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Versions: 00 01 02 03 04                                                
DNS Operations                                           S. Krishnaswamy
Internet-Draft                                               SPARTA Inc.
Expires: August 17, 2005                               February 13, 2005


                Split-View DNSSEC Operational Practices
           draft-krishnaswamy-dnsop-dnssec-split-view-00.txt

Status of this Memo

   This document is an Internet-Draft and is subject to all provisions
   of Section 3 of RFC 3667.  By submitting this Internet-Draft, each
   author represents that any applicable patent or other IPR claims of
   which he or she is aware have been or will be disclosed, and any of
   which he or she become aware will be disclosed, in accordance with
   RFC 3668.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on August 17, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   The security extensions to the Domain Name System (DNSSEC) allow for
   integrity protection, whereby it is possible to make a determination
   of the verity of data returned from the Domain Name System in
   response to a query.  Current operation of the Domain Name System
   also allows for the creation of multiple views of data, where the
   answer returned in response to a query is dependent on the origin of
   the query.  Data integrity and the ability to return possibly



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   conflicting values as in split-views may be construed to be mutually
   conflicting goals; but this apparent dichotomy is resolvable in
   practice through proper configuration.  This document provides
   recommendations for correctly configuring the split-view DNSSEC
   environment in a typical enterprise network.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Split-view DNS . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.1   Background . . . . . . . . . . . . . . . . . . . . . . . .  4
     2.2   Query Channeling . . . . . . . . . . . . . . . . . . . . .  5
     2.3   Controlling Errant Queries . . . . . . . . . . . . . . . .  7
     2.4   Name Server Requirements . . . . . . . . . . . . . . . . .  7
       2.4.1   Internal Recursive Forwarder . . . . . . . . . . . . .  7
       2.4.2   Second-level Recursive Name Server . . . . . . . . . .  7
       2.4.3   Authoritative Internal and External-view Name
               Servers  . . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Split-view DNSSEC  . . . . . . . . . . . . . . . . . . . . . .  7
     3.1   No internal validation . . . . . . . . . . . . . . . . . .  8
     3.2   Same Key Signing . . . . . . . . . . . . . . . . . . . . .  9
     3.3   Partial Decoupling of chains-of-trust  . . . . . . . . . . 10
     3.4   Complete Decoupling of chains-of-trust . . . . . . . . . . 11
     3.5   Multiple DS Records  . . . . . . . . . . . . . . . . . . . 12
     3.6   Name Server Requirements . . . . . . . . . . . . . . . . . 13
   4.  Packet Filtering Considerations  . . . . . . . . . . . . . . . 13
     4.1   Inner packet filter  . . . . . . . . . . . . . . . . . . . 13
     4.2   Outer packet filter  . . . . . . . . . . . . . . . . . . . 14
   5.  Summary  . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     9.1   Normative References . . . . . . . . . . . . . . . . . . . 17
     9.2   Informative References . . . . . . . . . . . . . . . . . . 17
       Author's Address . . . . . . . . . . . . . . . . . . . . . . . 17
   A.  Tracing the query flow . . . . . . . . . . . . . . . . . . . . 18
     A.1   No validation  . . . . . . . . . . . . . . . . . . . . . . 19
     A.2   Same key signing . . . . . . . . . . . . . . . . . . . . . 19
     A.3   Partial Decoupling of chains . . . . . . . . . . . . . . . 20
     A.4   Complete Decoupling of chains  . . . . . . . . . . . . . . 20
     A.5   Multiple DS records  . . . . . . . . . . . . . . . . . . . 21
       Intellectual Property and Copyright Statements . . . . . . . . 22








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1.  Introduction

   Split-view DNS is the term used to describe multiple views of DNS
   information for a domain based on where and by whom the query is
   sent.  Split-views help contain DNS names to only those portions of
   the network that need to see these names.  Although primarily meant
   to be a network management technique, the tailoring of the DNS to
   create an internal view of information hidden from the outside is
   also seen by some as improving their organization's security posture,
   by preventing the exposure of internal host names, knowledge of whose
   existence is deemed to be sensitive.

   Relying solely on split-view DNS to protect sensitive hosts from
   attacks has proven to be less than adequate in the past.  Attack
   vectors in recent Internet exploits have been able to successfully
   infect hosts with or without their IP addresses being published in
   the DNS.  Conversely, publishing the IP addresses of hosts that are
   otherwise secured does not necessarily increase their vulnerability
   to these attacks.  Name hiding through split-view DNS is primarily
   useful as part of a more comprehensive defense-in-depth strategy to
   provide one line of defense against name-based attacks.

