draft-ietf-dnsop-5966bis-05.txt   draft-ietf-dnsop-5966bis-06.txt 
dnsop J. Dickinson dnsop J. Dickinson
Internet-Draft S. Dickinson Internet-Draft S. Dickinson
Obsoletes: 5966 (if approved) Sinodun Obsoletes: 5966 (if approved) Sinodun
Updates: 1035,1123 (if approved) R. Bellis Updates: 1035,1123 (if approved) R. Bellis
Intended status: Standards Track ISC Intended status: Standards Track ISC
Expires: June 19, 2016 A. Mankin Expires: July 18, 2016 A. Mankin
D. Wessels D. Wessels
Verisign Labs Verisign Labs
December 17, 2015 January 15, 2016
DNS Transport over TCP - Implementation Requirements DNS Transport over TCP - Implementation Requirements
draft-ietf-dnsop-5966bis-05 draft-ietf-dnsop-5966bis-06
Abstract Abstract
This document specifies the requirement for support of TCP as a This document specifies the requirement for support of TCP as a
transport protocol for DNS implementations and provides guidelines transport protocol for DNS implementations and provides guidelines
towards DNS-over-TCP performance on par with that of DNS-over-UDP. towards DNS-over-TCP performance on par with that of DNS-over-UDP.
This document obsoletes RFC5966 and therefore updates RFC1035 and This document obsoletes RFC5966 and therefore updates RFC1035 and
RFC1123. RFC1123.
Status of This Memo Status of This Memo
skipping to change at page 1, line 39 skipping to change at page 1, line 39
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on June 19, 2016. This Internet-Draft will expire on July 18, 2016.
Copyright Notice Copyright Notice
Copyright (c) 2015 IETF Trust and the persons identified as the Copyright (c) 2016 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Requirements Terminology . . . . . . . . . . . . . . . . . . 3 2. Requirements Terminology . . . . . . . . . . . . . . . . . . 4
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4 3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 4 4. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . 4
5. Transport Protocol Selection . . . . . . . . . . . . . . . . 5 5. Transport Protocol Selection . . . . . . . . . . . . . . . . 5
6. Connection Handling . . . . . . . . . . . . . . . . . . . . . 6 6. Connection Handling . . . . . . . . . . . . . . . . . . . . . 6
6.1. Current practices . . . . . . . . . . . . . . . . . . . . 6 6.1. Current practices . . . . . . . . . . . . . . . . . . . . 6
6.1.1. Clients . . . . . . . . . . . . . . . . . . . . . . . 7 6.1.1. Clients . . . . . . . . . . . . . . . . . . . . . . . 7
6.1.2. Servers . . . . . . . . . . . . . . . . . . . . . . . 7 6.1.2. Servers . . . . . . . . . . . . . . . . . . . . . . . 7
6.2. Recommendations . . . . . . . . . . . . . . . . . . . . . 7 6.2. Recommendations . . . . . . . . . . . . . . . . . . . . . 7
6.2.1. Connection Re-use . . . . . . . . . . . . . . . . . . 7 6.2.1. Connection Re-use . . . . . . . . . . . . . . . . . . 8
6.2.1.1. Query Pipelining . . . . . . . . . . . . . . . . 8 6.2.1.1. Query Pipelining . . . . . . . . . . . . . . . . 8
6.2.2. Concurrent connections . . . . . . . . . . . . . . . 8 6.2.2. Concurrent connections . . . . . . . . . . . . . . . 8
6.2.3. Idle Timeouts . . . . . . . . . . . . . . . . . . . . 9 6.2.3. Idle Timeouts . . . . . . . . . . . . . . . . . . . . 9
6.2.4. Tear Down . . . . . . . . . . . . . . . . . . . . . . 9 6.2.4. Tear Down . . . . . . . . . . . . . . . . . . . . . . 9
7. Response Reordering . . . . . . . . . . . . . . . . . . . . . 10 7. Response Reordering . . . . . . . . . . . . . . . . . . . . . 10
8. TCP Message Length Field . . . . . . . . . . . . . . . . . . 10 8. TCP Message Length Field . . . . . . . . . . . . . . . . . . 10
9. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . . . 11 9. TCP Fast Open . . . . . . . . . . . . . . . . . . . . . . . . 11
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11 10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
11. Security Considerations . . . . . . . . . . . . . . . . . . . 11 11. Security Considerations . . . . . . . . . . . . . . . . . . . 12
12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12 12. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 13
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 13 13. References . . . . . . . . . . . . . . . . . . . . . . . . . 13
13.1. Normative References . . . . . . . . . . . . . . . . . . 13 13.1. Normative References . . . . . . . . . . . . . . . . . . 13
13.2. Informative References . . . . . . . . . . . . . . . . . 14 13.2. Informative References . . . . . . . . . . . . . . . . . 14
Appendix A. Summary of Advantages and Disadvantages to using TCP Appendix A. Summary of Advantages and Disadvantages to using TCP
for DNS . . . . . . . . . . . . . . . . . . . . . . 15 for DNS . . . . . . . . . . . . . . . . . . . . . . 15
Appendix B. Changes between revisions . . . . . . . . . . . . . 16 Appendix B. Changes between revisions . . . . . . . . . . . . . 16
B.1. Changes -03 to -04 . . . . . . . . . . . . . . . . . . . 16 B.1. Changes -05 to -06 . . . . . . . . . . . . . . . . . . . 16
B.2. Changes -02 to -03 . . . . . . . . . . . . . . . . . . . 16 B.2. Changes -04 to -05 . . . . . . . . . . . . . . . . . . . 17
B.3. Changes -01 to -02 . . . . . . . . . . . . . . . . . . . 17 B.3. Changes -03 to -04 . . . . . . . . . . . . . . . . . . . 17
B.4. Changes -00 to -01 . . . . . . . . . . . . . . . . . . . 17 B.4. Changes -02 to -03 . . . . . . . . . . . . . . . . . . . 18
Appendix C. Changes to RFC5966 . . . . . . . . . . . . . . . . . 18 B.5. Changes -01 to -02 . . . . . . . . . . . . . . . . . . . 18
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 19 B.6. Changes -00 to -01 . . . . . . . . . . . . . . . . . . . 19
Appendix C. Changes to RFC5966 . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 20
1. Introduction 1. Introduction
Most DNS [RFC1034] transactions take place over UDP [RFC0768]. TCP Most DNS [RFC1034] transactions take place over UDP [RFC0768]. TCP
[RFC0793] is always used for full zone transfers (AXFR) and is often [RFC0793] is always used for full zone transfers (AXFR) and is often
used for messages whose sizes exceed the DNS protocol's original used for messages whose sizes exceed the DNS protocol's original
512-byte limit. The growing deployment of DNSSEC and IPv6 has 512-byte limit. The growing deployment of DNSSEC and IPv6 has
increased response sizes and therefore the use of TCP. The need for increased response sizes and therefore the use of TCP. The need for
increased TCP use has also been driven by the protection it provides increased TCP use has also been driven by the protection it provides
against address spoofing and therefore exploitation of DNS in against address spoofing and therefore exploitation of DNS in
reflection/amplification attacks. It is now widely used in Response reflection/amplification attacks. It is now widely used in Response
Rate Limiting [RRL1][RRL2]. Rate Limiting [RRL1][RRL2]. Additionally, recent work on DNS privacy
solutions such as [DNS-over-TLS] is another motivation to re-visit
DNS-over-TCP requirements.
Section 6.1.3.2 of [RFC1123] states: Section 6.1.3.2 of [RFC1123] states:
DNS resolvers and recursive servers MUST support UDP, and SHOULD DNS resolvers and recursive servers MUST support UDP, and SHOULD
support TCP, for sending (non-zone-transfer) queries. support TCP, for sending (non-zone-transfer) queries.
