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Versions: 00 01 02 03 04 05 06                                          
DNS Operation Working Group                                      D.Senie
Internet-Draft                                    Amaranth Networks Inc.
Expires July 2, 2007                                         A. Sullivan
                                                         January 2, 2007

           Considerations for the use of DNS Reverse Mapping

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
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   This Internet-Draft will expire on July 2, 2007.


   Mapping of addresses to names is a feature of DNS.  Many sites
   implement it, many others do not.  Some applications attempt to use
   it as a part of a security strategy.  The goal of this document is to
   outline what should be taken into account when deciding whether to
   implement reverse mappings of addresses to names.

1. Introduction

   1.1  Overview

   The Domain Name System has provision for providing mapping of IP
   addresses to host names.  It is common practice to ensure both name

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   to address, and address to name mappings are provided for networks.
   This practice is documented, but without guidelines for those who
   control address blocks.  This document provides some such guidelines,
   and also offers other guidance for the use of this reverse-mapping

1.2  Terminology

   In the following, the general term "reverse mapping" is used to refer
   to the general capability of mapping IP addresses to host names, and
   "reverse tree" the portions of the DNS that provide the
   functionality.  The term "IN-ADDR" is used to refer to the feature
   only as it applies to IPv4 use, and IN-ADDR.ARPA to the portion of
   the DNS that provides such IPv4-specific functionality.  Similarly,
   "IP6" refers to the feature only as it applies to IPv6 use, and
   "IP6.ARPA" to the portion of the DNS that provides the IPv6-specific
   functionality.  In what follows, except where the text explicitly
   refers only to IN-ADDR or IP6, the document can and should be applied
   to both address spaces.

1.3  Motivation

   In recent years, some sites have come to rely on reverse mapping as
   part of their administrative policies even as other sites have
   stopped maintaining useful reverse mappings of their addresses.

   The widespread practice of "virtual hosting" -- using one machine and
   IP address to host many different domains -- means that reverse
   mappings become sometimes difficult to maintain or awkward to use.
   The large IPv6 address space exacerbates the difficulty of
   administering reverse mapping.  Finally, some administrators regard
   the data in the reverse tree as at best worthless and at worst a
   potential information leak, and so object to maintaining reverse

   At the same time, some sites have attempted to use reverse mappings
   as a part of a security- or abuse-prevention policy.  Moreover, some
   protocols that store data in the DNS, such as those described in
   [RFC4025], [RFC4255], and [RFC4322], could benefit from accurate
   reverse mapping data.

   In light of the above conflicting pressures, this document attempts
   to outline some considerations for the maintenance and use of reverse
   mappings so that users and administrators can make informed

2. Background

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   In the early days of the Domain Name System [RFC883] a special domain
   was been set aside for resolving mappings of IP addresses to domain
   names.  This was refined in [RFC1035], describing the .IN-ADDR.ARPA
   domain in use today.  For the IPv6 address space, .IP6.ARPA was added
   by [RFC3152].

   The assignment of blocks of IP Address space was delegated to
   (originally three) Regional Internet Registries (RIRs). Guidelines
   for the registries are specified in [RFC2050], which strictly
   requires RIRs to maintain reverse mapping records only on the large
   blocks of space issued to ISPs and others.

   Each RIR has its own policy for requirements for reverse-mapping
   maintenance; these policies may change from time to time.  It should
   be noted, also, that many address blocks were allocated before the
   creation of the regional registries, and thus it is unclear whether
   any of the policies of the registries are binding on those who hold
   blocks from that era.

3. Issues surrounding reverse mapping

3.1  Examples of effects of missing reverse mapping

   Following are some examples of some of the uses to which reverse
   mapping checks are put, and some of the difficulties that can be
   encountered because of missing reverse tree records.  It is important
   to note that some of these strategies are at best often ineffective.
   Nevertheless, their failure in each case produces additional load on
   systems and additional latency in network activity.