   The security extensions to DNS [1] provide for origin authenticity
   and data integrity.  These properties are determined by validating
   the chain-of-trust from the signed record to some trusted key
   configured at the end resolver.  In the case of split-view DNS every
   chains-of-trust in every view must validate properly.  Some names may
   be common between multiple views but contain different data.  Cache
   pollution is a possibility when data from the wrong view is returned
   in response to a query.  Building a chain-of-trust from a trusted key
   above the zone that has split views, to data in the internal view of
   a zone can be especially problematic, caching problems
   notwithstanding.

   The objective of this document is to describe approaches for
   configuring split-view DNSSEC environments with the additional
   requirement that no server be both authoritative and recursive at the
   same time.  Separation of authoritative and recursive name servers
   not only provides simple role separation, but is also an important
   security measure in DNS for protecting authoritative name servers
   against compromised caches.

   In cases where the different views of DNS information correspond to
   different physical networks, the name servers authoritative for the
   internal and external views of data are often separated by a
   firewall.  Among some of the frequently observed DNS resolution
   misbehaviour [3] is the problem of resolvers aggressively
   retransmitting queries from behind misconfigured firewalls that allow



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   queries out, but drop all returned responses.  This problem is
   exacerbated by a handful of errant queries that are sent by only a
   subset of internal resolvers, which makes problem isolation extremely
   difficult.  This document provides recommendations for reducing the
   impact of errant queries in the split-view DNS setup and also makes
   recommedations for DNS-related packet filtering rules required to
   support the proper operation of the suggested configuration.

   Section 2 describes the general approach for configuring split-view
   DNS, which by itself, is independent of DNSSEC.  Considerations for
   DNSSEC appear in Section 3 .

2.  Split-view DNS

2.1  Background

   Different views of the DNS can be created by a process of "query
   channeling".  Here, different servers are made authoritative for the
   different views of the DNS information and queries are channeled to
   these name servers based upon their origination address.

   It is also possible to use a single machine as the authoritative name
   server for both views of data by running multiple instances of the
   name server process on a machine with multiple network interfaces,
   and answering differently based on the query source.  Some name
   server implementations also directly support split-view DNS.
   Variants include the view-based approach and the data tagging
   approach.  In the former, the name server loads multiple zone
   databases and makes available answers from a particular zone based on
   the origin of the query.  The second approach tags the data in the
   database itself as either being internally, externally or globally
   available.

   The single name server approaches are susceptible to leakage of DNS
   information if the host on which they operate is compromised.
   Confidentiality of the namespace is directly tied to how resilient
   the name server is against such attacks.  An alternate way to protect
   a namespace of sensitive hosts is to have that entire namespace
   reside within a private delegation.  By doing so, it is possible to
   have the protection given to the name server that serves these names
   commensurate with the protection given to the hosts themselves.
   Since hosts in the private branch are explicitly marked as such by
   virtue of their domain name, this method also allows the network
   administrator to better classify hosts as being public or private and
   lessens the opportunity for sensitive hosts to be inadvertantly
   placed in public domains.  Private delegations are useful when name
   hiding is the only reason for namespace separation.  They have the
   drawback that they do not allow for transparency during name



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   resolution; queries have to be made for specific names in specific
   views.

   This document describes a generic configuration for split-view DNS
   using multiple nameservers without relying on any special capablities
   from any machine or name server implementation.  The architecture is
   modeled around a typical enterprise structure: the two views are for
   the internal and external portions of the network, with the external
   portion actually residing within the boundary network.  The two
   networks are separated by a packet filtering firewall.  A packet
   filtering firewall also separates the boundary network from the
   external Internet.  Name hiding is not an objective of this
   split-view setup, but avoiding cache pollution is.  Although the two
   concepts are related, this configuration is not recommended for
   hiding sensitive names because of the ease with which names can be
   leaked out due to trivial configuration errors.  Again, if name
   hiding is the main objective for providing split-views, the approach
   of using a private delegation for sensitive names is strongly
   encouraged.

   The suggested configuration uses a combination of multiple name
   servers and query forwarding.  One name server answers queries for
   the internal view and forwards all requests for external data to a
   second name server.  The second name server recursively answers
   queries but only if asked by the first.  Other name servers are
   configured in such a way so as to decouple the roles of the
   authoritative and recursive name servers.

2.2  Query Channeling

   The main DNS concern for split-views is that of preventing cache
   pollution.  Cache pollution can be avoided by carefully controlling
   how the queries are sent to different name servers.

