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DNS Transport over TCP - Implementation Requirements

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 7766.
Authors John Dickinson , Sara Dickinson , Ray Bellis , Allison Mankin , Duane Wessels
Last updated 2015-12-07 (Latest revision 2015-11-02)
Replaces draft-dickinson-dnsop-5966-bis
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Document shepherd Tim Wicinski
Shepherd write-up Show Last changed 2015-11-01
IESG IESG state Became RFC 7766 (Proposed Standard)
Consensus boilerplate Unknown
Telechat date (None)
Responsible AD Joel Jaeggli
Send notices to (None)
IANA IANA review state IANA OK - No Actions Needed
dnsop                                                       J. Dickinson
Internet-Draft                                              S. Dickinson
Obsoletes: 5966 (if approved)                                    Sinodun
Intended status: Standards Track                               R. Bellis
Expires: May 6, 2016                                                 ISC
                                                               A. Mankin
                                                              D. Wessels
                                                           Verisign Labs
                                                        November 3, 2015

          DNS Transport over TCP - Implementation Requirements


   This document specifies the requirement for support of TCP as a
   transport protocol for DNS implementations and provides guidelines
   towards DNS-over-TCP performance on par with that of DNS-over-UDP.
   This document obsoletes RFC5966.

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 May 6, 2016.

Copyright Notice

   Copyright (c) 2015 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

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   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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Terminology  . . . . . . . . . . . . . . . . . .   3
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   4
   5.  Transport Protocol Selection  . . . . . . . . . . . . . . . .   5
   6.  Connection Handling . . . . . . . . . . . . . . . . . . . . .   6
     6.1.  Current practices . . . . . . . . . . . . . . . . . . . .   6
       6.1.1.  Clients . . . . . . . . . . . . . . . . . . . . . . .   6
       6.1.2.  Servers . . . . . . . . . . . . . . . . . . . . . . .   7
     6.2.  Recommendations . . . . . . . . . . . . . . . . . . . . .   7
       6.2.1.  Connection Re-use . . . . . . . . . . . . . . . . . .   7  Query Pipelining  . . . . . . . . . . . . . . . .   8
       6.2.2.  Concurrent connections  . . . . . . . . . . . . . . .   8
       6.2.3.  Idle Timeouts . . . . . . . . . . . . . . . . . . . .   9
       6.2.4.  Tear Down . . . . . . . . . . . . . . . . . . . . . .   9
   7.  Response Reordering . . . . . . . . . . . . . . . . . . . . .  10
   8.  TCP Message Length Field  . . . . . . . . . . . . . . . . . .  10
   9.  TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . . .  11
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   11. Security Considerations . . . . . . . . . . . . . . . . . . .  11
   12. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  12
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     13.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Appendix A.  Summary of Advantages and Disadvantages to using TCP
                for DNS  . . . . . . . . . . . . . . . . . . . . . .  14
   Appendix B.  Changes between revisions  . . . . . . . . . . . . .  15
     B.1.  Changes -03 to -04  . . . . . . . . . . . . . . . . . . .  15
     B.2.  Changes -02 to -03  . . . . . . . . . . . . . . . . . . .  16
     B.3.  Changes -01 to -02  . . . . . . . . . . . . . . . . . . .  16
     B.4.  Changes -00 to -01  . . . . . . . . . . . . . . . . . . .  17
     B.5.  Changes to RFC 5966 . . . . . . . . . . . . . . . . . . .  17
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

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.  The growing deployment of DNSSEC and IPv6 has
   increased response sizes and therefore the use of TCP.  The need for

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   increased TCP use has also been driven by the protection it provides
   against address spoofing and therefore exploitation of DNS in
   reflection/amplification attacks.  It is now widely used in Response
   Rate Limiting [RRL].

   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 limited support for TCP restricts interoperability and hinders
   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.

   There are several advantages and disadvantages to the increased use
   of TCP (see Appendix A) as well as implementation details that need
   to be considered.  This document addresses these issues and presents
   TCP as a valid transport alternative for DNS.  It extends the content
   of [RFC5966], with additional considerations and lessons learned from
   research, developments and implementation of TCP in DNS and in other
   internet protocols.

   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.  Requirements Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Terminology

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   o  Persistent connection: a TCP connection that is not closed either
      by the server after sending the first response nor by the client
      after receiving the first response.

   o  Connection Reuse: the sending of multiple queries and responses
      over a single TCP connection.

   o  Idle DNS-over-TCP session: Clients and servers view application
      level idleness differently.  A DNS client considers an established
      DNS-over-TCP session to be idle when it has no pending queries to
      send and there are no outstanding responses.  A DNS server
      considers an established DNS-over-TCP session to be idle when it
      has sent responses to all the queries it has received on that

   o  Pipelining: the sending of multiple queries and responses over a
      single TCP connection but not waiting for any outstanding replies
      before sending another query.

   o  Out-Of-Order Processing: The processing of queries concurrently
      and the returning of individual responses as soon as they are
      available, possibly out-of-order.  This will most likely occur in
      recursive servers, however it is possible in authoritative servers
      that, for example, have different backend data stores.