   Some applications use DNS lookups for security checks.  To ensure
   validity of claimed names, some applications will look up records in
   the reverse tree to get names, and then look up the resultant name to
   see if it maps back to the address originally known.  Failure to
   resolve matching names is interpreted as a potential security

   Some popular FTP sites will simply reject user sessions, even for
   anonymous FTP, if the reverse mapping lookup fails or if the reverse
   mapping, when itself resolved, does not match.  Some Telnet servers
   also implement this check.

   Web sites sometimes use reverse mapping to verify whether the client
   is located within a certain geopolitical entity.  This approach has
   sometimes been employed for downloads of cryptographic software, for
   example, where export of that software is restricted to certain
   locales.  Site operators may choose to refuse to allow the connection

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   in the event they are not able to perform these checks.  Credit card
   anti-fraud systems also sometimes use similar methods for geographic
   placement purposes, and may generate false alarms in the event the
   reverse resolution is not possible.

   The popular TCP Wrappers program found on most Unix and Linux systems
   has options to perform reverse mapping checks and to reject any
   client that does not resolve.  The program also has a way to check to
   see that the name given by a PTR record then resolves back to the
   same IP address.  In the event that the checks fail, connections may
   be terminated.

   Poor or missing implementation of reverse mapping on dialup, CDPD and
   other such client-oriented portions of the Internet results in higher
   latency for queries (due to lack of negative caching), and higher
   name server load and DNS traffic.

   Some anti-spam (anti junk email) systems use the reverse tree to
   verify the presence of a PTR record, or validate the PTR value points
   back to the same address as the system originating the mail.  Some
   mail servers have the ability to perform such checks at the time of
   negotiation, and to reject all mail from hosts that do not have
   matching reverse mappings for their hostnames.  These PTR checks
   sometimes include databases of well-known conventions for "generic
   naming" conventions (for example, PTR records for dynamically-
   assigned hostnames and IP addresses), and sometimes allow complicated
   rules for quarantining or filtering mail from unknown or suspect
   sources.  Even very large ISPs may reserve the right to refuse mail
   from hosts without a reverse mapping.

   Many web servers query for reverse mappings for visitors, to be used
   in log analysis.  This adds to the server load, but in the case of
   reverse mapping unavailability, it can lead to delayed responses for
   users.  Moreover, some statistics packages perform such lookups in
   retrospect, and missing reverse mapping will prevent such packages
   from working as expected.

   Traceroute output with descriptive reverse mapping proves useful when
   debugging problems spanning large areas.  When this information is
   missing, the traceroutes take longer, and it takes additional steps
   to determine what network is the cause of problems.

3.2  The difficulty with blanket policies

   Some users have reported difficulty in ensuring correct reverse tree
   management by their upstream providers.  (This is the user's
   perspective of the "reachover problem" described in section 3.3,

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   below.)  Users without many choices among providers, especially, can
   become the needless victim of aggressive reverse mapping checks.

   Reverse mapping tests may also give the administrator a false sense
   of security.  There is little evidence that a reverse mapping test
   provides much in the way of security, and may make troubleshooting in
   the case of DNS failure more difficult.

   It is possible for there to be multiple PTRs at a single reverse tree
   node.  In extreme cases, these multiple PTRs could cause a DNS
   response to exceed the UDP limit, and fall back to TCP.  Such a case
   could be one where the advantages of reverse mapping are exceeded by
   the disadvantages of the additional burden.  This may be of
   particular significance for "mass virtual hosting" systems, where
   many hostnames are associated with a single IP.

3.3  Differences in IPv4 and IPv6 operations

   RIRs allocate address blocks on CIDR [RFC4632] boundaries.
   Unfortunately, the IN-ADDR zones are based on classful allocations.
   Guidelines [RFC2317] for delegating on non-octet-aligned boundaries
   exist, but are not always implemented.  There is not a similar
   concern for IP6.ARPA.