However, some implementors have taken the text quoted above to mean However, some implementors have taken the text quoted above to mean
that TCP support is an optional feature of the DNS protocol. that TCP support is an optional feature of the DNS protocol.
The majority of DNS server operators already support TCP and the The majority of DNS server operators already support TCP and the
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Regarding the choice of when to use UDP or TCP, Section 6.1.3.2 of Regarding the choice of when to use UDP or TCP, Section 6.1.3.2 of
RFC 1123 also says: RFC 1123 also says:
... a DNS resolver or server that is sending a non-zone-transfer ... a DNS resolver or server that is sending a non-zone-transfer
query MUST send a UDP query first. query MUST send a UDP query first.
This requirement is hereby relaxed. Stub resolvers and recursive This requirement is hereby relaxed. Stub resolvers and recursive
resolvers MAY elect to send either TCP or UDP queries depending on resolvers MAY elect to send either TCP or UDP queries depending on
local operational reasons. TCP MAY be used before sending any UDP local operational reasons. TCP MAY be used before sending any UDP
queries. If it already has an open TCP connection to the server it queries. If the resolver already has an open TCP connection to the
SHOULD reuse this connection. In essence, TCP ought to be considered server it SHOULD reuse this connection. In essence, TCP ought to be
a valid alternative transport to UDP, not purely a fallback option. considered a valid alternative transport to UDP, not purely a retry
option.
In addition it is noted that all Recursive and Authoritative servers In addition it is noted that all Recursive and Authoritative servers
MUST send responses using the same transport as the query arrived on. MUST send responses using the same transport as the query arrived on.
In the case of TCP this MUST also be the same connection. In the case of TCP this MUST also be the same connection.
6. Connection Handling 6. Connection Handling
6.1. Current practices 6.1. Current practices
Section 4.2.2 of [RFC1035] says: Section 4.2.2 of [RFC1035] says:
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connections as persistent (particularly after receiving an SOA), but connections as persistent (particularly after receiving an SOA), but
unfortunately does not provide enough detail for an unambiguous unfortunately does not provide enough detail for an unambiguous
interpretation of client behaviour for queries other than a SOA. interpretation of client behaviour for queries other than a SOA.
Additionally, DNS does not yet have a signalling mechanism for Additionally, DNS does not yet have a signalling mechanism for
connection timeout or close, although some have been proposed. connection timeout or close, although some have been proposed.
6.1.1. Clients 6.1.1. Clients
There is no clear guidance today in any RFC as to when a DNS client 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 should close a TCP connection, and there are no specific
recommendations with regard to DNS client idle timeouts. However it recommendations with regard to DNS client idle timeouts. However, at
is common practice for clients to close the TCP connection after the time of writing, it is common practice for clients to close the
sending a single request (apart from the SOA/AXFR case). TCP connection after sending a single request (apart from the SOA/
AXFR case).
6.1.2. Servers 6.1.2. Servers
Many DNS server implementations use a long fixed idle timeout and Many DNS server implementations use a long fixed idle timeout and
default to a small number of TCP connections. They also offer little default to a small number of TCP connections. They also offer little
by the way of TCP connection management options. The disadvantages by the way of TCP connection management options. The disadvantages
of this include: of this include:
o Operational experience has shown that long server timeouts can o Operational experience has shown that long server timeouts can
easily cause resource exhaustion and poor response under heavy easily cause resource exhaustion and poor response under heavy
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the next query. Clients SHOULD treat TCP and UDP equivalently when the next query. Clients SHOULD treat TCP and UDP equivalently when
considering the time at which to send a particular query. considering the time at which to send a particular query.
It is likely that DNS servers need to process pipelined queries It is likely that DNS servers need to process pipelined queries
concurrently and also send out-of-order responses over TCP in order concurrently and also send out-of-order responses over TCP in order
to provide the level of performance possible with UDP transport. If to provide the level of performance possible with UDP transport. If
TCP performance is of importance, clients might find it useful to use TCP performance is of importance, clients might find it useful to use
server processing times as input to server and transport selection server processing times as input to server and transport selection
algorithms. algorithms.