   Resolving outer data is straightforward since queries follow their
   normative paths.  For the internal view, a two level recursive server
   scheme is recommended.  One server functions as a recursive forwarder
   and is responsible for answering all internal queries.  This server
   forwards all queries for internal data to their respective
   authoritative name servers while recursively obtaining answers from
   the outside from a second-level name server.  The second-level name
   server is a simple caching name server that asks questions from the
   outside, but only if asked by the recursive forwarder.  The recursive
   forwarder and the name servers authoritative for the internal data
   reside in the internal network; the second-level recursive name
   server that is used for returning answers from the outside resides in
   the boundary network.




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   The two-level recursive scheme controls where queries are directed
   to.  Since queries for internal data are sent to authoritative name
   servers which are not also recursive, this scheme also controls where
   data is received from.  In this way internal data is kept totally
   separate from external data, thus preventing cache pollution.  Figure
   1 illustrates the above setup.

       ^                    root, TLD servers etc
       |                           ^ (queries)       EXTERNAL NAMESERVERS
    [Outside]                      |                    ^ (responses)
       |                           |                    |
       v                           |                    |
   ------------[O u t e r----P a c k e t---F i l t e r]-------------------
       ^                           |                    |
       |                           |                    v (queries)
       |                           |            AUTHORITATIVE EXT-VIEW SERV
       |                           v (responses)
     [Boundary]            RECURSIVE NAMESERVER
       Net                       ^ (queries from the
       |               CLIENTS   | recursive forwarder
       |       (responses ^      | protected by TSIG)
       |     for internal |      |
       v            data) |      |
   ------------[I n n e r----P a c k e t---F i l t e r]-------------------
       ^                  |      |
       |         (queries)|      |
       |                  v      v (external data)
       |            RECURSIVE FORWARDER <---------> INTERNAL
   [Internal]            ^ (internal data)   (query/   CLIENT
       |                 |                  response)
       |                 |
       |                 v
       |         AUTHORITATIVE INT-VIEW SERV
       v

                                Figure 1

   It is useful to note that the internal recursive forwarder must not
   attempt to recursively answer queries if the authoritative name
   server for internal-view data fails to respond.  If it did so,
   external data could be returned in such circumstances and lead to
   cache pollution.  Since neither the server authoritative for a
   forwarded zone nor the server doing the forwarding can recursively
   answer queries for delegations from that zone, the internal recursive
   forwarder must explicitly forward queries for every internal
   delegation to its respective authoritative name server.  This rule
   can be relaxed while forwarding queries to name servers that are
   simultaneously authoritative for the child as well as the parent



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   zone.

2.3  Controlling Errant Queries

   DNS queries that are sent from the internal recursive forwarder to
   the outside should only be directed towards the second-level
   recursive name server.  Since the second-level name server has no
   knowledge of internal-view data, internal resolvers must not use it
   directly for resolving queries.  Only properly configured internal
   recursive forwarders must be approved to send queries to this name
   server and solely for the purpose of resolving external answers.  It
   is conceivable that there are multiple approved name servers sending
   queries to the second-level name server and TSIG is the recommeded
   method for controlling which name servers can send these queries.
   Having these rules alternatively configured in the packet filter is
   also possible, but using TSIG for performing this authorization eases
   packet filter administration for DNS.

2.4  Name Server Requirements

   This section summarizes the list of requirements for the various name
   servers involved in the split-view configuration.

2.4.1  Internal Recursive Forwarder

   o  Ability to forward queries to specific name servers.
   o  Ability to control forwarding behaviour such that the recursive
      option is not tried, even if the name server that queries are
      normally forwarded to fails to respond.
   o  Ability to recursively answer queries.
   o  Ability to protect the integrity of messages using TSIG for
      selected destinations.

2.4.2  Second-level Recursive Name Server

   o  Ability to recursively answer queries.
   o  Ability to verify TSIG protection on messages.
   o  Ability to filter incoming queries based on the TSIG key used to
      protect the message.

2.4.3  Authoritative Internal and External-view Name Servers

   o  Ability to authoritatively answer queries for a zone.
   o  Ability to disable all recursive behaviour.

3.  Split-view DNSSEC

   The DNSSEC concern for split-view is ensuring that the internal and



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   external chains-of-trust validate properly.  This concern is
   addressed by making an appropriate choice of trusted and SEP keys.
   Some DNSSEC configurations may also make the split-view setup more
   resilient against cache pollution.

