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rfc:rfc7766

Internet Engineering Task Force (IETF) J. Dickinson Request for Comments: 7766 S. Dickinson Obsoletes: 5966 Sinodun Updates: 1035, 1123 R. Bellis Category: Standards Track ISC ISSN: 2070-1721 A. Mankin

                                                            D. Wessels
                                                         Verisign Labs
                                                            March 2016
        DNS Transport over TCP - Implementation Requirements

Abstract

 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 RFC 5966 and therefore updates RFC 1035 and
 RFC 1123.

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 in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7766.

Dickinson, et al. Standards Track [Page 1] RFC 7766 DNS over TCP March 2016

Copyright Notice

 Copyright (c) 2016 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
 (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.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Requirements Terminology  . . . . . . . . . . . . . . . . . .   4
 3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
 4.  Discussion  . . . . . . . . . . . . . . . . . . . . . . . . .   4
 5.  Transport Protocol Selection  . . . . . . . . . . . . . . . .   5
 6.  Connection Handling . . . . . . . . . . . . . . . . . . . . .   6
   6.1.  Current Practices . . . . . . . . . . . . . . . . . . . .   6
     6.1.1.  Clients . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.2.  Servers . . . . . . . . . . . . . . . . . . . . . . .   7
   6.2.  Recommendations . . . . . . . . . . . . . . . . . . . . .   8
     6.2.1.  Connection Reuse  . . . . . . . . . . . . . . . . . .   8
       6.2.1.1.  Query Pipelining  . . . . . . . . . . . . . . . .   8
     6.2.2.  Concurrent Connections  . . . . . . . . . . . . . . .   9
     6.2.3.  Idle Timeouts . . . . . . . . . . . . . . . . . . . .   9
     6.2.4.  Teardown  . . . . . . . . . . . . . . . . . . . . . .  10
 7.  Response Reordering . . . . . . . . . . . . . . . . . . . . .  10
 8.  TCP Message Length Field  . . . . . . . . . . . . . . . . . .  11
 9.  TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . . .  11
 10. Security Considerations . . . . . . . . . . . . . . . . . . .  12
 11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
   11.1.  Normative References . . . . . . . . . . . . . . . . . .  13
   11.2.  Informative References . . . . . . . . . . . . . . . . .  14
 Appendix A.  Summary of Advantages and Disadvantages to Using TCP
              for DNS  . . . . . . . . . . . . . . . . . . . . . .  16
 Appendix B.  Changes to RFC 5966  . . . . . . . . . . . . . . . .  16
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  17
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  18

Dickinson, et al. Standards Track [Page 2] RFC 7766 DNS over TCP March 2016

1. Introduction

 Most DNS [RFC1034] transactions take place over UDP [RFC768].  TCP
 [RFC793] is always used for full zone transfers (using AXFR) and is
 often used for messages whose sizes exceed the DNS protocol's
 original 512-byte limit.  The growing deployment of DNS Security
 (DNSSEC) and IPv6 has increased response sizes and therefore the use
 of TCP.  The need for 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 [RRL1] [RRL2].  Additionally,
 recent work on DNS privacy solutions such as [DNS-over-TLS] is
 another motivation to revisit DNS-over-TCP requirements.
 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 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) will probably result
 in resolution failure and/or application-level timeouts.

Dickinson, et al. Standards Track [Page 3] RFC 7766 DNS over TCP March 2016

2. Requirements Terminology

 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. Terminology

 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
    connection.
 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 that needs 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.

Dickinson, et al. Standards Track [Page 4] RFC 7766 DNS over TCP March 2016

 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 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 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

 Section 6.1.3.2 of [RFC1123] is updated: 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.

Dickinson, et al. Standards Track [Page 5] RFC 7766 DNS over TCP March 2016

 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
    servers.
 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.
 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 the resolver 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 retry
 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:
  1. 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.
  1. 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.

Dickinson, et al. Standards Track [Page 6] RFC 7766 DNS over TCP March 2016

 Other more modern protocols (e.g., HTTP/1.1 [RFC7230], HTTP/2
 [RFC7540]) 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
 recommendations with regard to DNS client idle timeouts.  However, at
 the time of writing, 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
 in 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
    load.
 o  Intentionally opening many connections and leaving them idle can
    trivially create a TCP denial of service (DoS) 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 DoS problem.
 Note that this DoS is only on the TCP service.  However, in these
 cases, it affects not only clients that wish to use TCP for their
 queries for operational reasons, but all clients that choose to fall
 back to TCP from UDP after receiving a TC=1 flag.