4.  Discussion

   In the absence of EDNS0 (Extension Mechanisms for DNS 0 [RFC6891])
   (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

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   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 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 many 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.

5.  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 the interoperability between their own clients and upstream

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

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

   This requirement is hereby relaxed.  Stub resolvers and recursive
   resolvers MAY elect to send either TCP or UDP queries depending on
   local operational reasons.  TCP MAY be used before sending any UDP
   queries.  If it already has an open TCP connection to the server it
   SHOULD reuse this connection.  In essence, TCP ought to be considered
   a valid alternative transport to UDP, not purely a fallback option.

   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.

6.  Connection Handling

6.1.  Current practices

   Section 4.2.2 of [RFC1035] says:

   o  The server should assume that the client will initiate connection
      closing, and should delay closing its end of the connection until
      all outstanding client requests have been satisfied.

   o  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 graceful close.

   Other more modern protocols (e.g., HTTP/1.1 [RFC7230]) have support
   by default for persistent TCP connections for all requests.
   Connections are then normally closed via a 'connection close' signal
   from one party.

   The description in [RFC1035] is clear that servers should view
   connections as persistent (particularly after receiving an SOA), but
   unfortunately does not provide enough detail for an unambiguous
   interpretation of client behaviour for queries other than a SOA.
   Additionally, DNS does not yet have a signalling mechanism for
   connection timeout or close, although some have been proposed.

6.1.1.  Clients

   There is no clear guidance today in any RFC as to when a DNS client
   should close a TCP connection, and there are no specific

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   recommendations with regard to DNS client idle timeouts.  However it
   is common practice for clients to close the TCP connection after
   sending a single request (apart from the SOA/AXFR case).

6.1.2.  Servers

   Many DNS server implementations use a long fixed idle timeout and
   default to a small number of TCP connections.  They also offer little
   by the way of TCP connection management options.  The disadvantages
   of this include:

   o  Operational experience has shown that long server timeouts can
      easily cause resource exhaustion and poor response under heavy

   o  Intentionally opening many connections and leaving them idle can
      trivially create a TCP "denial-of-service" attack as many DNS
      servers are poorly equipped to defend against this by modifying
      their idle timeouts or other connection management policies.

   o  A modest number of clients that all concurrently attempt to use
      persistent connections with non-zero idle timeouts to such a
      server could unintentionally cause the same "denial-of-service"

   Note that this denial-of-service is only on the TCP service.
   However, in these cases it affects not only clients wishing to use
   TCP for their queries for operational reasons, but all clients who
   choose to fall back to TCP from UDP after receiving a TC=1 flag.

6.2.  Recommendations

   The following sections include recommendations that are intended to
   result in more consistent and scalable implementations of DNS-over-

6.2.1.  Connection Re-use

   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 persistent
   TCP connection.

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   When sending multiple queries over a TCP connection clients MUST take
   care to avoid Message ID collisions.  In other words, they MUST NOT
   re-use the DNS Message ID of an in-flight query on the same TCP
   connection.  This is especially important if the server could be
   performing out-of-order processing (see Section 7).  Query Pipelining

   Due to the historical 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 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.

   It is likely that DNS servers need to process pipelined queries
   concurrently and also send out-of-order responses over TCP in order
   to provide the level of performance possible with UDP transport.  If
   TCP performance is of importance, clients might find it useful to use
   server processing times as input to server and transport selection

   DNS servers (especially recursive) SHOULD expect to receive pipelined
   queries.  The server SHOULD process TCP queries concurrently, just as
   it would for UDP.  The server SHOULD answer all pipelined queries,
   even if they are sent in quick succession.  The handling of responses
   to pipelined queries is covered in Section 7.

6.2.2.  Concurrent connections

   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.  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.  It is however noted that certain primary/secondary
   configurations with many busy zones might need to use more than one
   TCP connection for zone transfers for operational reasons.

   Similarly, servers MAY impose limits on the number of concurrent TCP
   connections being handled for any particular client IP address or
   subnet.  These limits SHOULD be much looser than the client
   guidelines above, because the server does not know, for example, if a
   client IP address belongs to a single client or is multiple resolvers

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   on a single machine, or multiple clients behind a device performing
   Network Address Translation (NAT).

