One document matched: draft-ietf-dnsext-dns-tcp-requirements-03.xml
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<rfc category="std" ipr="trust200902" docName="draft-ietf-dnsext-dns-tcp-requirements-03"
updates="1035, 1123">
<?rfc toc='yes' ?>
<?rfc tocompact='no' ?>
<?rfc compact='yes' ?>
<?rfc subcompact='yes' ?>
<?rfc sortrefs='yes' ?>
<?rfc symrefs='yes' ?>
<front>
<title abbrev="DNS over TCP">DNS Transport over TCP - Implementation Requirements</title>
<author initials="R.P." surname="Bellis" fullname="Ray Bellis">
<organization>Nominet UK</organization>
<address>
<postal>
<street>Edmund Halley Road</street>
<city>Oxford</city>
<code>OX4 4DQ</code>
<country>United Kingdom</country>
</postal>
<phone>+44 1865 332211</phone>
<email>ray.bellis@nominet.org.uk</email>
<uri>http://www.nominet.org.uk/</uri>
</address>
</author>
<date year="2010"/>
<area>Internet Area</area>
<workgroup>DNSEXT</workgroup>
<keyword>DNS</keyword>
<keyword>TCP/IP</keyword>
<abstract>
<t> This document updates the requirements for the support of TCP as a transport
protocol for DNS implementations.</t>
</abstract>
</front>
<middle>
<section title="Introduction" anchor="introduction">
<t> Most <xref target="RFC1035">DNS</xref> transactions take place over <xref
target="RFC0768">UDP</xref>. <xref target="RFC0793">TCP</xref> is always used
for zone transfers and is often used for messages whose sizes exceed the DNS
protocol's original 512 byte limit.</t>
<t> Section 6.1.3.2 of <xref target="RFC1123"/> states: <list>
<t><vspace/>DNS resolvers and recursive servers MUST support UDP, and SHOULD
support TCP, for sending (non-zone-transfer) queries.</t>
</list>
</t>
<t> However, some implementors have taken the text quoted above to mean that TCP support
is an optional feature of the DNS protocol.</t>
<t> 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.</t>
<t> This document therefore updates the core DNS protocol specifications such that
support for TCP is henceforth a REQUIRED part of a full DNS protocol implementation.</t>
<t> Whilst this document makes no specific recommendations to operators of DNS servers,
it should be noted that failure to support TCP (or blocking of DNS over TCP at the
network layer) may result in resolution failure and/or application-level timeouts.</t>
</section>
<section anchor="terminology" title="Terminology used in this document">
<t>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 <xref target="RFC2119"/>. </t>
</section>
<section title="Discussion">
<t> In the absence of EDNS0 (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.</t>
<t> RFC 1123 also says: <list>
<t><vspace/>... 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.</t>
</list>
</t>
<t> Existing deployments of <xref target="RFC4033">DNSSEC</xref> 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 <xref target="RFC5155"
>NSEC3</xref> is almost invariably larger than 512 bytes.</t>
<t> Since the original core specifications for DNS were written, the Extension
Mechanisms for DNS (<xref target="RFC2671">EDNS0</xref>) 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 truncation.</t>
<t> 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 <xref
target="RFC5625"/>.</t>
<t>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. </t>
<t> The future that was anticipated in RFC 1123 has arrived, and the only standardised
UDP-based mechanism which may have resolved the packet size issue has been found
inadequate.</t>
</section>
<section title="Transport Protocol Selection" anchor="selection">
<t> All general purpose DNS implementations MUST support both UDP and TCP transport.</t>
<t>
<list style="symbols">
<t> Authoritative server implementations MUST support TCP so that they do not
limit the size of responses.</t>
<t> Recursive resolver (or forwarder) implementations MUST support TCP so that
the do not prevent large responses from a TCP-capable server from reaching
its TCP-capable clients.</t>
<t> 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 upstream servers. </t>
</list>
</t>
<t> An exception may be made for proprietary stub resolver implementations. These MAY
omit support for TCP if operating in an environment where truncation can never
occur, or where DNS lookup failure is acceptable should truncation occur.</t>
<t> Regarding the choice of when to use UDP or TCP, RFC 1123 says:<list>
<t><vspace/>... a DNS resolver or server that is sending a non-zone-transfer
query MUST send a UDP query first.</t>
</list></t>
<t> 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.</t>
</section>
<section title="Connection Handling" anchor="timeouts">
<t> Section 4.2.2 of <xref target="RFC1035"/> says:<list>
<t><vspace/>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.