Dickinson, et al. Standards Track [Page 7] RFC 7766 DNS over TCP March 2016

6.2. Recommendations

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

6.2.1. Connection Reuse

 One perceived disadvantage to DNS over TCP is the added connection
 setup latency, generally equal to one RTT.  To amortise connection
 setup costs, both clients and servers SHOULD support connection reuse
 by sending multiple queries and responses over a single persistent
 TCP connection.
 When sending multiple queries over a TCP connection, clients MUST NOT
 reuse the DNS Message ID of an in-flight query on that connection in
 order to avoid Message ID collisions.  This is especially important
 if the server could be performing out-of-order processing (see
 Section 7).

6.2.1.1. 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
 algorithms.
 DNS servers (especially recursive) MUST 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 received in quick succession.  The handling of
 responses to pipelined queries is covered in Section 7.

Dickinson, et al. Standards Track [Page 8] RFC 7766 DNS over TCP March 2016

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).  However, it is 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
 (for example, to support concurrent transfers of multiple zones).
 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, is multiple resolvers
 on a single machine, or is 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 minimise 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,
 [edns-tcp-keepalive].
 To mitigate the risk of unintentional server overload, it is
 RECOMMENDED that the default server application-level idle period be
 on 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 they are
 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 reset the idle timeout on the receipt of a
 full DNS message, rather than on receipt of any part of a DNS
 message.

Dickinson, et al. Standards Track [Page 9] RFC 7766 DNS over TCP March 2016

6.2.4. Teardown

 Under normal operation DNS clients typically initiate connection
 closing on idle connections; however, DNS servers can close the
 connection if the idle timeout set by local policy is exceeded.
 Also, connections can be closed by either end under unusual
 conditions such as defending against an attack or system failure/
 reboot.
 DNS 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 DNS server finds that a DNS 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 DNS server MAY cache those
 responses.

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 than 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.

Dickinson, et al. Standards Track [Page 10] RFC 7766 DNS over TCP March 2016

8. TCP Message Length Field

 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 is for reasons of both efficiency and to avoid problems due to
 some DNS server implementations behaving undesirably when reading
 data from the TCP layer (due to a lack of clarity in previous
 documents).  For example, some DNS server implementations might abort
 a TCP session if the first "read" from the TCP layer does not contain
 both the length field and the entire message.
 To clarify, DNS servers MUST NOT close a connection simply because
 the first "read" from the TCP layer does not contain the entire DNS
 message, and servers SHOULD apply the connection timeouts as
 specified in Section 6.2.3.

9. TCP Fast Open

 This section is non-normative.
 TCP Fast Open (TFO) [RFC7413] allows data to be carried in the SYN
 packet, reducing the cost of reopening 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 behind load balancers 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 the regular
    3WHS.

Dickinson, et al. Standards Track [Page 11] RFC 7766 DNS over TCP March 2016

 When DNS-over-TCP is a transport for DNS private exchange, as in
 [DNS-over-TLS], the implementor needs to be aware of TFO and to
 ensure that data requiring protection (e.g. data for a DNS query) is
 not accidentally transported in the clear.  See [DNS-over-TLS] for
 discussion.

10. 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 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
 designed.
 Readers are advised to familiarise themselves with [CPNI-TCP], a
 security assessment of TCP that details known TCP attacks and
 countermeasures and that references most of the relevant RFCs on this
 topic.
 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, reusing 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 TCP stack of the
 operating system.  However, detailed advice on this is outside the
 scope of this document.

Dickinson, et al. Standards Track [Page 12] RFC 7766 DNS over TCP March 2016

 Operators of recursive servers are advised to ensure that they only
 accept connections from expected clients (for example, by the use of
 an Access Control List (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.