6.2.3.  Idle Timeouts

   To mitigate the risk of unintentional server overload, DNS clients
   MUST take care to minimize the idle time of established DNS-over-TCP
   sessions made to any individual server.  DNS clients SHOULD close the
   TCP connection of an idle session, unless an idle timeout has been
   established using some other signalling mechanism, for example,

   To mitigate the risk of unintentional server overload it is
   RECOMMENDED that the default server application-level idle period be
   of the order of seconds, but no particular value is specified.  In
   practice, the idle period can vary dynamically, and servers MAY allow
   idle connections to remain open for longer periods as resources
   permit.  A timeout of at least a few seconds is advisable for normal
   operations to support those clients that expect the SOA and AXFR
   request sequence to be made on a single connection as originally
   specified in [RFC1035].  Servers MAY use zero timeouts when
   experiencing heavy load or are under attack.

   DNS messages delivered over TCP might arrive in multiple segments.  A
   DNS server that resets its idle timeout after receiving a single
   segment might be vulnerable to a "slow read attack."  For this
   reason, servers SHOULD apply the idle timeout to the receipt of a
   full DNS message, rather than to receipt of any part of a DNS

6.2.4.  Tear Down

   Under normal operation clients typically initiate connection closing
   on idle connections however servers can close the connection if their
   local idle timeout policy is exceeded.  Connections can be also
   closed by either end under unusual conditions such as defending
   against an attack or system failure/reboot.

   Clients SHOULD retry unanswered queries if the connection closes
   before receiving all outstanding responses.  No specific retry
   algorithm is specified in this document.

   If a server finds that a client has closed a TCP session, or if the
   session has been otherwise interrupted, before all pending responses
   have been sent then the server MUST NOT attempt to send those
   responses.  Of course the server MAY cache those responses.

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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:

      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 preparing of responses in parallel and sending them 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 pipelined responses can arrive out-of-order, clients MUST match
   responses to outstanding queries on the same TCP connection using the
   Message ID.  If the response contains a question section the client
   MUST match the QNAME, QCLASS and QTYPE fields.  Failure by clients to
   properly match responses to outstanding queries can have serious
   consequences for interoperability.

8.  TCP Message Length Field

   For reasons of efficiency, DNS clients and servers SHOULD pass the
   two-octet length field, and the message described by that length
   field, to the TCP layer at the same time (e.g., in a single "write"
   system call) to make it more likely that all the data will be
   transmitted in a single TCP segment.

   This additionally avoids problems due to some DNS servers being very
   sensitive to timeout conditions on receiving messages (they might
   abort a TCP session if the first TCP segment does not contain both
   the length field and the entire message).  Such behavior is certainly
   undesirable.  As described in Section 6.2.3, servers SHOULD apply
   connection timeouts to the receipt of a full message and MUST NOT
   close a connection simply because the first "read" from the TCP layer
   does not contain the entire message.

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9.  TCP Fast Open

   This section is non-normative.

   TCP Fast Open [RFC7413] (TFO) allows data to be carried in the SYN
   packet, reducing the cost of re-opening TCP connections.  It also
   saves up to one RTT compared to standard TCP.

   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.

   DNS services taking advantage of IP anycast [RFC4786] might need to
   take additional steps when enabling TFO.  From [RFC7413]:

      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.

10.  IANA Considerations

   This memo includes no request to IANA.

11.  Security Considerations

   Some DNS server operators have expressed concern that wider promotion
   and use of DNS over TCP will expose them to a higher risk of denial-
   of-service (DoS) attacks on TCP (both accidental and deliberate).

   Although there is a higher risk of some specific attacks against TCP-
   enabled servers, techniques for the mitigation of DoS attacks at the
   network level have improved substantially since DNS was first

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   Readers are advised to familiarise themselves with [CPNI-TCP], a
   security assessment of TCP detailing known TCP attacks and
   countermeasures which references most of the relevant RFCs on this

   To mitigate the risk of DoS attacks, DNS servers are advised to
   engage in TCP connection management.  This could include maintaining
   state on existing connections, re-using existing connections and
   controlling request queues to enable fair use.  It is likely to be
   advantageous to provide configurable connection management options,
   for example:

   o  total number of TCP connections

   o  maximum TCP connections per source IP address or subnet

   o  TCP connection idle timeout

   o  maximum DNS transactions per TCP connection

   o  maximum TCP connection duration

   No specific values are recommended for these parameters.

   Operators are advised to familiarise themselves with the
   configuration and tuning parameters available in the operating system
   TCP stack.  However detailed advice on this is outside the scope of
   this document.

   Operators of recursive servers are advised to ensure that they only
   accept connections from expected clients (for example by the use of
   an ACL), and do not accept them from unknown sources.  In the case of
   UDP traffic, this will help protect against reflection 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 Francis Dupont and Paul Vixie for
   detailed review, Andrew Sullivan, Tony Finch, Stephane Bortzmeyer,
   Joe Abley, Tatuya Jinmei and the many others who contributed to the
   mailing list discussion.  Also Liang Zhu, Zi Hu, and John Heidemann
   for extensive DNS-over-TCP discussions and code.  Lucie Guiraud and
   Danny McPherson for reviewing early versions of this document.  We
   would also like to thank all those who contributed to RFC5966.