   DNSSEC forces one to think about the threat environment for DNS data
   in split-views.  Any view that is likely to be spoofed has to be
   signed.  Often two views of a split zone are administered separately,
   so having different zone signing keys for the different views is also
   desirable.  While validating external data is relatively
   straightforward, there are multiple approaches that can used for
   validating internal data.  The method of choice depends on what the
   threat environment for the internal view is perceived to be, the
   amount of end-resolver configuration overhead that is needed, the
   ease of debugging and the ability to have administrative separation
   between the two split-views.  The configuration overhead at end
   resolvers is mainly associated with the task of defining trust
   anchors at different recursive resolvers.  Having fewer keys is
   desirable in that it makes key management easier.  It is also
   desirable to reduce the amount of reconfiguration required for
   clients that  move between the two views of data, while still being
   able to tie an answer to a particular view.  The different options
   for internal data validation are further outlined below.

3.1  No internal validation

          (No trusted key)           parent zone (trusted)
                                          ^
                                          |
                                          |
             split zone               split zone
              (internal)               (external)
                                          ^
                                          |
                                          |
             sub split zone          sub split zone
              (internal)               (external)

                                Figure 2

   This is an option if the security requirements for the internal zone
   are more relaxed than the external zone.  The threat environment for
   the internal zone in this scenario does not include DNS compromise
   and validation results returned from the internal recursive forwarder
   is not important.  The internal recursive forwarder does not have any
   trusted key configured and does not perform any validation.





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3.2  Same Key Signing

                        -----------> parent zone (trusted)
                       /       ^          ^
                      /        |          |
                     /       (same key)-->|
                    /                     |
             split zone               split zone
              (internal)               (external)
                  ^                       ^
                  |                       |
                  |                       |
             sub split zone          sub split zone
              (internal)               (external)

                                Figure 3

   In this scenario, a single private key is used to sign both the
   internal and the external zone data.  The glue DS and NS records at
   the delegation point of the split zone all correspond to the external
   view data.  Validation proceeds by constructing two separate segments
   of the chain-of-trust.  In the first segment, data at the level of
   the split and below is validated by constructing a chain-of-trust
   contained entirely within the internal view.  If the trusted key is
   configured at or below the level of the split, validation stops at
   this point and queries are never sent to the outer view.  If not, a
   second validation chain segment is constructed from the DS record
   covering the split to the trusted key.  In forming the second
   validation segment all queries (including the query for the DS record
   of the split zone) are sent to the outer zone.  Since the same
   private key is used to sign both views of data, the DS record, even
   though pointing to the key in the outer view of the split zone
   applies to the key in the internal view also, thus completing the
   chain-of-trust.

   This approach allows flexibility in choosing the level at which the
   trusted key is configured, with the possibility of the same trusted
   key being used for validating answers on both views.

   Although easy to setup, this approach is difficult to troubleshoot.
   There is no easy way to identify if the record obtained for a query
   corresponds to the internal view or the external view.  Using the
   same key also makes administrative separation of the two views of
   data difficult.







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3.3  Partial Decoupling of chains-of-trust

                                     parent zone


   (trusted) split zone               split zone (trusted)
              (internal)               (external)
                  ^                       ^
                  |                       |
                  |                       |
             sub split zone          sub split zone
              (internal)               (external)

                                Figure 4

   With the DS record in the parent always pointing to a key in the
   outer view, the construction of the chain-of-trust becomes
   problematic when the keys used to sign data in the two views of the
   split are different.  The trusted key cannot be defined above the
   level of the split since there would be no way of linking the DS
   record in the outer zone to the apex DNSKEY set in the internal view
   of the split zone.

   A simple solution is to configure the trusted key at the level of the
   split such that the chains-of-trust for the internal and external
   zones share no common records that might cause any ambiguity.

   Having separate keys for the two views of data is useful for
   troubleshooting and in determining which view a given record belongs
   to.  Cache pollution can be detected because such cases would lead to
   validation failures.

   This configuration however involves more configuration overhead since
   trusted keys need to be configured for every zone that is split.
   This problem is more pronounced when dealing with validating stub
   resolvers on mobile nodes, where moving between the internal and
   external views would involve constant reconfiguration of all of these
   trusted keys.













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3.4  Complete Decoupling of chains-of-trust

   (trusted) parent zone             parent zone (trusted)
              (internal)              (external)
                  ^                       ^
                  |                       |
                  |                       |
             split zone               split zone
              (internal)              (external)
                  ^                       ^
                  |                       |
                  |                       |
             sub split zone          sub split zone
              (internal)              (external)

                                Figure 5

   One problem with Section 3.3 is that trusted keys need to be
   configured for every zone that is split under the parent.  An option
   to circumvent this while still retaining the advantages of the
   earlier setup is to split the parent also, and configure the trusted
   key at the level of the parent.  An internal name server is
   configured as the authoritative server for the internal view of this
   split and the internal recursive forwarder is modified to forward all
   internal queries for the parent zone to it.

