INTERNET-DRAFT                                                  D. Senie
Category: BCP                                     Amaranth Networks Inc.
Expires in six months                                         March 2003

                     Requiring DNS IN-ADDR Mapping

Status of this Memo

   This draft, is intended to be become a Best Current Practice RFC.
   Distribution of this document is unlimited. Comments should be sent
   to the Domain Name Server Operations working group mailing list
   <> or to the author.

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of [RFC2026].

   Internet-Drafts are working documents of the Internet Engineering
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   To view the list Internet-Draft Shadow Directories, see

Copyright Notice

   Copyright (C) The Internet Society (2000-2002). All Rights Reserved.


   Mapping of addresses to names has been a feature of DNS. Many sites,
   implement it, many others don't. Some applications attempt to use it
   as a part of a security strategy. The goal of this document is to
   encourage proper deployment of address to name mappings, and provide
   guidance for their use.

1. Introduction

   The Domain Name Service has provision for providing mapping of IP
   addresses to host names. It is common practice to ensure both name to
   address, and address to name mappings are provided for networks. This
   practice, while documented, has never been documented as a
   requirement placed upon those who control address blocks. This

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   document fills this gap.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2. Discussion

   From the early days of the Domain Name Service [RFC 883] a special
   domain has been set aside for resolving mappings of IP addresses to
   domain names.  This was refined in [RFC1035], describing the .IN-
   ADDR.ARPA in use today.

   The assignment of blocks of IP Address space was delegated to three
   regional registries. Guidelines for the registries are specified in
   [RFC2050], which requires regional registries to maintain IN-ADDR
   records on the large blocks of space issued to ISPs and others.

   ARIN's policy requires ISPs to maintain IN-ADDR for /16 or larger
   allocations. For smaller allocations, ARIN can provide IN-ADDR for
   /24 and shorter prefixes. [ARIN].  APNIC provides methods for ISPs to
   update IN-ADDR, however the present version of its policy document
   for IPv4 [APNIC] dropped the IN-ADDR requirements that were in draft
   copies of this document. As of this writing, it appears APNIC has no
   actual policy on IN-ADDR.  RIPE appears to have the strongest policy
   in this area [ripe-185] indicating Local Internet Registries are
   required to perform IN-ADDR services, and delegate those as
   appropriate when address blocks are delegated.

   As we can see, the regional registries have their own policies for
   requirements for IN-ADDR maintenance. It should be noted, however,
   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.

   Registries allocate address blocks on CIDR [RFC1519] 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. Providers SHOULD follow these
   guidelines and ensure their clients set up zone files to answer the

3. Effects of missing IN-ADDR

   Many applications use DNS lookups for security checks. To ensure
   validity of claimed names, some applications will look up IN-ADDR
   records to get names, and then look up the resultant name to see if

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   it maps back to the address originally known. Failure to resolve
   matching names is seen as a potential security concern.

   Some popular FTP sites will flat-out reject users, even for anonymous
   FTP, if the IN-ADDR lookup fails or if the result of the IN-ADDR
   lookup when itself resolved, does not match. Some Telnet servers also
   implement this check.

   Web sites are in some cases using IN-ADDR checks to verify whether
   the client is located within a certain geopolitical entity. This is
   being employed for downloads of crypto software, for example, where
   export of that software is prohibited to some locales.  Credit card
   anti-fraud systems also use these methods for geographic placement

   The popular TCP Wrappers program found on most Unix and Linux systems
   has options to enforce IN-ADDR checks and to reject any client that
   does not resolve.

   Wider-scale implementation of IN-ADDR on dialup, CDPD and other such
   client-oriented portions of the Internet would result in lower
   latency for queries (due to lack of negative caching), and lower name
   server load and DNS traffic.

   Some anti-spam (anti junk email) systems use IN-ADDR to verify return
   addresses before accepting email.

   Many web servers look up the IN-ADDR of visitors to be used in log
   analysis.  This adds to the server load, but in the case of IN-ADDR
   unavailability, it can lead to delayed responses for users.

   Traceroutes with descriptive IN-ADDR naming proves useful when
   debugging problems spanning large areas. When this information is
   missing, the traceroutes take longer, and it takes additional steps
   to determine that network is the cause of problems.

4. Requirements

   Applications SHOULD NOT rely on IN-ADDR for proper operation. The use
   of IN-ADDR, 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 IN-ADDR, users of those blocks are penalized.

   All IP address space assigned and in use SHOULD be resolved by IN-
   ADDR records. All PTR records MUST use canonical names.

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   Internet providers and other users to whom a block of addresses are
   delegated SHOULD provide for lookup of host names from IP addresses.
   This may be provided directly or by delegation to the user of the
   address block. The ISP is responsible for one or the other. In the
   event of delegation, the user is responsible for resolution.

   Only IP addresses not presently in use within a block, or that are
   not valid for use (zeros or ones broadcast addresses) are permitted
   to have no mapping.  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 a
   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 must therefore have an IN-ADDR mapping.

   Regional Registries and any Local Registries to whom they delegate
   SHOULD establish and convey a policy to those to whom they delegate
   blocks that IN-ADDR mappings are required. Internet providers and end
   users with address blocks must verify their own internal networks are
   properly represented in IN-ADDR records, either by providing that
   service themselves, or delegating it to others.

   Those to whom blocks have been delegated SHOULD convey a policy to
   delegatees requiring that they too provide IN-ADDR records and
   require and delegations below to do the same. ISPs may wish to
   provide IN-ADDR records for their clients if the customers are unable
   to provide this for themselves.

5. Security Considerations

   This document has no negative impact on security. While it could be
   argued that lack of PTR record capabilities provides a degree of
   anonymity, this is really not valid. Trace routes, whois lookups and
   other sources will still provide methods for discovering identity.

   By recommending applications avoid using IN-ADDR 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 authentication.

6. References

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

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

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   [RFC1519] V. Fuller, et. al., "Classless Inter-Domain Routing (CIDR):
   an Address Assignment and Aggregation Strategy," RFC 1519, September

   [RFC2026] S. Bradner, "The Internet Standards Process -- Revision 3",
   RFC 2026, BCP 9, October 1996.

   [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
   Requirement Levels", RFC 2119, BCP 14, March 1997.

   [RFC2050] K. Hubbard, et. al., "Internet Registry IP Allocation
   Guidelines", RFC2050, BCP 12, Novebmer 1996.

   [RFC2317] H. Eidnes, et. al., "Classless IN-ADDR.ARPA delegation,"
   RFC 2317, March 1998.

   [ARIN] "ISP Guidelines for Requesting Initial IP Address Space," date

   [APNIC] "Policies For IPv4 Address Space Management in the Asia
   Pacific Region," APNIC-086, 13 January 2003.

   [RIPE185] "European Internet Registry Policies and Procedures,"
   ripe-185, October 26, 1998.

7. Acknowledgements

   Thanks to Peter Koch and Gary Miller for their input, and to many
   people who encouraged me to write this document.

8. Author's Address

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

   Phone: (978) 779-6813


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