11. References

11.1. Normative References

 [RFC768]   Postel, J., "User Datagram Protocol", STD 6, RFC 768,
            DOI 10.17487/RFC0768, August 1980,
            <http://www.rfc-editor.org/info/rfc768>.
 [RFC793]   Postel, J., "Transmission Control Protocol", STD 7,
            RFC 793, DOI 10.17487/RFC0793, September 1981,
            <http://www.rfc-editor.org/info/rfc793>.
 [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
            STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
            <http://www.rfc-editor.org/info/rfc1034>.
 [RFC1035]  Mockapetris, P., "Domain names - implementation and
            specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
            November 1987, <http://www.rfc-editor.org/info/rfc1035>.
 [RFC1123]  Braden, R., Ed., "Requirements for Internet Hosts -
            Application and Support", STD 3, RFC 1123,
            DOI 10.17487/RFC1123, October 1989,
            <http://www.rfc-editor.org/info/rfc1123>.
 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC4033]  Arends, R., Austein, R., Larson, M., Massey, D., and S.
            Rose, "DNS Security Introduction and Requirements",
            RFC 4033, DOI 10.17487/RFC4033, March 2005,
            <http://www.rfc-editor.org/info/rfc4033>.
 [RFC4786]  Abley, J. and K. Lindqvist, "Operation of Anycast
            Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
            December 2006, <http://www.rfc-editor.org/info/rfc4786>.

Dickinson, et al. Standards Track [Page 13] RFC 7766 DNS over TCP March 2016

 [RFC5155]  Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
            Security (DNSSEC) Hashed Authenticated Denial of
            Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
            <http://www.rfc-editor.org/info/rfc5155>.
 [RFC5358]  Damas, J. and F. Neves, "Preventing Use of Recursive
            Nameservers in Reflector Attacks", BCP 140, RFC 5358,
            DOI 10.17487/RFC5358, October 2008,
            <http://www.rfc-editor.org/info/rfc5358>.
 [RFC5625]  Bellis, R., "DNS Proxy Implementation Guidelines",
            BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009,
            <http://www.rfc-editor.org/info/rfc5625>.
 [RFC5966]  Bellis, R., "DNS Transport over TCP - Implementation
            Requirements", RFC 5966, DOI 10.17487/RFC5966, August
            2010, <http://www.rfc-editor.org/info/rfc5966>.
 [RFC6891]  Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
            for DNS (EDNS(0))", STD 75, RFC 6891,
            DOI 10.17487/RFC6891, April 2013,
            <http://www.rfc-editor.org/info/rfc6891>.
 [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
            Protocol (HTTP/1.1): Message Syntax and Routing",
            RFC 7230, DOI 10.17487/RFC7230, June 2014,
            <http://www.rfc-editor.org/info/rfc7230>.
 [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
            Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
            DOI 10.17487/RFC7540, May 2015,
            <http://www.rfc-editor.org/info/rfc7540>.

11.2. Informative References

 [Connection-Oriented-DNS]
            Zhu, L., Hu, Z., Heidemann, J., Wessels, D., Mankin, A.,
            and N. Somaiya, "Connection-Oriented DNS to Improve
            Privacy and Security", 2015 IEEE Symposium on Security and
            Privacy (SP), DOI 10.1109/SP.2015.18,
            <http://ieeexplore.ieee.org/xpl/
            articleDetails.jsp?arnumber=7163025>.
 [CPNI-TCP]
            CPNI, "Security Assessment of the Transmission Control
            Protocol (TCP)", 2009, <http://www.gont.com.ar/papers/
            tn-03-09-security-assessment-TCP.pdf>.

Dickinson, et al. Standards Track [Page 14] RFC 7766 DNS over TCP March 2016

 [DNS-over-TLS]
            Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
            and P. Hoffman, "Specification for DNS over TLS", Work in
            Progress, draft-ietf-dprive-dns-over-tls-06, February
            2016.
 [edns-tcp-keepalive]
            Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
            edns-tcp-keepalive EDNS0 Option", Work in Progress,
            draft-ietf-dnsop-edns-tcp-keepalive-03, September 2015.
 [fragmentation-considered-poisonous]
            Herzberg, A. and H. Shulman, "Fragmentation Considered
            Poisonous", May 2012, <http://arxiv.org/abs/1205.4011>.
 [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
            for Application Designers", BCP 145, RFC 5405,
            DOI 10.17487/RFC5405, November 2008,
            <http://www.rfc-editor.org/info/rfc5405>.
 [RFC6824]  Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
            "TCP Extensions for Multipath Operation with Multiple
            Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
            <http://www.rfc-editor.org/info/rfc6824>.
 [RFC7413]  Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
            Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
            <http://www.rfc-editor.org/info/rfc7413>.
 [RRL1]     Vixie, P. and V. Schryver, "DNS Response Rate Limiting
            (DNS RRL)", ISC-TN 2012-1-Draft1, April 2012,
            <https://ftp.isc.org/isc/pubs/tn/isc-tn-2012-1.txt>.
 [RRL2]     ISC Support, "Using the Response Rate Limiting Feature in
            BIND 9.10", ISC Knowledge Base AA-00994, June 2013,
            <https://kb.isc.org/article/AA-00994/>.