   Although this option reduces the number of trusted keys at the end
   resolver, the trusted key still needs to change when moving between
   the two views.  Since splitting the parent essentially creates two
   new zones, records in the parent that were previously common in both
   views would now need to be duplicated in the two split zones.  The
   number of such records is typically not very large, but the overhead
   and complexity in maintaining duplicate records can still be a
   burden.

















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3.5  Multiple DS Records

                        -----------> parent zone (trusted)
                       /    (DS1)         ^
                      /                   |
                     /                    | (DS2)
                    /                     |
             split zone               split zone
              (internal)               (external)
                  ^                       ^
                  |                       |
                  |                       |
             sub split zone          sub split zone
              (internal)               (external)

                                Figure 6

   In this approach, the parent is not split; however, DS records
   corresponding to each of the two different views are published at the
   delegation point.  The glue and NS records at the delegation point
   still corresponds to the external zone but this information is never
   used by the internal zone.  As in option 4, the trusted key is
   configured at the level of the parent zone.  The chain-of-trust from
   this trusted key to either zone is formed by using one of the two DS
   records, which ever is applicable at that view.

   Since some coordination between the split zone and the parent is
   required to publish multiple DS records, this approach is most
   suitable when the split is made at a level lower than the zone apex
   (e.g.  for example.com, the split is made at a level lower than
   example.com).  This approach lends itself to using different keys in
   different views while still allowing for minimal configuration at the
   end resolvers; trusted keys need not be changed even if the nodes are
   mobile across the two views.  This approach has the advantage that
   administrative separation of the two views of the split can be
   maintained while still having a single key configured at the end
   resolvers.  Identifying which view a given record belongs to can be
   done by tracing back the keys used to form the chain-of-trust.

   While most of the internal zone contents can be kept private to the
   internal view, the DS record must still be exposed.  This should not
   be a problem since data hiding is not be the objective of the
   split-view setup as was mentioned before.  An attendent problem with
   multiple DS records is that since the validation algorithm
   iteratively looks for a DS record in the parent while completing the
   chain-of-trust there is some added computational overhead which
   increases as the number of DS records in the delegation point grows.
   It may also be difficult for the parent to include all DS records



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   that the child sends especially when the two entities are located in
   different organizations.  Lastly, the internal view is still
   susceptible to an insider "attack", where data from the outside view
   injected in response to internal queries can corrupt the cache.  This
   attack is common to all scenarios that use a common key for
   validating internal and external zone contents.  Any cache pollution
   introduced due to administrator errors can also escape detection for
   the same reason.

3.6  Name Server Requirements

   All name servers listed below must conform to the specifications
   given in [2].  Additionally, the Internal Recursive Forwarder must
   support the following:

   o  Ability to validate DNSSEC responses.
   o  Support for configurable DNSSEC trusted keys.  It should be
      possible to configure more than one trusted key.

4.  Packet Filtering Considerations

   The following subsections define the rules that must be configured in
   the two packet filters depicted in Figure 1 in order to support the
   split-view configuration.

4.1  Inner packet filter

   In order to allow the above configuration to work, any packet
   filtering system between the internal network and the boundary
   network must allow all of the following types of packets.

   DNS queries from any internal server to the second-level recursive
   name server (Finer level access control is done by TSIG):


    Proto  SrcIP       SrcPort     DestIP          DstPort    AckBit
    UDP    internal    >1023       rec.srv           53         N/A
    UDP    internal    53          rec.srv           53         N/A
    TCP    internal    >1023       rec.srv           53         Any


   Other queries originating from internal servers but not destined to
   the second-level recursive name server must be denied.








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   Responses to the above queries from the second-level recursive name
   server to any internal server:


    Proto  SrcIP          SrcPort     DestIP          DstPort    AckBit
    UDP    rec.srv        53          internal        >1023      N/A
    UDP    rec.srv        53          internal        53         N/A
    TCP    rec.srv        53          internal        >1023      Set


   Queries from clients in the boundary network to any internal name
   server:


    Proto  SrcIP       SrcPort     DestIP          DstPort    AckBit
    UDP    client      >1023       internal        53         N/A
    TCP    client      >1023       internal        53         Any


   Responses to the above queries from the (any) recursive forwarder to
   clients in the boundary network:


    Proto  SrcIP       SrcPort     DestIP          DstPort    AckBit
    UDP    internal    53          client          >1023      N/A
    TCP    internal    53          client          >1023      Set



4.2  Outer packet filter

   Any packet filtering system configured between the boundary network
   and the external network needs to allow the following.