   RIRs may delegate address space to Local Internet Registries (LIRs),
   who may perform further delegation.  Reverse mapping only works if
   all the intermediate delegations are correctly maintained.  As a
   result, RIRs find they cannot enforce policies requiring reverse
   mappings, because they sometimes do not have any relationship with
   the intermediate party on whom some end-point reverse mapping
   depends.  It may be supposed that IPv6 will make this "reachover
   problem" worse, because of the likelihood of longer delegation chains
   in IPv6.

   The much larger address space of IPv6 makes administration of reverse
   mapping somewhat daunting, in the absence of good tools to help
   administrators.  Some discussion of this issue can be found in
   [RFC4472], particularly section 7.

   The larger address space of IPv6 also makes possible "hiding" active
   hosts within a large address block: the impracticability of scanning
   an entire IPv6 network for running network services means that an
   administrator could effectively conceal running services in an IPv6
   network in a way not possible in an IPv4 network.  Such hiding would
   be prevented by a reverse mapping that revealed only existing hosts.
   If such "hiding" is desirable, it is possible nevertheless to provide
   reverse mapping for (a large segment of) the network in question, and

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   then use only a small number of the so-mapped hosts.  This approach
   is consistent with the suggestion outlined in section 4.1, below.

4. Recommendations

4.1 Delegation considerations

   In general, the DNS response to a reverse map query for an address
   ought to reflect what is supposed to be seen at the address by the
   machine initiating the query.

   It is desirable that Regional Registries and any Local Registries to
   whom they delegate encourage reverse mappings.

   Network operators should define and implement policies and procedures
   which delegate reverse mappings to their clients who wish to run
   their own reverse tree DNS services.  By the same token, network
   operators should provide reverse mapping for those users who do not
   have the resources to do it themselves.

   All IP addresses in use within a block should have a reverse mapping.
   Those addresses not in use, and those that are not valid for use
   (zeros or ones broadcast addresses within a CIDR block) need not have
   mappings, although it may be useful to indicate that a given block is
   unassigned.   This principle is not intended, however, to create new
   reverse mapping considerations for addresses discussed in [RFC3330]
   (and more specifically, the [RFC1918] addresses).  While these
   private use addresses are "assigned", they are assigned in a local
   way; so policy around reverse mappings for these addresses is also a
   local issue.

   It should be noted that due to CIDR, many addresses that appear to be
   otherwise valid host addresses may actually be zeroes or ones
   broadcast addresses.  As such, attempting to audit a site's degree of
   compliance can only be done with knowledge of the internal routing
   structure of the site.  However, any host that originates an IP
   packet necessarily will have a valid host address, and ought
   therefore to have a reverse mapping.

4.2 Application considerations

   Applications should not rely on reverse mapping for proper operation,
   although functions that depend on reverse mapping will obviously not
   work in its absence.  Operators and users are reminded that the use
   of the reverse tree, sometimes in conjunction with a lookup of the
   name resulting from the PTR record, provides no real security, can

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   lead to erroneous results and generally just increases load on DNS
   servers. Further, in cases where address block holders fail to
   properly configure reverse mapping, users of those blocks are

4.3  Usage and deployment considerations

   Site administrators are encouraged to think carefully before adopting
   any test of reverse delegation, particularly when that test is
   intended to improve security.  The use of reverse mapping does not
   usually improve security, and should not be a default policy.  In
   particular, some users continue to report difficulty in ensuring
   correct management of the reverse tree by upstream providers.  This
   situation can be corrected by the provision by those providers of
   reverse mapping; but until the day reverse mapping is universal,
   complete connection rejection on the basis of missing reverse mapping
   should be regarded as a last resort.  At the same time, site
   administrators are cautioned that administrators at other sites
   sometimes use reverse mapping as one of several pieces of evidence in
   evaluating connection traffic, particularly in the context of mail
   systems and anti-spam efforts.

