INTERNET-DRAFT D. Senie
Category: BCP Amaranth Networks Inc.
Expires in six months July 2005
Encouraging the use of DNS IN-ADDR Mapping
draft-ietf-dnsop-inaddr-required-07.txt
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Abstract
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.
Copyright Notice
Copyright (C) The Internet Society. (2005)
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 required, though it is
generally encouraged. This document both encourages the presence of
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these mappings and discourages reliance on such mappings for security
checks.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
2. Discussion
From the early days of the Domain Name Service [RFC883] 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. For the in the IPv6 address space, .IP6.ARPA
was added [RFC3152]. This document uses IPv4 CIDR block sizes and
allocation strategy where there are differences and uses IPv4
terminology. Aside from these differences, this document can and
should be applied to both address spaces.
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 [RIPE302] indicating Local Internet Registries should
provide 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
recommendations and/or 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.
3. Examples of impact of missing IN-ADDR
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These are some examples of problems that may be introduced by
reliance on IN-ADDR.
Some 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
it maps back to the address originally known. Failure to resolve
matching names is seen as a potential security concern.
Some 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
approach has been 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 purposes.
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. This 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. This method provdes more comfort but no appreciable
additional security.
Some anti-spam (anti junk email) systems use IN-ADDR to verify the
presence of a PTR record, or validate the PTR value points back to
the same address.
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.
Wider-scale implementation of IN-ADDR on dialup, wireless access 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.
4. Recommendations
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4.1 Delegation Recommendations
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 recommended. Policies should
recommend those receiving delegations to provide IN-ADDR service
and/or delegate to downstream customers.
Network operators should define and implement policies and procedures
which delegate IN-ADDR to their clients who wish to run their own IN-
ADDR DNS services, and provide IN-ADDR services for those who do not
have the resources to do it themselves. Delegation mechanisms should
permit the downstream customer to implement and comply with IETF
recommendations application of IN-ADDR to CIDR [RFC2317].
All IP address space assigned and in use should be resolved by IN-
ADDR records. All PTR records must use canonical names.
All IP addresses in use within a block should have an IN-ADDR
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.
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 may only be done with knowledge of the internal subnet
architecture of the site. It can be assumed, however, any host that
originates an IP packet necessarily will have a valid host address,
and must therefore have an IN-ADDR mapping.
4.2 Application Recommendations
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.
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.
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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. IANA Considerations
There are no IANA considerations for this document.
7. References
7.1 Normative References
[RFC883] P.V. Mockapetris, "Domain names: Implementation
specification," RFC883, November 1983.
[RFC1035] P.V. Mockapetris, "Domain Names: Implementation
Specification," RFC 1035, November 1987.
[RFC1519] V. Fuller, et. al., "Classless Inter-Domain Routing (CIDR):
an Address Assignment and Aggregation Strategy," RFC 1519, September
1993.
[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.
[RFC3152] R. Bush, "Delegation of IP6.ARPA," RFC 3152, BCP 49, August
2001.
7.2 Informative References
[ARIN] "ISP Guidelines for Requesting Initial IP Address Space," date
unknown, http://www.arin.net/regserv/initial-isp.html
[APNIC] "Policies For IPv4 Address Space Management in the Asia
Pacific Region," APNIC-086, 13 January 2003.
[RIPE302] "Policy for Reverse Address Delegation of IPv4 and IPv6
Address Space in the RIPE NCC Service Region", RIPE-302, April 26,
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2004. http://www.ripe.net//ripe/docs/rev-del.html
8. Acknowledgements
Thanks to Peter Koch and Gary Miller for their input, and to many
people who encouraged me to write this document.
9. Author's Address
Daniel Senie
Amaranth Networks Inc.
324 Still River Road
Bolton, MA 01740
Phone: (978) 779-5100
EMail: dts@senie.com
10. Full Copyright Statement
Copyright (C) The Internet Society (2005).
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Internet Engineering Task Force (IETF), its areas, and its
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working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of
six months and may be updated, replaced, or obsoleted by
other documents at any time. It is inappropriate to use
Internet-Drafts as reference material or to cite them other
than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be
accessed at http://www.ietf.org/shadow.html
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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