   Queries from the recursive name server in the boundary network to the
   outside network:


    Proto  SrcIP           SrcPort     DestIP          DstPort    AckBit
    UDP    rec.srv         >1023       outside        53         N/A
    UDP    rec.srv         53          outside        53         N/A
    TCP    rec.srv         >1023       outside        53         Any









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   Responses to the above queries from the outside to the boundary
   network recursive name server:


    Proto  SrcIP      SrcPort     DestIP          DstPort    AckBit
    UDP    outside    53          rec.srv         >1023      N/A
    UDP    outside    53          rec.srv         53         N/A
    TCP    outside    53          rec.srv         >1023      Set


   Queries from outside clients to the external-view authoritative
   servers:


    Proto  SrcIP      SrcPort    DestIP                DstPort    AckBit
    UDP    outside    >1023      auth.serv(ext view)   53         N/A
    UDP    outside    53         auth.serv(ext view)   53         N/A
    TCP    outside    >1023      auth.serv(ext view)   53         Any


   Responses to the above queries from the external-view authoritative
   server to the outside:


    Proto  SrcIP                  SrcPort     DestIP          DstPort    AckBit
    UDP    auth.serv(ext view)    53          outside        >1023      N/A
    UDP    auth.serv(ext view)    53          outside        53         N/A
    TCP    auth.serv(ext view)    53          outside        >1023      Set


   Note that in this configuration, queries from all recursive name
   servers in the boundary network for any external view information
   would need to transit outward through the second-level packet filter
   and then back again into the boundary network.  If the existing
   packet filter policy prevents such traffic patterns, all such
   recursive name servers would need additional forwarding statements to
   forward these queries directly to their respective authoritative name
   servers without going through the packet filter.

5.  Summary

   This document describes an approach for configuring split-view
   DNSSEC.  The approach uses a two level recursive scheme where an
   internal recursive forwarder resolves inside answers and marshalls
   all outside queries to a second-level recursive name server.  TSIG
   between the internal and the second-level name servers protects
   against errant queries.




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   The recommended configuration has been shown to be adjustable for
   various needs and security consideration levels.  Differences in
   these approaches make trade-offs between configuration overhead and
   validation overhead.  Trading-off in favour of minimal operator
   overhead is useful for overall maintainability of the system,
   especially when split-view DNS is considered in the context of nodes
   that are mobile across the two views.

   Although split-view DNSSEC is possible using the recommended setup,
   it still involves significant effort: for configuring the various
   name servers, for setting up zone forwarding, for configuring and
   distributing shared keys for TSIG, and, depending on the
   configuration, for performing DS (or keyset) exchanges for every view
   of a split zone.  Some configurations may also require multiple
   trusted keys in end resolvers which may change between views.  Proper
   care must be taken to ensure that correct split-view behavior is
   consistently maintained.

6.  IANA Considerations

   This document has no actions for IANA.

7.  Security Considerations

   Since the second-level name server has no knowledge of internal view
   data, internal resolvers must not use it directly for resolving
   queries.  Only properly configured internal recursive forwarders must
   be approved to send queries to this name server and solely for the
   purpose of resolving external answers.  DNS queries that are sent
   from the internal view name servers to the outside should only be
   destined to the second-level name server.  The internal recursive
   forwarder must additionally not attempt to recursively answer queries
   if the authoritative name server for internal-view data fails to
   respond.  If it did so, external data could be returned in such
   circumstances and lead to cache pollution.

   The above examples are a quick reminder for the different ways in
   which it is possible to incorrectly configure the split-view DNS
   setup.  Any misconfigurations in the three different types of name
   servers or the two packet filters can either result in cache
   pollution and cause incorrect results to be returned, or impede the
   ability for end resolvers to validate data returned in response to
   queries.  An improperly configured packet filter that allows errant
   DNS traffic through or denies legitimate responses can lead to
   aggressive retransmission of queries.

   Each of the validation options outlined in Section 3 also introduce
   their own security considerations.  Using a common key between both



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   views of the split does not allow one to differentiate between
   internal and external data and troubleshooting is greatly encumbered.
   All approaches that use a common key for validating internal and
   external data are also susceptible to an insider attack where data
   from the outside view injected in response to internal queries can
   corrupt the cache.  On the other hand, using a multitude of keys at
   end resolvers only increases the operator overhead and thus the
   chances for configuration errors.

