices. The process begins at step  1000 . Since the Network Configuration Model already exists on the analysis platform, at step  1010 , the Network Configuration Model is retrieved. The network administrator can edit one or more configuration files with his/her proposed changes and upload the new configuration file(s) to the analysis platform in step  1020 . In step  1030 , the analysis platform updates the Network Configuration Model in response to the new configuration file(s). 
   Then, in step  1040 , the analysis platform analyzes the changed Network Configuration Model against the network policy and, in step  1050 , generates a report indicating the violation(s) against the network policy (if any) caused by the changes to the configuration files. For example, an error in a configuration update for a firewall device can cause all employees at a given corporate site to lose access to the Internet. Without the use of the analysis platform, the administrator would only detect such an error after receiving phone calls from these employees. With the analysis platform, the network administrator can submit configuration changes to the analysis platform to see the effects of the changes, before actually committing these changes to the network device(s), thereby avoiding the above situation. 
   If there are violations of the network policy (Yes in step  1060 ), the process returns to step  1020  to allow the network administrator to correct the configuration file(s) that are in error. If there are no violations, the process can continue to an optional step  1070  wherein the analysis platform functions as a telnet pass-through to allow change(s) to the configuration file(s) to be uploaded to the corresponding network device(s). Instead of telnetting directly into a network device, the network administrator can telnet into the analysis platform and upload the changed configuration file (step  1020 ). The analysis platform then updates the ER model accordingly (step  1030 ), runs the analyzer (step  1040 ), and if there are no new violations against the existing policy (No in step  1060 ), a distributor module on the analysis platform transfers the updated configuration file to the corresponding network device in the network (step  1070 ), and the process ends at step  1080 . 
   Changes in business relationships and corporate structures may necessitate a change in the network policy. For example, a new external business partner might need direct access from its site to some of the company&#39;s sub-networks. In this case, the network administrator needs to change the network policy in addition to changing the configuration files of some devices. Similarly to the previous variation, the network administrator can submit changes to the network policy (either by using a Policy Wizard Module, or by directly changing the PML and/or TML programs stored on the analysis platform) and corresponding changes to configuration files (of the network devices) for the existing Network Configuration Model on the analysis platform. The analysis platform then restarts the analysis, using the changed network policy and configuration files. The analysis platform can then generate a report as before. This report now pinpoints the configurations of those network devices (either freshly submitted or original) that need to be changed in order for the network to adhere to the changed network policy. As discussed in the previous section, the information provided by the report greatly improves the speed and quality of implementing the required changes in the network configuration to support evolving business and corporate functions. 
   Although the present invention has been fully described by way of examples and with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art without departing from the spirit and scope of the invention. Therefore, unless such changes and modifications depart from the scope of the present invention, they should be construed as being included therein.