DNS Operation Working Group                                      D.Senie
INTERNET-DRAFT                                    Amaranth Networks Inc.
Expires: September 12, 2008                                  A. Sullivan
Intended Status: BCP                                 Command Prompt Inc.
                                                          March 12, 2008

           Considerations for the use of DNS Reverse Mapping

Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
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   aware will be disclosed, in accordance with Section 6 of BCP 79.

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   This Internet-Draft will expire on September 12, 2008.

   Discussion of this Internet Draft is being pursued on the
   dnsop@ietf.org mail list.  The editors solicit comments.


   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, suggests that site
   administrators implement reverse mappings if there are no strong
   considerations against such mappings, and provides considerations to
   be taken into account when using reverse mappings.

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1. Introduction

   1.1  Overview

   The Domain Name System allows for providing mapping of IP addresses
   to host names.  The feature allows administrators to provide both
   name to address, and address to name mappings for networks.  This
   practice is documented, but without guidelines for those who control
   address blocks.  This document provides some such guidelines,
   suggests that site administrators implement reverse mappings in the
   absence of strong counter-considerations, and also offers other
   guidance for the use of the reverse-mapping capability.

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.

   Starting from a given IPv4 address (possibly the result of a query
   for an A RR), the term "existing reverse data" means that a query for
   <reversed-ip4-address>.in-addr.arpa. type PTR results in a response
   other than Name Error.

   Starting from a given IPv6 address (possibly the result of a query
   for an AAAA RR), the term "existing reverse data" means that a query
   for <reversed-ip6-address>.ip6.arpa. type PTR results in a response
   other than Name Error.

   The term "matching reverse data" means that the query for existing
   reverse data results in a response containing 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 the query for existing
   reverse data results in a response of Name Error.

   So, for example, a query for

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   that resulted in a response of Name Error 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


   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 either
   stopped maintaining matching reverse mappings of their addresses, or
   else stopped implementing reverse mappings altogether.

   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] 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

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2. Background

   In the early days of the Domain Name System [RFC883] a special domain
   was 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].

   [RFC1912] suggests that it is an operational or configuration error
   not to have matching PTR and A records.

   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.  Some RIRs
   have policies that actively encourage reverse mapping.  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.

2.1 Historical origins of reverse mapping use in security

   The growth of the Internet in the late 1980s and early 1990s brought
   with it attackers who acquired access to machines without
   authorization.  Many systems attached to the Internet up to that time
   were poorly prepared for such attacks, and administrators were forced
   to react using available resources rather than to redesign the
   network to meet the new security challenges.

   The popular TCP Wrapper package was originally conceived to discover
   the network location of an attacker [Venema1992].  It used the
   reverse mapping of a connecting host to provide the hostname of that
   host in its output.

   During the same period, the so-called "UNIX r* commands", like rlogin
   [RFC1282], were widely used, in spite of warnings that they were
   prone to abuse [Reid1987]. The r* commands allowed users to employ a
   list of trusted hosts, from which connections would be accepted and
   authenticated without password (sometimes called the "rhosts
   authentication" mechanism). The mechanism remained in widespread use
   (in spite of known flaws) because of its convenience. Since the list
   of trusted hosts was a simple list of hostnames or addresses, an
   attacker could acquire access either by by putting the target host

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   name in the PTR record in the IN-ADDR zone for the attacking IP
   address; or, by intercepting the DNS query for a hostname, and
   replying with the IP address from which the attacker was making the
   rhosts authentication attempt. Different implementations of the r*
   commands authenticated differently, but none of them actually checked
   for matching reverse data; the exact method of attack depended on the
   version of the r* commands being attacked and the configuration in
   use. (This weakness was not the only one in the mechanism, but it is
   the most relevant to reverse mapping.)

   In an effort to strengthen the rhosts authentication mechanism, the
   TCP Wrapper package soon offered the ability to perform reverse
   mapping matching checks.  If the reverse and forward mappings did not
   match, the wrapper program would terminate the connection before
   checking any of its other permissions.  This mechanism could be used
   for all connections, on the grounds that forward and reverse
   mismatches were an indication either that an attack was in progress;
   or else that the network was badly managed, and therefore a likely
   origin for attack.

   Other protocols than the r* commands implemented rhosts-style
   authentication mechanisms.  In many but not all cases, this was
   implemented by employing features from the TCP Wrapper package.

3. Issues surrounding reverse mapping

   This section 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 outlines some
   issues, but 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 data.  The utility of each of
   these methods is discussed in section 3.2, below.  Irrespective of
   whether they are useful, their failure in each case produces
   additional load on systems and additional latency in network

   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

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   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
   placement purposes, and may generate false alarms in the event the
   reverse resolution is not possible.

   The popular TCP Wrapper 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.

   Some anti-spam 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 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 some very large
   ISPs are reported 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

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   debugging problems spanning large areas.  When this information is
   missing, the traceroutes can take longer, and those performing
   troubleshooting are left without useful hints.

