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DNS Transport over TCP - Implementation Requirements

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This is an older version of an Internet-Draft that was ultimately published as RFC 7766.
Authors John Dickinson , Ray Bellis , Allison Mankin , Duane Wessels
Last updated 2014-12-04
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dnsop                                                       J. Dickinson
Internet-Draft                             Sinodun Internet Technologies
Updates: 5966 (if approved)                                    R. Bellis
Intended status: Standards Track                                 Nominet
Expires: June 7, 2015                                          A. Mankin
                                                              D. Wessels
                                                           Verisign Labs
                                                        December 4, 2014

          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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   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."

   This Internet-Draft will expire on June 7, 2015.

Copyright Notice

   Copyright (c) 2014 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.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Transport Protocol Selection  . . . . . . . . . . . . . . . .   4
   5.  Connection Handling . . . . . . . . . . . . . . . . . . . . .   5
   6.  Query Pipelining  . . . . . . . . . . . . . . . . . . . . . .   6
   7.  Response Reordering . . . . . . . . . . . . . . . . . . . . .   6
   8.  TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . . .   7
   9.  Summary of Advantages and Disadvantages to using TCP for DNS    8
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .   9
   11. Security Considerations . . . . . . . . . . . . . . . . . . .   9
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .   9
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     13.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     13.2.  Informative References . . . . . . . . . . . . . . . . .  10
   Appendix A.  Changes to RFC 5966  . . . . . . . . . . . . . . . .  11
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  11

1.  Introduction

   Most DNS [RFC1034] transactions take place over UDP [RFC0768].  TCP
   [RFC0793] is always used for full zone transfers (AXFR) and is often
   used for messages whose sizes exceed the DNS protocol's original
   512-byte 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.

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   There are several advantages and disadvantages to the increased use
   of TCP as well as implementation details that need to be considered.
   This document addresses these issues and updates [RFC5966], with
   additional considerations and lessons learned from new research and
   implementations [Connection-Oriented-DNS].

   Whilst this document makes no specific requirements for operators of
   DNS servers to meet, it does offer some suggestions to operators to
   help ensure that support for TCP on their servers and network is
   optimal.  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

   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 (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 [RFC6891]) have been introduced.
   These extensions can be used to indicate that the client is prepared

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   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-
   signed responses.

   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
      upstream servers.

   Regarding the choice of when to use UDP or TCP, Section of
   RFC 1123 also says:

      ... a DNS resolver or server that is sending a non-zone-transfer
      query MUST send a UDP query first.

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   This requirement is hereby relaxed.  A resolver MAY elect to send
   either TCP or UDP queries depending on local operational reasons.  If
   it already has an open TCP connection to the server it SHOULD reuse
   this connection.

   In essence, TCP SHOULD be considered as valid a transport as UDP.  It
   SHOULD NOT be used only for zone transfers and as a fallback.

   In addition it is noted that all Recursive and Authoritative servers
   MUST send responses using the same transport as the query arrived on.
   In the case of TCP this MUST also be the same connection.

5.  Connection Handling

   One perceived disadvantage to DNS over TCP is the added connection
   setup latency, generally equal to one RTT.  To amortize connection
   setup costs, both clients and servers SHOULD support connection reuse
   by sending multiple queries and responses over a single TCP

   DNS currently has no connection signaling mechanism.  Clients and
   servers may close a connection at any time.  Clients MUST be prepared
   to retry failed queries on broken connections.

   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/1.1 [RFC7230]) 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"

   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 practice, the idle period may vary dynamically, and
   servers MAY allow dormant connections to remain open for longer
   periods as resources permit.

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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.  Similarly, servers MAY impose limits
   on the number of concurrent TCP connections being handled for any
   particular client.  It is RECOMMENDED that for any given client -
   server interaction there SHOULD be no more than one connection for
   regular queries, one for zone transfers and one for each protocol
   that is being used on top of TCP, for example, if the resolver was
   using TLS.  The server MUST NOT enforce these rules for a particular
   client because it does not know if the client IP address belongs to a
   single client or is, for example, multiple clients behind NAT.

6.  Query Pipelining

   Due to the use of TCP primarily for zone transfer and truncated
   responses, no existing RFC discusses the idea of pipelining DNS
   queries over a TCP connection.

   In order to achieve performance on par with UDP, it is therefore
   RECOMMENDED that DNS clients should pipeline their queries.  When a
   DNS client sends multiple queries to a server, it should not wait for
   an outstanding reply before sending the next query.  Clients should
   treat TCP and UDP equivalently when considering the time at which to
   send a particular query.

   DNS servers (especially recursive) SHOULD expect to receive pipelined
   queries.  The server should process TCP queries in parallel, just as
   it would for UDP.  The handling of responses to pipelined queries is
   covered in the following section.

   When pipelining queries over TCP it is very easy to send more DNS
   queries than there are DNS Message ID's.  Implementations MUST take
   care to check their list of outstanding DNS Message ID's before
   sending a new query over an existing TCP connection.  This is
   especially important if the server could be performing out-of-order
   processing.  In addition, when sending multiple queries over TCP it
   is very easy for a name server to overwhelm its own network
   interface.  Implementations MUST take care to manage buffer sizes or
   to throttle writes to the network interface.

7.  Response Reordering

   RFC 1035 is ambiguous on the question of whether TCP responses may be
   reordered -- the only relevant text is in Section 4.2.1, which
   relates to UDP:

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      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.
   Authoritative servers and recursive resolvers are RECOMMENDED to
   support the sending of responses in parallel and/or out-of-order,
   regardless of the transport protocol in use.  Stub and recursive
   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.

   In order to achieve performance on par with UDP, recursive resolvers
   SHOULD process TCP queries in parallel and return individual
   responses as soon as they are available, possibly out-of-order.