DNS servers (especially recursive) SHOULD expect to receive pipelined DNS servers (especially recursive) MUST expect to receive pipelined
queries. The server SHOULD process TCP queries concurrently, just as queries. The server SHOULD process TCP queries concurrently, just as
it would for UDP. The server SHOULD answer all pipelined queries, it would for UDP. The server SHOULD answer all pipelined queries,
even if they are received in quick succession. The handling of even if they are received in quick succession. The handling of
responses to pipelined queries is covered in Section 7. responses to pipelined queries is covered in Section 7.
6.2.2. Concurrent connections 6.2.2. Concurrent connections
To mitigate the risk of unintentional server overload, DNS clients To mitigate the risk of unintentional server overload, DNS clients
MUST take care to minimize the number of concurrent TCP connections MUST take care to minimize the number of concurrent TCP connections
made to any individual server. It is RECOMMENDED that for any given made to any individual server. It is RECOMMENDED that for any given
client/server interaction there SHOULD be no more than one connection client/server interaction there SHOULD be no more than one connection
for regular queries, one for zone transfers and one for each protocol 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 that is being used on top of TCP, for example, if the resolver was
using TLS. It is however noted that certain primary/secondary using TLS. It is however noted that certain primary/secondary
configurations with many busy zones might need to use more than one configurations with many busy zones might need to use more than one
TCP connection for zone transfers for operational reasons. 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 Similarly, servers MAY impose limits on the number of concurrent TCP
connections being handled for any particular client IP address or connections being handled for any particular client IP address or
subnet. These limits SHOULD be much looser than the client subnet. These limits SHOULD be much looser than the client
guidelines above, because the server does not know, for example, if a guidelines above, because the server does not know, for example, if a
client IP address belongs to a single client or is multiple resolvers client IP address belongs to a single client or is multiple resolvers
on a single machine, or multiple clients behind a device performing on a single machine, or multiple clients behind a device performing
Network Address Translation (NAT). Network Address Translation (NAT).
6.2.3. Idle Timeouts 6.2.3. Idle Timeouts
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properly match responses to outstanding queries can have serious properly match responses to outstanding queries can have serious
consequences for interoperability. consequences for interoperability.
8. TCP Message Length Field 8. TCP Message Length Field
DNS clients and servers SHOULD pass the two-octet length field, and 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 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 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. likely that all the data will be transmitted in a single TCP segment.
This is both for reasons of efficiency and to avoid problems due to This is both for reasons of efficiency and to avoid problems due to
some DNS server implementations behaving undesirably when processing some DNS server implementations behaving undesirably when reading
TCP segments (due to a lack of clarity in previous standards). For data from the TCP layer (due to a lack of clarity in previous
example, some DNS server implementations might abort a TCP session if standards). For example, some DNS server implementations might abort
the first TCP segment does not contain both the length field and the a TCP session if the first "read" from the TCP layer does not contain
entire message. both the length field and the entire message.
To clarify, DNS servers MUST NOT close a connection simply because To clarify, DNS servers MUST NOT close a connection simply because
the first "read" from the TCP layer does not contain the entire DNS the first "read" from the TCP layer does not contain the entire DNS
message, and servers SHOULD apply the connection timeouts as message, and servers SHOULD apply the connection timeouts as
specified in Section 6.2.3. specified in Section 6.2.3.
9. TCP Fast Open 9. TCP Fast Open
This section is non-normative. This section is non-normative.
