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Versions: 00 01 02 03 04                                                
DNS Operations                                           S. Krishnaswamy
Internet-Draft                                               SPARTA Inc.
Expires: March 11, 2006                                September 7, 2005


                Split-View DNSSEC Operational Practices
           draft-krishnaswamy-dnsop-dnssec-split-view-01.txt

Status of this Memo

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   This Internet-Draft will expire on March 11, 2006.

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
   conflicting values as in split-views may be construed to be mutually
   conflicting goals; but this apparent dichotomy is resolvable in



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   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  . . . . . . . . . . . . . . . . . . . . . .  8
     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  . . . . . . . . . . . . . . . . . . . 11
     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 . . . . . . . . . . . . . . . . . . . . . . . 16
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
     9.1.  Normative References . . . . . . . . . . . . . . . . . . . 17
     9.2.  Informative References . . . . . . . . . . . . . . . . . . 17
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 18
   Intellectual Property and Copyright Statements . . . . . . . . . . 19















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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
   chain-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.

   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.  A much better alternative
   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.



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   They have the drawback that they do not allow for transparency during
   name 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 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 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

   Query channeling is the process of carefully controlling how the
   queries are sent to different name servers so as to avoid cache
   pollution.

   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 external
   answers from a second-level name server.  The authoritative name
   servers do not perform any recursion themselves.  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 external answers



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   resides in the boundary network.

   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



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   can be relaxed while forwarding queries to name servers that are
   simultaneously authoritative for the child as well as the parent
   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 that are approved to send queries to this name
   server must do so, and that too solely for the purpose of resolving
   external answers.  TSIG is the recommended method for controlling
   which recursive forwarders are approved for sending queries to the
   second-level name server.  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.





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3.  Split-View DNSSEC

   Any data in any view that is likely to be spoofed has to be signed.
   The DNSSEC concern for split-view is ensuring that the internal and
   external chains-of-trust validate properly.  This concern is
   addressed by making an appropriate choice of trusted and Secure Entry
   Point (SEP) keys.

   While validating external data is relatively straightforward, there
   are multiple approaches that can be 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
   validating 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.  Often two views of a split zone are administered
   separately, so having different zone signing keys for the different
   views is also desirable.  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 key
   referenced in the DS record is present in the apex key-set of both
   views, the chain-of-trust can be completed.

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

   Although easy to setup, this approach can be 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 configured 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 its 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.

3.5.  Multiple DS Records















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                        -----------> 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
   organization 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 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.  Since data
   hiding is not the objective of the split-view setup this should not
   really be a problem in most cases.  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.
   In some circumstances there may not be sufficient flexibility to
   include all DS records in the parent, especially if the child is a
   part of a different organization.  Lastly, the internal view is still



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   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 address 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


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


    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



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   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


   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










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   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 may
   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.

   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,



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   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

   The configuration suggested in this document tries to minimize cache
   pollution, but misconfigurations are still easily possible.  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
   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.





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9.  References

9.1.  Normative References

   [1]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
        "DNS Security Introduction and Requirements", RFC 4033,
        March 2005.

   [2]  Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
        "Protocol Modifications for the DNS Security  Extensions",
        RFC 4035, March 2005.

9.2.  Informative References

   [3]  Larson, M. and P. Barber, "Observed DNS Resolution Misbehavior",
        Work in Progress ietf-dnsop-bad-dns-res, July 2005.



































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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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