8.  Acknowledgements

   The contributions, suggestions and remarks of the following persons
   to this draft are particularly acknowledged: Wesley Griffin, John
   Kelley, Russ Mundy and Sam Weiler.  The two-level name server scheme
   described in this document builds upon work that was originally
   performed by Ed Lewis.

9.  References

9.1  Normative References

   [1]  Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose,
        "DNS Security Introduction and Requirements", Internet
        Draft ietf-dnsext-dnssec-intro, October 2004.

   [2]  Arends, R., Austein, R., Larson, M., Massey, D. and S. Rose,
        "Protocol Modifications for the DNS Security  Extensions",
        Internet Draft ietf-dnsext-dnssec-proto, October 2004.

9.2  Informative References

   [3]  Larson, M. and P. Barber, "Observed DNS Resolution Misbehavior",
        Internet Draft ietf-dnsop-bad-dns-res, July 2004.


Author's Address

   Suresh Krishnaswamy
   SPARTA Inc.
   7075 Samuel Morse Dr.
   Columbia, MD  21046
   US

   Email: suresh@tislabs.com
   URI:   http://www.sparta.com






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Appendix A.  Tracing the query flow

       ^                     root, com.
       |                     name servers
       |                          ^
    [Outside]                     |
       |                          +<--  --  --  -- -- -+
       v                          |                    |
   ------------[O u t e r----P a c k e t---F i l t e r--------------------
       ^                          |                    |
       |                          |                    v
       |                          |                   10.0.0.1
       |                          v                  example.com
   [10.0.0.0/8]            RECURSIVE NAMESERVER   sub.example.com(ext)
       |                       (10.0.0.2)
       |     CLIENT (10.0.0.3)     ^
       |                  ^        |
       v                  |        |
   ------------[I n n e r----P a c k e t---F i l t e r--------------------
       ^                  |        |
       |                  v        v
       |               INT RECURSIVE FORWARDER <---------> 192.168.2.3
       |                     (192.168.2.2)
   [192.168.0.0/16]                ^
       |                           |
       |                           v
       |                      192.168.2.1
       v                  sub.example.com(int)

                               Figure 15

   Consider a fictitious domain "example.com" containing a delegation
   "sub.example.com", which is split into an internal (int) and an
   external view (ext).  The name servers authoritative for the internal
   view data are located in the 192.168.0.0/16 network while the name
   servers authoritative for the external view is located in the
   boundary network (10.0.0.0/8).  The name server at 10.0.0.1 is
   authoritative for both the example.com zone and the external view of
   sub.example.com.  10.0.0.3 is a DNS client configured to directly
   query the recursive forwarder at 192.168.2.1 for internal-view data.

   [Note: while the above example lists example.com as residing with an
   RFC-1918 private-address space, in real-world situations, the
   boundary network would normally reside in a publically routable
   address space.]

   The internal recursive forwarder 192.168.2.2 is configured to forward
   all queries for sub.example.com to the name server at 192.168.2.1



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   (which is authoritative for the inner view of this zone) and forwards
   all other queries to the recursive server at 10.0.0.2.  Communication
   between 192.168.2.2 and 10.0.0.2 is protected using TSIG.  10.0.0.2
   is configured to only answer those queries that are protected using
   the TSIG key that it shares with 192.168.2.2.

   The packet filter between the internal network and boundary network
   only allows DNS queries from the internal network destined to
   10.0.0.2, queries from 10.0.0.3 to the internal name servers and
   their corresponding responses, and denies all other DNS traffic.  The
   packet filter between the external network and the boundary network
   allows all outbound queries from 10.0.0.2, all inbound queries to
   10.0.0.1 and their respective responses while disallowing all other
   DNS traffic.  There are no rules that affect forward and reverse
   traffic that result from queries that are sent by clients in the
   boundary network to authoritative external view name servers.

   Unless stated otherwise, the "example." zone is assumed to contain
   the following delegation and glue:

   sub.example.com         NS serv.sub.example.com
   serv.sub.example.com    A  10.0.0.1

   The following subsections trace the query and response flow that
   occurs in order to validate the query "a.sub.example.com/A"
   originating at a client on 192.168.2.3 and directed at 192.168.2.2,
   for each of the different approaches outlined in Section 3.

