Network Working Group D. Lawrence
Request for Comments: 3425 Nominum
Updates: 1035 November 2002
Category: Standards Track
Status of this Memo
This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2002). All Rights Reserved.
The IQUERY method of performing inverse DNS lookups, specified in RFC
1035, has not been generally implemented and has usually been
operationally disabled where it has been implemented. Both reflect a
general view in the community that the concept was unwise and that
the widely-used alternate approach of using pointer (PTR) queries and
reverse-mapping records is preferable. Consequently, this document
deprecates the IQUERY operation, declaring it entirely obsolete.
This document updates RFC 1035.
1 - Introduction
As specified in RFC 1035 (section 6.4), the IQUERY operation for DNS
queries is used to look up the name(s) which are associated with the
given value. The value being sought is provided in the query's
answer section and the response fills in the question section with
one or more 3-tuples of type, name and class.
As noted in [RFC1035], section 6.4.3, inverse query processing can
put quite an arduous burden on a server. A server would need to
perform either an exhaustive search of its database or maintain a
separate database that is keyed by the values of the primary
database. Both of these approaches could strain system resource use,
particularly for servers that are authoritative for millions of
Response packets from these megaservers could be exceptionally large,
and easily run into megabyte sizes. For example, using IQUERY to
find every domain that is delegated to one of the nameservers of a
large ISP could return tens of thousands of 3-tuples in the question
section. This could easily be used to launch denial of service
Operators of servers that do support IQUERY in some form (such as
very old BIND 4 servers) generally opt to disable it. This is
largely due to bugs in insufficiently-exercised code, or concerns
about exposure of large blocks of names in their zones by probes such
as inverse MX queries.
IQUERY is also somewhat inherently crippled by being unable to tell a
requester where it needs to go to get the information that was
requested. The answer is very specific to the single server that was
queried. This is sometimes a handy diagnostic tool, but apparently
not enough so that server operators like to enable it, or request
implementation where it is lacking.
No known clients use IQUERY to provide any meaningful service. The
only common reverse mapping support on the Internet, mapping address
records to names, is provided through the use of pointer (PTR)
records in the in-addr.arpa tree and has served the community well
for many years.
Based on all of these factors, this document recommends that the
IQUERY operation for DNS servers be officially obsoleted.
2 - Requirements
The key word "SHOULD" in this document is to be interpreted as
described in BCP 14, RFC 2119, namely that there may exist valid
reasons to ignore a particular item, but the full implications must
be understood and carefully weighed before choosing a different
3 - Effect on RFC 1035
The effect of this document is to change the definition of opcode 1
from that originally defined in section 4.1.1 of RFC 1035, and to
entirely supersede section 6.4 (including subsections) of RFC 1035.
The definition of opcode 1 is hereby changed to:
"1 an inverse query (IQUERY) (obsolete)"
The text in section 6.4 of RFC 1035 is now considered obsolete. The
following is an applicability statement regarding the IQUERY opcode:
Inverse queries using the IQUERY opcode were originally described as
the ability to look up the names that are associated with a
particular Resource Record (RR). Their implementation was optional
and never achieved widespread use. Therefore IQUERY is now obsolete,
and name servers SHOULD return a "Not Implemented" error when an
IQUERY request is received.
4 - Security Considerations
Since this document obsoletes an operation that was once available,
it is conceivable that someone was using it as the basis of a
security policy. However, since the most logical course for such a
policy to take in the face of a lack of positive response from a
server is to deny authentication/authorization, it is highly unlikely
that removing support for IQUERY will open any new security holes.
Note that if IQUERY is not obsoleted, securing the responses with DNS
Security (DNSSEC) is extremely difficult without out-on-the-fly
5 - IANA Considerations
The IQUERY opcode of 1 should be permanently retired, not to be
assigned to any future opcode.
6 - Acknowledgments
Olafur Gudmundsson instigated this action. Matt Crawford, John
Klensin, Erik Nordmark and Keith Moore contributed some improved
wording in how to handle obsoleting functionality described by an
7 - References
[RFC1035] Mockapetris, P., "Domain Names - Implementation and
Specification", STD 13, RFC 1035, November 1987.
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", BCP 9, RFC 2026, October 1996.
[RFC2119] Bradner, S., "Key Words for Use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
9 - Full Copyright Statement
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