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Internet Engineering Task Force (IETF)                         R. BellisRequest for Comments: 5966                                    Nominet UKUpdates:1035,1123                                          August 2010Category: Standards TrackISSN: 2070-1721DNS Transport over TCP - Implementation RequirementsAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5966.Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) 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.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Bellis                       Standards Track                    [Page 1]

RFC 5966                      DNS over TCP                   August 2010Table of Contents1.  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   limit.Section 6.1.3.2 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   the blocking of DNS over TCP at the network layer) may result in   resolution failure and/or application-level timeouts.Bellis                       Standards Track                    [Page 2]

RFC 5966                      DNS over TCP                   August 20102.  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].3.  Discussion   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   truncation.   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].Bellis                       Standards Track                    [Page 3]

RFC 5966                      DNS over TCP                   August 2010   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 inRFC 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   transport.   o  Authoritative server implementations MUST support TCP so that they      do not limit the size of responses to what fits in a single UDP      packet.   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      upstream servers.   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   acceptable.   Regarding the choice of when to use UDP or TCP,Section 6.1.3.2 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.Bellis                       Standards Track                    [Page 4]

RFC 5966                      DNS over TCP                   August 20105.  Connection HandlingSection 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   particular client.   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 ReorderingRFC 1035 is ambiguous on the question of whether TCP queries may be   reordered -- the only relevant text is inSection 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.Bellis                       Standards Track                    [Page 5]

RFC 5966                      DNS over TCP                   August 20107.  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.   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   with [CPNI-TCP].   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.8.  Acknowledgements   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   Nicholas Weaver.9.  References9.1.  Normative References   [RFC0768]   Postel, J., "User Datagram Protocol", STD 6,RFC 768,               August 1980.   [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.Bellis                       Standards Track                    [Page 6]

RFC 5966                      DNS over TCP                   August 2010   [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   [CPNI-TCP]  CPNI, "Security Assessment of the Transmission Control               Protocol (TCP)", 2009, <http://www.cpni.gov.uk/Docs/tn-03-09-security-assessment-TCP.pdf>.   [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.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/Bellis                       Standards Track                    [Page 7]