A.1  No validation

   192.168.2.2 forwards the query to the authoritative internal-view
   name server (192.168.2.1), which returns an authoritative answer.  No
   trusted key is configured at 192.168.2.2 so no validation of this
   response is done.  The answer is returned to the client at
   192.168.2.3 without any validation.

A.2  Same key signing

   The example.com/DNSKEY record is configured as a trusted key at
   192.168.2.2 and forms one end of the chain-of-trust.  The delegations
   sub.example.com(ext) and sub.example.com(int) are signed using the
   same key; the DS record in example.com points to this key.

   The answer for a.sub.example.com/A is returned from 192.168.2.1 as
   described in the previous section.  192.168.2.2  is able to build the
   chain-of-trust for a.sub.example.com/A from the following additional
   data  -- sub.example.com/DNSKEY, sub.example.com/DS and
   example.com/DNSKEY.



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   The query for sub.example.com/DNSKEY is forwarded to the
   authoritative server at 192.168.2.1.  Since sub.example.com/DS and
   example.com/DNSKEY are both external-view data, queries for them are
   forwarded by 192.168.2.2 to the second-level recursive server at
   10.0.0.2.

   192.168.2.2 determines the final answer based on all received
   responses and returns this to the client at 192.168.2.3.

A.3  Partial Decoupling of chains

   The sub.example.com/DNSKEY (internal) record is configured as a
   trusted key at 192.168.2.2 and forms one end of the chain-of-trust.

   The answer for a.sub.example.com/A is returned from 192.168.2.1 as
   described in the previous section.  In order to complete the
   chain-of-trust for a.sub.example.com/A, 192.168.2.2 only needs to
   fetch the data for sub.example.com/DNSKEY.

   It does this according to its forwarding rules by sending a query
   directly to the authoritative server for this data at 192.168.2.1,
   before finally returning the answer to the client at 192.168.2.3.

A.4  Complete Decoupling of chains

   In this option, example.com is also split into an internal and
   external view.

   [Note: For this sub-section, the name server at 192.168.2.1 is
   assumed to be authoritative for example.com(int) in addition to
   sub.example.com(int).

   The delegation point for sub.example.com at example.com(int) is also
   assumed to contain glue that points to the name server at
   192.168.2.1.

   sub.example.com         NS serv.sub.example.com
   serv.sub.example.com    A  192.168.2.1

   The delegation point for sub.example.com at example.com(ext) contains
   glue that points to the name server at 10.0.0.1 and does not change.]

   The configuration at 192.168.2.2 is modified to include a rule to
   forward queries for all data under example.com to 192.168.2.1, which
   is the name server authoritative for the internal view of this zone.
   The trusted key for both views of data is configured at the level of
   example.com but the exact key is different for the internal and the
   external view.  The internal example.com/DNSKEY record is configured



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   as the trusted key at 192.168.2.2 and forms one end of the
   chain-of-trust.

   The answer for a.sub.example.com/A is returned from 192.168.2.1 as
   before.  In order to complete the chain-of-trust for
   a.sub.example.com/A, 192.168.2.2 validates the following data in
   succession -- sub.example.com/DNSKEY, sub.example.com/DS and
   example.com/DNSKEY.  All of these queries are sent according to the
   forwarding rules in 192.168.2.2 to 192.168.2.1, which is the
   authoritative server for all of them.

   192.168.2.2 determines the final answer based on all received
   responses and sends this to the client at 192.168.2.3.

A.5  Multiple DS records

   sub.example.com(int) and sub.example.com(ext) are signed using
   different keys.  The corresponding DS records are both present in
   example.com, which is not split.  The trusted key is at the level of
   example.com and is the same for both views.  This is configured as
   the trusted key at 192.168.2.2 and forms one end of the
   chain-of-trust.

   The answer to a.sub.example.com/A is returned from 192.168.2.1 as
   before.  In order to complete the chain-of-trust for
   a.sub.example.com/A, 192.168.2.2 validates the following data in
   succession -- sub.example.com/DNSKEY, sub.example.com/DS and
   example.com/DNSKEY.

   The query for sub.example.com/DNSKEY is forwarded to the
   authoritative server at 192.168.2.1 while the the queries for
   sub.example.com/DS and example.com/DNSKEY are sent to the external
   recursive server at 10.0.0.2.

   In constructing the chain-of-trust, the recursive server at
   192.168.2.2 iteratively checks each DS record returned from the
   sub.example.com/DS and uses the one that successfully completes the
   chain for sub.example.com/DNSKEY (internal).  192.168.2.2 determines
   the final answer based on all reveived responses and sends this to
   the client at 192.168.2.3.











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