   Administrators are advised to keep in mind the effects of adding a
   very large number of PTR records for a given reverse mapping.  In
   particular, sites where a very large number of "virtual" host names
   resolve to the same host may, if the foregoing advice is followed too
   rigorously, force DNS responses to use TCP.  Such cases should be
   treated as unusual exceptions to the usual rule that reverse mapping
   entries are to be added for hosts on the Internet.

5. Security Considerations

   This document has no negative impact on security. While it may be
   argued that lack of PTR record capabilities provides a degree of
   anonymity, the same goal can be achieved by providing reverse
   mappings that are opaque to remote users, for all the assigned IP
   address space.  To the extent that forward delegations are already
   published in the DNS, the anonymity cannot be realized anyway; and
   delegations not published in the forward zone cannot be distinguished
   if an opacity strategy is adopted.

   By recommending applications avoid using reverse mapping as a
   security mechanism this document points out that this practice,
   despite its use by many applications, is an ineffective form of
   security. Applications should use better mechanisms of

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6. IANA Considerations

     There are no IANA considerations or implications that arise from

7. References

7.1 Normative References

   [RFC883] Mockapetris, P.V., "Domain names: Implementation
   specification," RFC883, November 1983.

   [RFC1035] Mockapetris, P.V., "Domain Names: Implementation
   Specification," RFC 1035, November 1987.

   [RFC1918] Rekhter, Y., B. Moskowitz, D. Karrenberg, G. J. de Groot,
   and E. Lear, "Address Allocation for Private Internets," RFC 1918,
   BCP 5, February 1996.

   [RFC2050] Hubbard, K., M. Kosters, D. Conrad, D. Karrenberg, J.
   Postel, "Internet Registry IP  Allocation Guidelines", RFC2050, BCP
   12, Novebmer 1996.

   [RFC2317] Eidnes, H., G. de Groot, P. Vixie, "Classless IN-ADDR.ARPA
   delegation," RFC 2317, March 1998.

   [RFC3152] Bush, R., "Delegation of IP6.ARPA," RFC 3152, BCP 49,
   August 2001.

   [RFC3330] Internet Assigned Numbers Authority, "Special-Use IPv4
   Addresses," RFC 3330, September 2002.

   [RFC4632] Fuller, V., T. Li, "Classless Inter-Domain Routing (CIDR):
   The Internet Address Assignment and Aggregation Plan," RFC 4632,
   August 2006.

7.2 Informative References

   [RFC4025] Richardson, M., "A Method for Storing IPsec Keying Material
   in DNS," RFC 4025, February 2005.

   [RFC4255] Schlyter, J. and W. Griffin, "Using DNS to Securely Publish
   Secure Shell (SSH) Key Fingerprints," RFC4255, January 2006.

   [RFC4322] Richardson, M. and D.H. Redelmeier, "Opportunistic
   Encryption using the Internet Key Exchange (IKE)," RFC 4322, December

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   [RFC4472] Durand, A., J. Ihren, and P. Savola, "Operational
   Considerations and Issues with IPv6 DNS," RFC 4472, April 2006.

8. Acknowledgements

   Thanks to Joe Abley, Steven Champeon, Kim Davies, Tatuya Jinmei,
   Shane Kerr, Peter Koch, Ed Lewis, George Michaelson, Gary Miller,
   Pekka Savola, and Paul Wouters for their input, and to many people
   who encouraged the writing of this document.

9. Authors' Addresses

   Daniel Senie
   Amaranth Networks Inc.
   324 Still River Road
   Bolton, MA 01740

   Phone: +1 978 779 5100

   EMail: dts@senie.com

   Andrew Sullivan
   204-4141 Yonge Street
   Toronto, ON, CA
   M2P 2A8

   Phone: +1 416 673 4110

   EMail: andrew@ca.afilias.info

9.  Full Copyright Statement

   Copyright (C) The IETF Trust (2007).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an

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