   Since responses may arrive out-of-order, clients must take care to
   match responses to outstanding queries, using the ID field, port
   number, query name/type/class, and any other relevant protocol

8.  TCP Fast Open

   This section is non-normative.

   TCP fastopen [I-D.ietf-tcpm-fastopen] (TFO) allows data to be carried
   in the SYN packet.  It also saves up to one RTT compared to standard

   TFO mitigates the security vulnerabilities inherent in sending data
   in the SYN, especially on a system like DNS where amplification
   attacks are possible, by use of a server-supplied cookie.  TFO
   clients request a server cookie in the initial SYN packet at the
   start of a new connection.  The server returns a cookie in its SYN-
   ACK.  The client caches the cookie and reuses it when opening
   subsequent connections to the same server.

   The cookie is stored by the client's TCP stack (kernel) and persists
   if either the client or server processes are restarted.  TFO also
   falls back to a regular TCP handshake gracefully.

   Adding support for this to existing name server implementations is
   relatively easy, but does require source code modifications.  On the
   client, the call to connect() is replaced with a TFO aware version of
   sendmsg() or sendto().  On the server, TFO must be switched into
   server mode by changing the kernel parameter (net.ipv4.tcp_fastopen
   on Linux) to enable the server bit (Set the integer value to 2

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   (server only) or 3 (client and server)) and setting a socket option
   between the bind() and listen() calls.

   DNS services taking advantage of IP anycast [RFC4786] may need to
   take additional steps when enabling TFO.  From

      Servers that accept connection requests to the same server IP
      address should use the same key such that they generate identical
      Fast Open Cookies for a particular client IP address.  Otherwise a
      client may get different cookies across connections; its Fast Open
      attempts would fall back to regular 3WHS.

9.  Summary of Advantages and Disadvantages to using TCP for DNS

   The TCP handshake generally prevents address spoofing and, therefore,
   the reflection/amplification attacks which plague UDP.

   TCP does not suffer from UDP's issues with fragmentation.
   Middleboxes are known to block IP fragments, leading to timeouts and
   forcing client implementations to "hunt" for EDNS0 reply size values
   supported by the network path.  Additionally, fragmentation may lead
   to cache poisoning [fragmentation-considered-poisonous].

   TCP setup costs an additional RTT compared to UDP queries.  Setup
   costs can be amortized by reusing connections, pipelining queries,
   and enabling TCP Fast Open.

   TCP imposes additional state-keeping requirements on clients and
   servers.  The use of TCP Fast Open reduces the cost of closing and
   re-opening TCP connections.

   Long-lived TCP connections to anycast servers may be disrupted due to
   routing changes.  Clients utilizing TCP for DNS must always be
   prepared to re-establish connections or otherwise retry outstanding
   queries.  It may also possible for TCP Multipath [RFC6824] to allow a
   server to hand a connection over from the anycast address to a
   unicast address.

   There are many "Middleboxes" in use today that interfere with TCP
   over port 53 [RFC5625].  This document does not propose any
   solutions, other than to make it absolutely clear that TCP is a valid
   transport for DNS and must be supported by all implementations.

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10.  IANA Considerations

   This memo includes no request to IANA.

11.  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.

   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.

12.  Acknowledgements

   The authors would like to thank Liang Zhu, Zi Hu, and John Heidemann
   for extensive DNS-over-TCP discussions and code; and Lucie Guiraud
   and Danny McPherson for reviewing early versions of this document.
   We would also like to thank all those who contributed to RFC 5966.

13.  References

13.1.  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.

   [RFC1123]  Braden, R., "Requirements for Internet Hosts - Application
              and Support", STD 3, RFC 1123, October 1989.

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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
              Rose, "DNS Security Introduction and Requirements", RFC
              4033, March 2005.

   [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
              Services", BCP 126, RFC 4786, December 2006.

   [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.

   [RFC5966]  Bellis, R., "DNS Transport over TCP - Implementation
              Requirements", RFC 5966, August 2010.

   [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
              for DNS (EDNS(0))", STD 75, RFC 6891, April 2013.

   [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230, June

13.2.  Informative References

              CPNI, "Security Assessment of the Transmission Control
              Protocol (TCP)", 2009, <

              Zhu, L., Hu, Z., Heidemann, J., Wessels, D., Mankin, A.,
              and N. Somaiya, "T-DNS: Connection-Oriented DNS to Improve
              Privacy and Security (extended)",

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              Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", draft-ietf-tcpm-fastopen-10 (work in
              progress), September 2014.

   [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
              "TCP Extensions for Multipath Operation with Multiple
              Addresses", RFC 6824, January 2013.

              Herzberg, A. and H. Shulman, "Fragmentation Considered
              Poisonous", May 2012, <>.

Appendix A.  Changes to RFC 5966

   This document differs from RFC 5966 in four additions:

   1.  DNS implementations are recommended not only to support TCP but
       to support it on an equal footing with UDP

   2.  DNS implementations are recommended to support reuse of TCP

   3.  DNS implementations are recommended to support pipelining and out
       of order processing of the query stream

   4.  A non-normative discussion of use of TCP Fast Open is added

Authors' Addresses

   John Dickinson
   Sinodun Internet Technologies
   Magdalen Centre
   Oxford Science Park
   Oxford  OX4 4GA


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   Ray Bellis
   Edmund Halley Road
   Oxford  OX4 4DQ

   Phone: +44 1865 332211

   Allison Mankin
   Verisign Labs
   12061 Bluemont Way
   Reston, VA  20190

   Phone: +1 703 948-3200

   Duane Wessels
   Verisign Labs
   12061 Bluemont Way
   Reston, VA  20190

   Phone: +1 703 948-3200

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