Commit 169b9003 authored by Mark Andrews's avatar Mark Andrews
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DNSEXT R. Bellis
Internet-Draft Nominet UK
Updates: 1123, 1035 October 6, 2009
(if approved)
Intended status: Standards Track
Expires: April 9, 2010
DNS Transport over TCP
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
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
The list of Internet-Draft Shadow Directories can be accessed at
This Internet-Draft will expire on April 9, 2010.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
This document updates the requirements for the support of the TCP
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protocol for the transport of DNS traffic.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology used in this document . . . . . . . . . . . . . . . 3
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Transport Protocol Selection . . . . . . . . . . . . . . . . . 4
5. Dormant Connection Handling . . . . . . . . . . . . . . . . . . 5
6. Response re-ordering . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . . 7
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . . 7
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
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1. Introduction
Most DNS [RFC1035] transactions take place over the UDP [RFC0792]
protocol. The TCP [RFC0793] protocol is used for zone transfers and
is supported by some implementations for the transfer of other
packets which exceed the protocol's original 512 byte packet-size
Section of [RFC1123] states:
DNS resolvers and recursive servers MUST support UDP, and SHOULD
support TCP, for sending (non-zone-transfer) queries.
This document normatively updates the core DNS protocol
specifications such that (except in very limited circumstances)
support for the TCP protocol is henceforth REQUIRED.
2. Terminology used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
document are to be interpreted as described in [RFC2119].
3. Discussion
Some implementors have taken the [RFC1123] text quoted above to mean
that TCP support is truly optional for typical DNS operation.
However, whilst RFC 1123 predates the current RFC 2119 terminology
document it uses exactly the same text:
SHOULD - This word, or the adjective "RECOMMENDED", mean that
there may exist valid reasons in particular circumstances to
ignore a particular item, but the full implications must be
understood and carefully weighed before choosing a different
In the absence of EDNS0 (see below) the normal behaviour of any DNS
server needing to send a UDP response that exceeds that 512 limit is
for the server to truncate the response at the 512 byte limit and set
the TC flag in the response header. When the client receives such a
response it takes the TC flag as notice that it should retry over TCP
RFC 1123 also says:
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... it is also clear that some new DNS record types defined in the
future will contain information exceeding the 512 byte limit that
applies to UDP, and hence will require TCP. Thus, resolvers and
name servers should implement TCP services as a backup to UDP
today, with the knowledge that they will require the TCP service
in the future.
Existing deployments of DNSSEC [RFC4033] have shown that truncation
at the 512 byte boundary is now commonplace. For example an NXDOMAIN
(RCODE == 3) response from a DNSSEC signed zone using NSEC3 [RFC5155]
is almost invariably longer than 512 bytes.
Since the original core specifications for DNS were written the
Extension Mechanisms for DNS EDNS0 [RFC2671] have been introduced.
These extensions can be used to indicate that the client is prepared
to receive UDP responses longer than 512 bytes. An EDNS0 compatible
server receiving a request from an EDNS0 compatible client may send
UDP packets up to that client's announced buffer size without
However, transport of UDP packets which exceed the size of the path
MTU has been found to be unreliable in some circumstances because of
IP packet fragmentation. Many firewalls routinely block fragmented
IP packets, and some implementations lack the software logic
necessary to reassemble a fragmented datagram. Worse still, some
devices deliberately refuse to handle DNS packets containing EDNS0
options. Other issues relating to UDP transport and packet size are
discussed in [RFC5625].
The MTU most commonly found in the core of the Internet is around
1500 bytes, and even that limit is routinely exceeded by DNSSEC
signed responses.
The future that was anticipated in RFC 1123 is now here, and the only
standardised mechanism which may have resolved the packet size issue
has been found inadequate.
4. Transport Protocol Selection
On a case by case basis, authoritative DNS server operators MAY elect
to disable DNS transport over TCP if all of the conditions below are
o the server is authoritative
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o the server does not support AXFR
o the server does not support DNSSEC
o all requests and responses are guaranteed to be <= 512 bytes
A general purpose stub resolver implementation (e.g. an operating
system's DNS resolution library) MUST support TCP since to do
otherwise would limit its interoperability with its own clients and
with upstream servers.
A proprietary stub resolver implementation MAY omit support for TCP
if it is operating in an environment where truncation will not occur,
or if it is prepared to accept a DNS lookup failure should truncation
A recursive resolver or forwarder MUST support TCP so that it does
not prevent long responses from a TCP-capable server from reaching
its TCP-capable clients.
Otherwise, all DNS implementations MUST support TCP transport.
Regarding the choice of when to use UDP or TCP, RFC 1123 says:
... a DNS resolver or server that is sending a non-zone-transfer
query MUST send a UDP query first.
This requirement is no longer mandatory. A resolver SHOULD send a
UDP query first, but MAY elect to send a TCP query instead if it has
good reason to expect the response would be truncated if it were sent
over UDP, or other operational considerations suggest otherwise.
5. Dormant Connection Handling
Section 4.2.2 of [RFC1035] says:
If the server needs to close a dormant connection to reclaim
resources, it should wait until the connection has been idle for a
period on the order of two minutes.
Other more modern protocols (e.g. HTTP [RFC2616]) have support for
persistent TCP connections and operational experience has shown that
long timeouts can easily cause resource exhaustion and poor response
under heavy load. Intentionally opening many connections and leaving
them dormant can trivially create a "denial of service" attack.
This document therefore RECOMMENDS that the idle period should be of
the order of TBD seconds. With modern high performance networks 2 to
4 seconds should be sufficient to allow significant numbers (i.e.
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thousands) of concurrent dormant connections without impacting
service performance.
Servers MAY allow idle connections to remain open for longer periods,
but for the avoidance of doubt persistent DNS connections should
generally be considered to be as much for the server's benefit as for
the client's. Therefore if the server needs to unilaterally close a
dormant TCP connection it MUST be free to do so whenever required.
6. Response re-ordering
[Potential text to be added regarding whether TCP responses can come
back in a different order to requests. I'm not aware whether this is
specified anywhere]
7. Security Considerations
Some DNS server operators have expressed concern that wider use of
DNS over TCP will expose them to a higher risk of "denial of service"
Many large authoritative DNS operators including all but one of the
root servers and the vast majority of TLDs already support TCP and
attacks against them are infrequent and very rarely successful.
Operators of recursive servers should ensure that they only accept
connections from expected clients, and do not accept them from
unknown sources. In the case of UDP traffic this will protect
against reflector attacks [RFC5358] and in the case of TCP traffic it
will prevent an unknown client from exhausting the server's limits on
the number of concurrent connections.
8. IANA Considerations
This document requests no IANA actions.
9. References
9.1. Normative References
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
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RFC 793, September 1981.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
9.2. Informative References
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, March 2008.
[RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive
Nameservers in Reflector Attacks", BCP 140, RFC 5358,
October 2008.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
BCP 152, RFC 5625, August 2009.
Appendix A. Change Log
NB: to be removed by the RFC Editor before publication.
Initial draft
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Author's Address
Ray Bellis
Nominet UK
Edmund Halley Road
Oxford OX4 4DQ
United Kingdom
Phone: +44 1865 332211
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