Dickinson, et al. Standards Track [Page 15] RFC 7766 DNS over TCP March 2016

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

           DNS
 The TCP handshake generally prevents address spoofing and, therefore,
 the reflection/amplification attacks that plague UDP.
 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; this 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 amortised 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
 reopening 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 Multipath TCP [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
 implementations.
 A more in-depth discussion of connection-oriented DNS can be found
 elsewhere [Connection-Oriented-DNS].

Appendix B. Changes to RFC 5966

 This document obsoletes [RFC5966] and differs from it in several
 respects.  An overview of the most substantial changes/updates that
 implementors should take note of is given below.
 1.   A Terminology section (Section 3) is added defining several new
      concepts.

Dickinson, et al. Standards Track [Page 16] RFC 7766 DNS over TCP March 2016

 2.   Paragraph 3 of Section 5 puts TCP on a more equal footing with
      UDP than RFC 5966 does.  For example, it states:
      1.  TCP MAY be used before sending any UDP queries.
      2.  TCP ought to be considered a valid alternative transport to
          UDP, not purely a fallback option.
 3.   Section 6.2.1 adds a new recommendation that TCP connection
      reuse SHOULD be supported.
 4.   Section 6.2.1.1 adds a new recommendation that DNS clients
      SHOULD pipeline their queries and DNS servers SHOULD process
      pipelined queries concurrently.
 5.   Section 6.2.2 adds new recommendations on the number and usage
      of TCP connections for client/server interactions.
 6.   Section 6.2.3 adds a new recommendation that DNS clients SHOULD
      close idle sessions unless using a signalling mechanism.
 7.   Section 7 clarifies that servers are RECOMMENDED to prepare TCP
      responses in parallel and send answers out of order.  It also
      clarifies how TCP queries and responses should be matched by
      clients.
 8.   Section 8 adds a new recommendation about how DNS clients and
      servers should handle the 2-byte message length field for TCP
      messages.
 9.   Section 9 adds a non-normative discussion of the use of TCP Fast
      Open.
 10.  Section 10 adds new advice regarding DoS mitigation techniques.

Acknowledgements

 The authors would like to thank Francis Dupont and Paul Vixie for
 their detailed reviews, as well as Andrew Sullivan, Tony Finch,
 Stephane Bortzmeyer, Joe Abley, Tatuya Jinmei, and the many others
 who contributed to the mailing list discussion.  Also, the authors
 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 draft versions of this document.  We would also like
 to thank all those who contributed to RFC 5966.

Dickinson, et al. Standards Track [Page 17] RFC 7766 DNS over TCP March 2016

Authors' Addresses

 John Dickinson
 Sinodun Internet Technologies
 Magdalen Centre
 Oxford Science Park
 Oxford  OX4 4GA
 United Kingdom
 Email: jad@sinodun.com
 URI:   http://sinodun.com
 Sara Dickinson
 Sinodun Internet Technologies
 Magdalen Centre
 Oxford Science Park
 Oxford  OX4 4GA
 United Kingdom
 Email: sara@sinodun.com
 URI:   http://sinodun.com
 Ray Bellis
 Internet Systems Consortium, Inc
 950 Charter Street
 Redwood City, CA  94063
 United States
 Phone: +1 650 423 1200
 Email: ray@isc.org
 URI:   http://www.isc.org
 Allison Mankin
 Verisign Labs
 12061 Bluemont Way
 Reston, VA  20190
 United States
 Phone: +1 301 728 7198
 Email: allison.mankin@gmail.com

Dickinson, et al. Standards Track [Page 18] RFC 7766 DNS over TCP March 2016

 Duane Wessels
 Verisign Labs
 12061 Bluemont Way
 Reston, VA  20190
 United States
 Phone: +1 703 948 3200
 Email: dwessels@verisign.com

Dickinson, et al. Standards Track [Page 19]

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