</t>
</list></t>
<t> Other more modern protocols (e.g. <xref target="RFC2616">HTTP</xref>) 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.</t>
<t> This document therefore RECOMMENDS that the default application-level idle period
should be of the order of seconds, but does not specify any particular value. In
practise the idle period may vary dynamically, and servers MAY allow dormant
connections to remain open for longer periods as resources permit.</t>
<t> 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 particular client.</t>
<t> 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.</t>
</section>
<section title="Response re-ordering" anchor="re-ordering">
<t> RFC 1035 is ambiguous on the question of whether TCP queries may be re-ordered - the
only relevant text is in Section 4.2.1 which relates to UDP:<list>
<t><vspace/> 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.</t>
</list></t>
<t> For the avoidance of future doubt, this requirement is clarified. Client resolvers
MUST be able to process responses which arrive in a different order to that in which
the requests were sent, regardless of the transport protocol in use.</t>
<t/>
</section>
<section title="Security Considerations" anchor="security">
<t> 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 (DoS) attacks.</t>
<t> 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.</t>
<t> At the time of writing the vast majority of 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.</t>
<t> That notwithstanding, readers are advised to familiarise themselves with <xref
target="CPNI-TCP"/>. </t>
<t> 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 <xref target="RFC5358">reflector
attacks</xref> 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.</t>
</section>
<section title="IANA Considerations" anchor="iana">
<t>This document requests no IANA actions.</t>
</section>
<section title="Acknowledgements">
<t> 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, Jim
Reid, Paul Vixie and Nicholas Weaver.</t>
</section>
</middle>
<back>
<references title="Normative References"> &rfc768; &rfc793; &rfc1035;
&rfc1123; &rfc2119; &rfc2671; </references>
<references title="Informative References"> &rfc2616; &rfc4033; &rfc5155;
&rfc5358; &rfc5625; <reference anchor="CPNI-TCP"
target="http://www.cpni.gov.uk/Docs/tn-03-09-security-assessment-TCP.pdf">
<front>
<title>Security Assessment of the Transmission Control Protocol (TCP)</title>
<author>
<organization>CPNI</organization>
</author>
<date year="2009"/>
</front>
</reference>
</references>
<section title="Change Log" anchor="changelog">
<t>NB: to be removed by the RFC Editor before publication.</t>
<t>draft-ietf-dnsext-dns-tcp-requirements-03<list>
<t>Editorial nits from WGLC</t>
<t>Clarification on "general purpose"</t>
<t>Fixed ref to UDP (RFC 768)</t>
<t>Included more ยง4.2.2 text from RFC 1035 and removed some from this draft
relating to connection resets.</t>
<t>s/long/large/ for packet sizes</t>
</list>
</t>
<t>draft-ietf-dnsext-dns-tcp-requirements-02<list>
<t>Change of title - more focus on implementation and not operation</t>
<t>Re-write of some of the security section</t>
<t>Added recommendation for minimal concurrent connections</t>
<t>Minor editorial nits from Alfred Hoenes</t>
</list>
</t>
<t>draft-ietf-dnsext-dns-tcp-requirements-01<list>
<t>Addition of response ordering section</t>
<t>Various minor editorial changes from WG reviewers</t>
</list>
</t>
<t>draft-ietf-dnsext-dns-tcp-requirements-00<list>
<t>Initial draft</t>
</list>
</t>
</section>
</back>
</rfc>
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