DNS Transport over TCP - Implementation Requirements

Versions: 00 01 02 03 rfc5966                                           
DNSEXT                                                         R. Bellis
Internet-Draft                                                Nominet UK
Updates: 1035, 1123                                      January 6, 2010
(if approved)
Intended status: Standards Track
Expires: July 10, 2010

          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

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   provisions of BCP 78 and BCP 79.

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   carefully, as they describe your rights and restrictions with respect
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   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 3

   2.  Terminology used in this document . . . . . . . . . . . . . . . 3

   3.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . . . 3

   4.  Transport Protocol Selection  . . . . . . . . . . . . . . . . . 4

   5.  Connection Handling . . . . . . . . . . . . . . . . . . . . . . 5

   6.  Response re-ordering  . . . . . . . . . . . . . . . . . . . . . 6

   7.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6

   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7

   9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     9.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     9.2.  Informative References  . . . . . . . . . . . . . . . . . . 7

   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . . . 8

   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8

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1.  Introduction

   Most DNS [RFC1035] transactions take place over UDP [RFC0792].  The
   TCP [RFC0793] is used for zone transfers and for the transfer of
   other packets which exceed the protocol's original 512 byte packet-
   size limit.

   Section 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
   protocol implementation.

   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 application-level timeouts.

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

   In the absence of EDNS0 (see below) the normal behaviour of any DNS
   server needing to send a UDP response that exceeds that 512 byte
   limit is for the server to truncate the response so that it fits
   within 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 an
   indication that it should retry over TCP instead.

   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 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 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 has arrived, and the only
   standardised UDP-based mechanism which may have resolved the packet
   size issue has been found inadequate.

4.  Transport Protocol Selection

   All DNS implementations MUST support both UDP and TCP transport.

   o  Authoritative resolver implementations MUST support TCP so that
      they may serve any long responses that they are configured to

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   o  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.
   o  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.

   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.

   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.

   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.

   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 application-level idle
   period should be of the order of TBD seconds.

   Servers MAY allow dormant 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

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   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.

   Further recommendations for the tuning of TCP parameters to allow
   higher throughput or improved resiliency against denial of service
   attacks are outside the scope of this document.

6.  Response re-ordering

   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:

      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 which 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"
   (DoS) attacks.

   Whilst there is a theoretically 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

   The vast majority of TLD authority servers and all but one of the
   root name servers already support TCP and the author knows of no
   evidence to suggest that TCP-based DoS attacks against existing DNS
   infrastructure are commonplace.

   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.

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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,
              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.

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Appendix A.  Change Log

   NB: to be removed by the RFC Editor before publication.

      Change of title - more focus on implementation and not operation
      Re-write of some of the security section
      Added recommendation for minimal concurrent connections
      Minor editorial nits from Alfred Hoenes

      Addition of response ordering section
      Various minor editorial changes from WG reviewers

      Initial draft

Author's Address

   Ray Bellis
   Nominet UK
   Edmund Halley Road
   Oxford  OX4 4DQ
   United Kingdom

   Phone: +44 1865 332211
   Email: ray.bellis@nominet.org.uk
   URI:   http://www.nominet.org.uk/

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