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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.

   [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.

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   [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, "Connection-Oriented DNS to Improve
              Privacy and Security",

   [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
              for Application Designers", BCP 145, RFC 5405, DOI
              10.17487/RFC5405, November 2008,

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

   [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
              Fast Open", RFC 7413, December 2014.

   [RRL]      Vixie, P. and V. Schryver, "DNS Response Rate Limiting
              (DNS RRL)", ISC-TN 2012-1-Draft1, April 2012.

              Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
              edns-tcp-keepalive EDNS0 Option", draft-ietf-dnsop-edns-
              tcp-keepalive-04 (work in progress), Oct 2015.

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

Appendix A.  Summary of Advantages and Disadvantages to using TCP for

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

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   IP fragmentation is less of a problem for TCP than it is for UDP.
   TCP stacks generally implement Path MTU Discovery so they can avoid
   IP fragmentation of TCP segments.  UDP, on the other hand, does not
   provide reassembly, which means datagrams that exceed the path MTU
   size must experience fragmentation [RFC5405].  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 might be disrupted due
   to routing changes.  Clients utilizing TCP for DNS need to always be
   prepared to re-establish connections or otherwise retry outstanding
   queries.  It might also be 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 support for it is a requirement for all

   A more in-depth discussion of connection orientated DNS can be found
   elsewhere [Connection-Oriented-DNS].

Appendix B.  Changes between revisions

   [Note to RFC Editor: please remove this section prior to

B.1.  Changes -03 to -04

   o  Re-stated how messages received over TCP should be mapped to

   o  Added wording to cover timeouts for server side behaviour for when
      receiving TCP messages.

   o  Added sentence to abstract stating this obsoletes RFC5966.

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   o  Moved reference to RFC6891 earlier in Discussion section.

   o  Several minor wording updates to improve clarity.

   o  Corrected nits and updated references.

B.2.  Changes -02 to -03

   o  Replaced certain lower case RFC2119 keywords to improve clarity.

   o  Updated section 6.2.2 to recognise requirements for concurrent
      zone transfers.

   o  Changed 'client IP address' to 'client IP address or subnet' when
      discussing restrictions on TCP connections from clients.

   o  Added reference to edns-tcp-keepalive draft.

   o  Added wording to introduction to reference Appendix A and state
      TCP is a valid transport alternative for DNS.

   o  Improved description of CPNI-TCP as a general reference source on
      TCP security related RFCs.

B.3.  Changes -01 to -02

   o  Added more text to Introduction as background to TCP use.

   o  Added definitions of Persistent connection and Idle session to
      Terminology section.

   o  Separated Connection Handling section into Current Practice and
      Recommendations.  Provide more detail on current practices and
      divided Recommendations up into more granular sub-sections.

   o  Add section on Idle time with new text on recommendations for
      client idle behaviour.

   o  Move TCP message field length discussion to separate section.

   o  Removed references to system calls in TFO section.

   o  Added more discussion on DoS mitigation in Security Considerations

   o  Added statement that servers MAY use 0 idle timeout.

   o  Re-stated position of TCP as an alternative to UDP in Discussion.

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   o  Updated text on server limits on concurrent connections from a
      particular client.

   o  Added text that client retry logic is outside the scope of this

   o  Clarified that servers should answer all pipelined queries even if
      sent very close together.

B.4.  Changes -00 to -01

   o  Changed updates to obsoletes RFC 5966.

   o  Improved text in Section 4 Transport Protocol Selection to change
      "TCP SHOULD NOT be used only for the transfers and as a fallback"
      to make the intention clearer and more consistent.

   o  Reference to TCP FASTOPEN updated now that it is an RFC.

   o  Added paragraph to say that implementations MUST NOT send the TCP
      framing 2 byte length field in a separate packet to the DNS

   o  Added Terminology section.

   o  Changed should and RECOMMENDED in reference to parallel processing
      to SHOULD in sections 7 and 8.

   o  Added text to address what a server should do when a client closes
      the TCP connection before pending responses are sent.

   o  Moved the Advantages and Disadvantages section to an appendix.

B.5.  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

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Authors' Addresses

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


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


   Ray Bellis
   Internet Systems Consortium, Inc
   950 Charter Street
   Redwood City  CA  94063

   Phone: +1 650 423 1200

   Allison Mankin
   Verisign Labs
   12061 Bluemont Way
   Reston, VA  20190

   Phone: +1 703 948-3200

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   Duane Wessels
   Verisign Labs
   12061 Bluemont Way
   Reston, VA  20190

   Phone: +1 703 948-3200

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