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DNS services taking advantage of IP anycast [RFC4786] might need to DNS services taking advantage of IP anycast [RFC4786] might need to
take additional steps when enabling TFO. From [RFC7413]: take additional steps when enabling TFO. From [RFC7413]:
Servers that accept connection requests to the same server IP Servers that accept connection requests to the same server IP
address should use the same key such that they generate identical address should use the same key such that they generate identical
Fast Open Cookies for a particular client IP address. Otherwise a Fast Open Cookies for a particular client IP address. Otherwise a
client may get different cookies across connections; its Fast Open client may get different cookies across connections; its Fast Open
attempts would fall back to regular 3WHS. attempts would fall back to regular 3WHS.
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. IANA Considerations 10. IANA Considerations
This memo includes no request to IANA. This memo includes no request to IANA.
11. Security Considerations 11. Security Considerations
Some DNS server operators have expressed concern that wider promotion 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- 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). of-service (DoS) attacks on TCP (both accidental and deliberate).
skipping to change at page 13, line 9 skipping to change at page 13, line 23
Joe Abley, Tatuya Jinmei and the many others who contributed to the Joe Abley, Tatuya Jinmei and the many others who contributed to the
mailing list discussion. Also Liang Zhu, Zi Hu, and John Heidemann mailing list discussion. Also Liang Zhu, Zi Hu, and John Heidemann
for extensive DNS-over-TCP discussions and code. Lucie Guiraud and for extensive DNS-over-TCP discussions and code. Lucie Guiraud and
Danny McPherson for reviewing early versions of this document. We Danny McPherson for reviewing early versions of this document. We
would also like to thank all those who contributed to RFC5966. would also like to thank all those who contributed to RFC5966.
13. References 13. References
13.1. Normative References 13.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768, DOI [RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
10.17487/RFC0768, August 1980, DOI 10.17487/RFC0768, August 1980,
<http://www.rfc-editor.org/info/rfc768>. <http://www.rfc-editor.org/info/rfc768>.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, RFC [RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
793, DOI 10.17487/RFC0793, September 1981, RFC 793, DOI 10.17487/RFC0793, September 1981,
<http://www.rfc-editor.org/info/rfc793>. <http://www.rfc-editor.org/info/rfc793>.
[RFC1034] Mockapetris, P., "Domain names - concepts and facilities", [RFC1034] Mockapetris, P., "Domain names - concepts and facilities",
STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987, STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,
<http://www.rfc-editor.org/info/rfc1034>. <http://www.rfc-editor.org/info/rfc1034>.
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, DOI 10.17487/RFC1035, specification", STD 13, RFC 1035, DOI 10.17487/RFC1035,
November 1987, <http://www.rfc-editor.org/info/rfc1035>. November 1987, <http://www.rfc-editor.org/info/rfc1035>.
[RFC1123] Braden, R., Ed., "Requirements for Internet Hosts - [RFC1123] Braden, R., Ed., "Requirements for Internet Hosts -
Application and Support", STD 3, RFC 1123, DOI 10.17487/ Application and Support", STD 3, RFC 1123,
RFC1123, October 1989, DOI 10.17487/RFC1123, October 1989,
<http://www.rfc-editor.org/info/rfc1123>. <http://www.rfc-editor.org/info/rfc1123>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/ Requirement Levels", BCP 14, RFC 2119,
RFC2119, March 1997, DOI 10.17487/RFC2119, March 1997,
<http://www.rfc-editor.org/info/rfc2119>. <http://www.rfc-editor.org/info/rfc2119>.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S. [RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements", RFC Rose, "DNS Security Introduction and Requirements",
4033, DOI 10.17487/RFC4033, March 2005, RFC 4033, DOI 10.17487/RFC4033, March 2005,
<http://www.rfc-editor.org/info/rfc4033>. <http://www.rfc-editor.org/info/rfc4033>.
[RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast [RFC4786] Abley, J. and K. Lindqvist, "Operation of Anycast
Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786, Services", BCP 126, RFC 4786, DOI 10.17487/RFC4786,
December 2006, <http://www.rfc-editor.org/info/rfc4786>. December 2006, <http://www.rfc-editor.org/info/rfc4786>.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008, Existence", RFC 5155, DOI 10.17487/RFC5155, March 2008,
<http://www.rfc-editor.org/info/rfc5155>. <http://www.rfc-editor.org/info/rfc5155>.
[RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive [RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive
Nameservers in Reflector Attacks", BCP 140, RFC 5358, DOI Nameservers in Reflector Attacks", BCP 140, RFC 5358,
10.17487/RFC5358, October 2008, DOI 10.17487/RFC5358, October 2008,
<http://www.rfc-editor.org/info/rfc5358>. <http://www.rfc-editor.org/info/rfc5358>.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines", BCP [RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
152, RFC 5625, DOI 10.17487/RFC5625, August 2009, BCP 152, RFC 5625, DOI 10.17487/RFC5625, August 2009,
<http://www.rfc-editor.org/info/rfc5625>. <http://www.rfc-editor.org/info/rfc5625>.
[RFC5966] Bellis, R., "DNS Transport over TCP - Implementation [RFC5966] Bellis, R., "DNS Transport over TCP - Implementation
Requirements", RFC 5966, DOI 10.17487/RFC5966, August Requirements", RFC 5966, DOI 10.17487/RFC5966, August
2010, <http://www.rfc-editor.org/info/rfc5966>. 2010, <http://www.rfc-editor.org/info/rfc5966>.
[RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms [RFC6891] Damas, J., Graff, M., and P. Vixie, "Extension Mechanisms
for DNS (EDNS(0))", STD 75, RFC 6891, DOI 10.17487/ for DNS (EDNS(0))", STD 75, RFC 6891,
RFC6891, April 2013, DOI 10.17487/RFC6891, April 2013,
<http://www.rfc-editor.org/info/rfc6891>. <http://www.rfc-editor.org/info/rfc6891>.
[RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer [RFC7230] Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
Protocol (HTTP/1.1): Message Syntax and Routing", RFC Protocol (HTTP/1.1): Message Syntax and Routing",
7230, DOI 10.17487/RFC7230, June 2014, RFC 7230, DOI 10.17487/RFC7230, June 2014,
<http://www.rfc-editor.org/info/rfc7230>. <http://www.rfc-editor.org/info/rfc7230>.
[RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext [RFC7540] Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
Transfer Protocol Version 2 (HTTP/2)", RFC 7540, DOI Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
10.17487/RFC7540, May 2015, DOI 10.17487/RFC7540, May 2015,
<http://www.rfc-editor.org/info/rfc7540>. <http://www.rfc-editor.org/info/rfc7540>.
13.2. Informative References 13.2. Informative References
[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>.
[Connection-Oriented-DNS] [Connection-Oriented-DNS]
Zhu, L., Hu, Z., Heidemann, J., Wessels, D., Mankin, A., Zhu, L., Hu, Z., Heidemann, J., Wessels, D., Mankin, A.,
and N. Somaiya, "Connection-Oriented DNS to Improve and N. Somaiya, "Connection-Oriented DNS to Improve
Privacy and Security", Privacy and Security",
<http://www.isi.edu/~johnh/PAPERS/Zhu15b.pdf>. <http://www.isi.edu/~johnh/PAPERS/Zhu15b.pdf>.
[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>.
[DNS-over-TLS]
Hu, Z., Zhu, L., Heidemann, J., Mankin, A., Wessels, D.,
and P. Hoffman, "TLS for DNS: Initiation and Performance
Considerations", draft-ietf-dprive-dns-over-tls (work in
progress), January 2016.
[edns-tcp-keepalive]
Wouters, P., Abley, J., Dickinson, S., and R. Bellis, "The
edns-tcp-keepalive EDNS0 Option", draft-ietf-dnsop-edns-
tcp-keepalive-05 (work in progress), Jan 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 [RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405, DOI for Application Designers", BCP 145, RFC 5405,
10.17487/RFC5405, November 2008, DOI 10.17487/RFC5405, November 2008,
<http://www.rfc-editor.org/info/rfc5405>. <http://www.rfc-editor.org/info/rfc5405>.
[RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure, [RFC6824] Ford, A., Raiciu, C., Handley, M., and O. Bonaventure,
"TCP Extensions for Multipath Operation with Multiple "TCP Extensions for Multipath Operation with Multiple
Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013, Addresses", RFC 6824, DOI 10.17487/RFC6824, January 2013,
<http://www.rfc-editor.org/info/rfc6824>. <http://www.rfc-editor.org/info/rfc6824>.
[RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP [RFC7413] Cheng, Y., Chu, J., Radhakrishnan, S., and A. Jain, "TCP
Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014, Fast Open", RFC 7413, DOI 10.17487/RFC7413, December 2014,
<http://www.rfc-editor.org/info/rfc7413>. <http://www.rfc-editor.org/info/rfc7413>.
[RRL1] Vixie, P. and V. Schryver, "DNS Response Rate Limiting [RRL1] Vixie, P. and V. Schryver, "DNS Response Rate Limiting
(DNS RRL)", ISC-TN 2012-1-Draft1, August 2014, (DNS RRL)", ISC-TN 2012-1-Draft1, August 2014,
<http://ss.vix.su/~vixie/isc-tn-2012-1.txt>. <http://ss.vix.su/~vixie/isc-tn-2012-1.txt>.
[RRL2] "BIND RRL", ISC Knowledge Base AA-00994, April 2012, [RRL2] "BIND RRL", ISC Knowledge Base AA-00994, April 2012,
<https://deepthought.isc.org/article/AA-00994/0/Using-the- <https://deepthought.isc.org/article/AA-00994/0/Using-the-
Response-Rate-Limiting-Feature-in-BIND-9.10.html>. Response-Rate-Limiting-Feature-in-BIND-9.10.html>.
[edns-tcp-keepalive]
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.
[fragmentation-considered-poisonous]
Herzberg, A. and H. Shulman, "Fragmentation Considered
Poisonous", May 2012, <http://arxiv.org/abs/1205.4011>.
Appendix A. Summary of Advantages and Disadvantages to using TCP for Appendix A. Summary of Advantages and Disadvantages to using TCP for
DNS DNS
The TCP handshake generally prevents address spoofing and, therefore, The TCP handshake generally prevents address spoofing and, therefore,
the reflection/amplification attacks which plague UDP. the reflection/amplification attacks which plague UDP.
IP fragmentation is less of a problem for TCP than it is for 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 TCP stacks generally implement Path MTU Discovery so they can avoid
IP fragmentation of TCP segments. UDP, on the other hand, does not IP fragmentation of TCP segments. UDP, on the other hand, does not
provide reassembly, which means datagrams that exceed the path MTU provide reassembly, which means datagrams that exceed the path MTU
skipping to change at page 16, line 22 skipping to change at page 16, line 44
implementations. implementations.
A more in-depth discussion of connection orientated DNS can be found A more in-depth discussion of connection orientated DNS can be found
elsewhere [Connection-Oriented-DNS]. elsewhere [Connection-Oriented-DNS].
Appendix B. Changes between revisions Appendix B. Changes between revisions
[Note to RFC Editor: please remove this section prior to [Note to RFC Editor: please remove this section prior to
publication.] publication.]
B.1. Changes -03 to -04 B.1. Changes -05 to -06
Introduction: Add reference to DNS-over-TLS
Section 5: 's/it/the resolver/' and 's/fallback/retry/'
Section 6.1.1: Make clear behaviour is 'at the time of writing', not
a recommendation
Section 6.2.1.1: Change SHOULD to MUST.
Section 6.2.2: Clarify 'operational reasons' for zone transfers.
Section 8: Re-word to remove reference to TCP segments.
Section 9: Add sentence about use of TFO with DNS privacy solutions.
