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DNS Operation Working Group                                      D.Senie
Internet-Draft                                    Amaranth Networks Inc.
Expires August 18, 2007                                      A. Sullivan
                                                       February 18, 2007

           Considerations for the use of DNS Reverse Mapping

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   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.  This document outlines what
   should be taken into account when deciding whether to implement
   reverse mappings of addresses to names, and recommends that site
   administrators implement reverse mappings if there are no strong
   considerations against such mappings.

1. Introduction

   1.1  Overview

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   The Domain Name System allows for providing mapping of IP addresses
   to host names.  The feature allows administrators to ensure both name
   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 overall 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.

   The term "existing reverse data" means that a reverse query for Q
   results in a response other than NXDOMAIN.

   The term "matching reverse data" means that a reverse query returns a
   set of one or more names which, when each queried themselves in the
   forward zone for A or AAAA RRs (as appropriate) return one or more
   results, one of which corresponds to the original query.

   The term "missing reverse data" means that a reverse query for Q
   results in a response of NXDOMAIN.

   So, for example, a query for


   that resulted in a response of NXDOMAIN would be a case of missing
   reverse data.  A query for

   that resulted in a response containing a PTR record to
   example1.example.org would be a case of existing reverse data.  If a
   corresponding query for

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   resulted in a response containing an A record, then it
   would be a case of matching reverse data.  If, however, the forward
   query did not result in a response containing an A record,
   then the reverse data could be said to exist, but not to match.

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 matching
   reverse mapping data, particularly when combined with the use of the
   DNS security extensions ([RFC4033],[RFC4034],[RFC4035]).

   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

   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], and its use is codified in [RFC3596].

   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.

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   Each RIR has its own policy for requirements for reverse-mapping
   maintenance; these policies may change from time to time.  Some RIRs
   have policies that actively encourage reverse mapping.  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

   The following discusses some of the ways in which reverse mapping is
   used; the effects for users of reverse mappings when those mappings
   are missing or do not match; and the effects on users when strong
   reverse mapping checks are in place, when users are unable or
   unwilling to implement reverse mappings.  This section merely
   outlines some issues, and should not be interpreted as either
   approval or disapproval of a given practice.

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 these strategies are at best often ineffective, and are
   occasionally considered harmful.  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 find
   matching reverse mappings is interpreted as a potential security

   Some popular FTP sites will simply reject user sessions, even for
   anonymous FTP, if there is a missing reverse mapping or if matching
   reverse mapping does not exist.  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
   in the event they are not able to perform these checks.  Credit card
   anti-fraud systems also sometimes use similar methods for geographic

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   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 with a missing reverse mapping.  The program also has a way to
   check for matching reverse mapping.  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 existing reverse mapping, or to check for matching reverse
   mapping.  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 names (for example, PTR records for dynamically-assigned
   hostnames and IP addresses), and may 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.  Often, the reverse map check is not used
   on its own, but is used as part of a scoring system in an attempt to
   indicate the probability that a given email message is spam.

   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 may take longer, and it may require
   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 reverse tree
   maintenance by their upstream providers.  (This is the user's
   perspective of the "reachover problem" described in section 3.3,
   below.)  Users without many choices among providers, especially, can
   become the needless victim of aggressive reverse mapping checks.

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   Reverse mapping tests can 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 ranges of numbers that may be
   expressed in CIDR [RFC4632] notation. Unfortunately, the IN-ADDR
   zones were originally based on classful allocations. Guidelines
   [RFC2317] for delegating on non-octet-aligned boundaries exist, but
   are not always implemented.  There is a similar issue for IP6.ARPA,
   although in practical terms it is less pressing.

   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 is possible that IPv6 will make this "reachover problem"
   worse, because of the opportunity for longer delegation chains in

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

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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, or continue to encourage, reverse

   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.

   Unless there are strong counter-considerations, such as a high
   probability of forcing large numbers of queries to use TCP, all IP
   addresses in use within a range 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 range 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.  Therefore, policy with respect to 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.  Nevertheless, 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
   lead to erroneous results and generally just increases load on DNS

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   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.  This
   is especially true of reverse checks that try to detect matching
   reverse data.  In the absence of the DNS security extensions
   ([RFC4033],[RFC4034],[RFC4035]) it is possible for a determined
   attacker to falsify the reverse data.

   In the context of anti-spam efforts, administrators are reminded that
   complete rejection of a connection (on the basis of missing or non-
   matching reverse mapping) is extremely controversial.  It may
   interrupt or prevent the transmission of legitimate mail.

   Some users continue to report difficulty in ensuring complete
   population 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.
   It may be that such evaluations will not cause complete connection
   failure, but that the evaluations will cause recipients of messages
   to disregard them as spam.

   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

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   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

6. IANA Considerations

     There are no IANA considerations or implications that arise from

7. References

7.1 Normative References

   [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, November 1996.

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

   [RFC3596] Thompson, S., C. Huitema, V. Ksinant, M. Souissi, "DNS
   Extensions to Support IP Version 6," RFC 3596, October 2003.

   [RFC4033] Arends, R., R. Austein, M. Larson, D. Massey, S. Rose, "DNS
   Security Introduction and Requirements," RFC 4033, March 2005.

   [RFC4034] Arends, R., R. Austein, M. Larson, D. Massey, S. Rose,
   "Resource Records for the DNS Security Extensions," RFC 4034, March

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   [RFC4035] Arends, R., R. Austein, M. Larson, D. Massey, S. Rose,
   "Protocol Modifications for the DNS Security Extensions," RFC 4035,
   March 2005.

   [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

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

   [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.

   [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

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

8. Acknowledgments

   Thanks to Joe Abley, Dean Anderson, Steven Champeon, Kim Davies,
   Tatuya Jinmei, Shane Kerr, Peter Koch, Ed Lewis, George Michaelson,
   Gary Miller, Pekka Savola, and Paul Wouters for their specific 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

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   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

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