DNS Operation Working Group D.Senie
Internet-Draft Amaranth Networks Inc.
Expires August 18, 2007 A. Sullivan
Afilias
February 18, 2007
Considerations for the use of DNS Reverse Mapping
draft-ietf-dnsop-reverse-mapping-considerations-02
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Abstract
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
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.
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
b.a.9.8.7.6.5.0.4.0.0.0.3.0.0.0.2.0.0.0.1.0.0.0.0.0.0.0.1.2.3.4.
IP6.ARPA.
that resulted in a response of NXDOMAIN would be a case of missing
reverse data. A query for
3.2.0.192.IN-ADDR.ARPA.
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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EXAMPLE1.EXAMPLE.ORG
resulted in a response containing an A record 192.0.2.3, 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 192.0.2.3,
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
mappings.
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
decisions.
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
concern.
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
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
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
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.
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
penalized.
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
authentication.
6. IANA Considerations
There are no IANA considerations or implications that arise from
this
document.
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
2005.
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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
2005.
[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
Afilias
204-4141 Yonge Street
Toronto, ON, CA
M2P 2A8
Phone: +1 416 673 4110
EMail: andrew@ca.afilias.info
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