B.2. Changes -04 to -05
Added second RRL reference to introduction
Introduction, paragraph 5: s/may result/will probably result/
Section 5: Strengthened wording on update of RFC1123
Section 5: Added reference to HTTP/2
Section 6.2.1: Simplify wording of Message ID collisions
Section 6.2.2: Clarify wording on idle timeout reset
Section 6.2.4: Use DNS Server/client for consistency
Section 8: Re-word to reduce confusion of timeout vs TCP reads
Appendix C: Updated differences to RFC5966.
B.3. Changes -03 to -04
o Re-stated how messages received over TCP should be mapped to o Re-stated how messages received over TCP should be mapped to
queries. queries.
o Added wording to cover timeouts for server side behaviour for when o Added wording to cover timeouts for server side behaviour for when
receiving TCP messages. receiving TCP messages.
o Added sentence to abstract stating this obsoletes RFC5966. o Added sentence to abstract stating this obsoletes RFC5966.
o Moved reference to RFC6891 earlier in Discussion section. o Moved reference to RFC6891 earlier in Discussion section.
o Several minor wording updates to improve clarity. o Several minor wording updates to improve clarity.
o Corrected nits and updated references. o Corrected nits and updated references.
B.2. Changes -02 to -03 B.4. Changes -02 to -03
o Replaced certain lower case RFC2119 keywords to improve clarity. o Replaced certain lower case RFC2119 keywords to improve clarity.
o Updated section 6.2.2 to recognise requirements for concurrent o Updated section 6.2.2 to recognise requirements for concurrent
zone transfers. zone transfers.
o Changed 'client IP address' to 'client IP address or subnet' when o Changed 'client IP address' to 'client IP address or subnet' when
discussing restrictions on TCP connections from clients. discussing restrictions on TCP connections from clients.
o Added reference to edns-tcp-keepalive draft. o Added reference to edns-tcp-keepalive draft.
o Added wording to introduction to reference Appendix A and state o Added wording to introduction to reference Appendix A and state
TCP is a valid transport alternative for DNS. TCP is a valid transport alternative for DNS.
o Improved description of CPNI-TCP as a general reference source on o Improved description of CPNI-TCP as a general reference source on
TCP security related RFCs. TCP security related RFCs.
B.3. Changes -01 to -02 B.5. Changes -01 to -02
o Added more text to Introduction as background to TCP use. o Added more text to Introduction as background to TCP use.
o Added definitions of Persistent connection and Idle session to o Added definitions of Persistent connection and Idle session to
Terminology section. Terminology section.
o Separated Connection Handling section into Current Practice and o Separated Connection Handling section into Current Practice and
Recommendations. Provide more detail on current practices and Recommendations. Provide more detail on current practices and
divided Recommendations up into more granular sub-sections. divided Recommendations up into more granular sub-sections.
skipping to change at page 17, line 42 skipping to change at page 19, line 8
o Updated text on server limits on concurrent connections from a o Updated text on server limits on concurrent connections from a
particular client. particular client.
o Added text that client retry logic is outside the scope of this o Added text that client retry logic is outside the scope of this
document. document.
o Clarified that servers should answer all pipelined queries even if o Clarified that servers should answer all pipelined queries even if
sent very close together. sent very close together.
B.4. Changes -00 to -01 B.6. Changes -00 to -01
o Changed updates to obsoletes RFC 5966. o Changed updates to obsoletes RFC 5966.
o Improved text in Section 4 Transport Protocol Selection to change o Improved text in Section 4 Transport Protocol Selection to change
"TCP SHOULD NOT be used only for the transfers and as a fallback" "TCP SHOULD NOT be used only for the transfers and as a fallback"
to make the intention clearer and more consistent. to make the intention clearer and more consistent.
o Reference to TCP FASTOPEN updated now that it is an RFC. o Reference to TCP FASTOPEN updated now that it is an RFC.
o Added paragraph to say that implementations MUST NOT send the TCP o Added paragraph to say that implementations MUST NOT send the TCP
 End of changes. 34 change blocks. 
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