Internet Engineering Task Force (IETF) R. Bellis
Request for Comments: 5966 Nominet UK
Updates: 1035, 1123 August 2010
Category: Standards Track
DNS Transport over TCP - Implementation Requirements
This document updates the requirements for the support of TCP as a
transport protocol for DNS implementations.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 32. Terminology Used in This Document . . . . . . . . . . . . . . . 33. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 34. Transport Protocol Selection . . . . . . . . . . . . . . . . . 45. Connection Handling . . . . . . . . . . . . . . . . . . . . . . 56. Response Reordering . . . . . . . . . . . . . . . . . . . . . . 67. Security Considerations . . . . . . . . . . . . . . . . . . . . 68. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 79. References . . . . . . . . . . . . . . . . . . . . . . . . . . 79.1. Normative References . . . . . . . . . . . . . . . . . . . 79.2. Informative References . . . . . . . . . . . . . . . . . . 71. Introduction
Most DNS [RFC1034] transactions take place over UDP [RFC0768]. TCP
[RFC0793] is always used for zone transfers and is often used for
messages whose sizes exceed the DNS protocol's original 512-byte
Section 126.96.36.199 of [RFC1123] states:
DNS resolvers and recursive servers MUST support UDP, and SHOULD
support TCP, for sending (non-zone-transfer) queries.
However, some implementors have taken the text quoted above to mean
that TCP support is an optional feature of the DNS protocol.
The majority of DNS server operators already support TCP and the
default configuration for most software implementations is to support
TCP. The primary audience for this document is those implementors
whose failure to support TCP restricts interoperability and limits
deployment of new DNS features.
This document therefore updates the core DNS protocol specifications
such that support for TCP is henceforth a REQUIRED part of a full DNS
Whilst this document makes no specific recommendations to operators
of DNS servers, it should be noted that failure to support TCP (or
the blocking of DNS over TCP at the network layer) may result in
resolution failure and/or application-level timeouts.
2. Terminology Used in This Document
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 [RFC2119].
In the absence of EDNS0 (Extension Mechanisms for DNS 0) (see below),
the normal behaviour of any DNS server needing to send a UDP response
that would exceed the 512-byte limit is for the server to truncate
the response so that it fits within that limit and then set the TC
flag in the response header. When the client receives such a
response, it takes the TC flag as an indication that it should retry
over TCP instead.
RFC 1123 also says:
... 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 DNS Security (DNSSEC) [RFC4033] have shown
that truncation at the 512-byte boundary is now commonplace. For
example, a Non-Existent Domain (NXDOMAIN) (RCODE == 3) response from
a DNSSEC-signed zone using NextSECure 3 (NSEC3) [RFC5155] is almost
invariably larger 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 larger 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 that exceed the size of the path
MTU causes IP packet fragmentation, which has been found to be
unreliable in some circumstances. Many firewalls routinely block
fragmented IP packets, and some do not implement the algorithms
necessary to reassemble fragmented packets. Worse still, some
network 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-
The future that was anticipated in RFC 1123 has arrived, and the only
standardised UDP-based mechanism that may have resolved the packet
size issue has been found inadequate.
4. Transport Protocol Selection
All general-purpose DNS implementations MUST support both UDP and TCP
o Authoritative server implementations MUST support TCP so that they
do not limit the size of responses to what fits in a single UDP
o Recursive server (or forwarder) implementations MUST support TCP
so that they do not prevent large responses from a TCP-capable
server from reaching its TCP-capable clients.
o Stub resolver implementations (e.g., an operating system's DNS
resolution library) MUST support TCP since to do otherwise would
limit their interoperability with their own clients and with
Stub resolver implementations MAY omit support for TCP when
specifically designed for deployment in restricted environments where
truncation can never occur or where truncated DNS responses are
Regarding the choice of when to use UDP or TCP, Section 188.8.131.52 of
RFC 1123 also says:
... a DNS resolver or server that is sending a non-zone-transfer
query MUST send a UDP query first.
That requirement is hereby relaxed. 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 (with or without EDNS0) or for other operational reasons, in
particular, if it already has an open TCP connection to the server.
5. 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. In particular, the server
should allow the SOA and AXFR request sequence (which begins a
refresh operation) to be made on a single connection. Since the
server would be unable to answer queries anyway, a unilateral
close or reset may be used instead of a graceful close.
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.
It is therefore RECOMMENDED that the default application-level idle
period should be of the order of seconds, but no particular value is
specified. In practise, the idle period may vary dynamically, and
servers MAY allow dormant connections to remain open for longer
periods as resources permit.
To mitigate the risk of unintentional server overload, DNS clients
MUST take care to minimize the number of concurrent TCP connections
made to any individual server. Similarly, servers MAY impose limits
on the number of concurrent TCP connections being handled for any
Further recommendations for the tuning of TCP stacks to allow higher
throughput or improved resiliency against denial-of-service attacks
are outside the scope of this document.
6. Response Reordering
RFC 1035 is ambiguous on the question of whether TCP queries may be
reordered -- the only relevant text is in Section 4.2.1, which
relates to UDP:
Queries or their responses may be reordered by the network, or by
processing in name servers, so resolvers should not depend on them
being returned in order.
For the avoidance of future doubt, this requirement is clarified.
Client resolvers MUST be able to process responses that arrive in a
different order to that in which the requests were sent, regardless
of the transport protocol in use.
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
Although there is a higher risk of such attacks against TCP-enabled
servers, techniques for the mitigation of DoS attacks at the network
level have improved substantially since DNS was first designed.
At the time of writing, the vast majority of Top Level Domain (TLD)
authority servers and all of the root name servers support TCP and
the author knows of no evidence to suggest that TCP-based DoS attacks
against existing DNS infrastructure are commonplace.
That notwithstanding, readers are advised to familiarise themselves
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 help 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.
The author would like to thank the document reviewers from the DNSEXT
Working Group, and in particular, George Barwood, Alex Bligh, Alfred
Hoenes, Fernando Gont, Olafur Gudmondsson, Jim Reid, Paul Vixie, and
9.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1034] Mockapetris, P., "Domain names - concepts and
facilities", STD 13, RFC 1034, November 1987.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.