3.2 Utility and effectiveness of some reverse mapping uses

   Especially in the absence of strong anti-spoofing mechanisms, like
   the DNS Security Extensions, a check for matching reverse DNS mapping
   should be regarded as an extremely weak form of authentication.  Even
   moderately skilled attackers have available to them tools to spoof
   DNS responses.  Because of the dearth of experience with the DNS
   Security Extensions, it is currently unknown whether they add any
   additional security to what will always be fundamentally a weak form
   of authentication.  The use of the DNS Security extensions also does
   nothing to indicate the intentions behind the attempted connection.
   In any case, there are stronger mechanisms for authentication

   Especially given the widespread deployment of Virtual Private
   Networks [RFC2764] and Network Address Translation [RFC3022], reverse
   mapping is not a reliable indicator of actual geopolitical location.
   In the context of fraud prevention and export restriction, false
   rejection may be an acceptable compromise, but administrators and
   policy makers should be aware of the unreliability of the measure.

   Reports from operators suggest that scoring mail on the basis of
   missing or non-matching reverse mapping remains an imperfect but
   useful measure of the likelihood that a given message is spam,
   particularly in combination with other measures.  It is clear that
   the presence of reverse mapping, and a match between the forward and
   reverse zones, is neither a necessary nor sufficient condition for a
   candidate message to be spam.

   The reliance on reverse mapping for logging may result in undesirable
   delays for users.  To the extent that reverse mappings are not widely
   implemented, it is also likely to produce poor data.  Performing the
   reverse lookup in retrospect may introduce errors, because in the
   period of dynamic assignment of IP addresses, it is possible that the
   reverse data at different times will not be the same.

3.3 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 3.4,
   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 (see above), 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 or otherwise
   exceed the DNS protocol limits.  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.

3.4  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 because the number of
   addresses affected is different.

   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

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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 4.2, below.

4. Recommendations

   4.1 General

   There are two sets of actors in respect of reverse mapping: producers
   of data, who are network operators; and consumers of data, who are
   users of the Internet.  It is desirable that operators of networks
   produce and maintain reverse mappings.  At the same time, consumers
   of reverse mapping should be careful in relying on reverse mappings.
   Reverse mappings can be useful, but only when they are used with the
   appropriate degree of caution about their reliability.

4.2 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, IP
   addresses in use within a range and referenced in a forward mapping
   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.  This principle is also not intended to impose undue burden on
   network operators.  It is nevertheless worth considering that not all
   benefit from an administration practice accrue to the administrator
   of a network.  The consumers of reverse mapping data are often not

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   the operators of the network that provides the reverse mappings.
   Users of reverse mapping data report that it is valuable to them.

   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.

4.3 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
   servers. Further, in cases where address block holders fail to
   properly configure reverse mapping, users of those blocks are

4.4  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 not hard for an 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

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   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 exceptions to the usual rule that reverse mapping entries
   are to be added for each entry in a forward zone on the Internet,
   notwithstanding the apparent advice in [RFC1912] that failing to have
   matching PTR and A records is a configuration or operational error.

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

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", STD13, RFC 1035, November 1987.

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

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

[RFC2317] Eidnes, H., G. de Groot, P. Vixie, "Classless IN-ADDR.ARPA
   delegation", BCP 20, 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

[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", BCP 122, RFC 4632,
   August 2006.

7.2 Informative References

[Reid1987] Reid, B., "Reflections on Some Recent Widespread Computer
   Break-Ins", Communications of the ACM, v. 30 no. 2, February 1987, pp

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

[RFC1282] Kantor, B., "BSD Rlogin," RFC 1282, December 1991.

[RFC1912] Barr, D., "Common DNS Operational and Configuration Errors",
   RFC 1912, February 1996.

[RFC2764] Gleeson, B, A. Lin, J. Heinanen, G. Armitage, A. Malis, "A
   Framework for IP Based Virtual Private Networks", RFC 2764, February

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[RFC3022] Srisuresh, P., K. Egevang, "Traditional IP Network Address
   Translator (Traditional NAT)", RFC 3022, January 2001.

[RFC3152] Bush, R., "Delegation of IP6.ARPA", BCP 49, RFC 3152, August
   2001. (RFC 3152 is obsoleted by RFC 3596, which is not a BCP

[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.

[RFC4322] Richardson, M. and D.H. Redelmeier, "Opportunistic Encryption
   using the Internet Key Exchange (IKE)," RFC 4322, December 2005.

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

[Venema1992] Venema, W., "TCP Wrapper: Network monitoring, access
   control, and booby traps," Proceedings of UNIX Security III
   Symposium, USENIX: Berkeley, 1992, pp 85-92.

8. Acknowledgments

   Thanks to Joe Abley, Dean Anderson, Mark Andrews, Stephane
   Bortzmeyer, Steven Champeon, Kevin Darcy, Kim Davies, John Dickinson,
   Bruce Gingery, Olafur Gudmundsson, Alfred Hoenes, Tatuya Jinmei,
   Shane Kerr, Peter Koch, Ed Lewis, George Michaelson, Gary Miller,
   Russ Mundy, 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

   Phone: +1 978 779 5100

   EMail: dts@senie.com

   Andrew Sullivan
   Command Prompt Inc.
   176 E Jewett Boulevard
   POB 50 PMB 161

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   White Salmon, WA 98672

   Phone: +1 503 667 4564

   EMail: ajs@commandprompt.com

9.  Full Copyright Statement

   Copyright (C) The IETF Trust (2008).

   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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   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at

Senie and Sullivan                                             [Page 14]

Internet-Draft   Considerations for DNS Reverse Mapping       March 2008


      Funding for the RFC Editor function is currently provided by the
      Internet Society.

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