draft-ietf-dnsext-dns-tcp-requirements-02.txt   draft-ietf-dnsext-dns-tcp-requirements-03.txt 
DNSEXT R. Bellis DNSEXT R. Bellis
Internet-Draft Nominet UK Internet-Draft Nominet UK
Updates: 1035, 1123 January 6, 2010 Updates: 1035, 1123 March 22, 2010
(if approved) (if approved)
Intended status: Standards Track Intended status: Standards Track
Expires: July 10, 2010 Expires: September 23, 2010
DNS Transport over TCP - Implementation Requirements DNS Transport over TCP - Implementation Requirements
draft-ietf-dnsext-dns-tcp-requirements-02 draft-ietf-dnsext-dns-tcp-requirements-03
Abstract Abstract
This document updates the requirements for the support of TCP as a This document updates the requirements for the support of TCP as a
transport protocol for DNS implementations. transport protocol for DNS implementations.
Status of this Memo Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79. provisions of BCP 78 and BCP 79.
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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."
The list of current Internet-Drafts can be accessed at The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt. http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html. http://www.ietf.org/shadow.html.
This Internet-Draft will expire on July 10, 2010. This Internet-Draft will expire on September 23, 2010.
Copyright Notice Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the Copyright (c) 2010 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
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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4. Transport Protocol Selection . . . . . . . . . . . . . . . . . 4 4. Transport Protocol Selection . . . . . . . . . . . . . . . . . 4
5. Connection Handling . . . . . . . . . . . . . . . . . . . . . . 5 5. Connection Handling . . . . . . . . . . . . . . . . . . . . . . 5
6. Response re-ordering . . . . . . . . . . . . . . . . . . . . . 6 6. Response re-ordering . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6 7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7 9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . . 7 10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
10.1. Normative References . . . . . . . . . . . . . . . . . . . 7
10.2. Informative References . . . . . . . . . . . . . . . . . . 7
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . . 8 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction 1. Introduction
Most DNS [RFC1035] transactions take place over UDP [RFC0792]. The Most DNS [RFC1035] transactions take place over UDP [RFC0768]. TCP
TCP [RFC0793] is used for zone transfers and for the transfer of [RFC0793] is always used for zone transfers and is often used for
other packets which exceed the protocol's original 512 byte packet- messages whose sizes exceed the DNS protocol's original 512 byte
size limit. limit.
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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whose failure to support TCP restricts interoperability and limits whose failure to support TCP restricts interoperability and limits
deployment of new DNS features. deployment of new DNS features.
This document therefore updates the core DNS protocol specifications This document therefore updates the core DNS protocol specifications
such that support for TCP is henceforth a REQUIRED part of a full DNS such that support for TCP is henceforth a REQUIRED part of a full DNS
protocol implementation. protocol implementation.
Whilst this document makes no specific recommendations to operators Whilst this document makes no specific recommendations to operators
of DNS servers, it should be noted that failure to support TCP (or of DNS servers, it should be noted that failure to support TCP (or
blocking of DNS over TCP at the network layer) may result in blocking of DNS over TCP at the network layer) may result in
resolution failure and application-level timeouts. resolution failure and/or application-level timeouts.
2. Terminology used in this document 2. Terminology used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119]. document are to be interpreted as described in [RFC2119].
3. Discussion 3. Discussion
In the absence of EDNS0 (see below) the normal behaviour of any DNS In the absence of EDNS0 (see below) the normal behaviour of any DNS
server needing to send a UDP response that exceeds that 512 byte 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 limit is for the server to truncate the response so that it fits
within the 512 byte limit and set the TC flag in the response header. 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 When the client receives such a response it takes the TC flag as an
indication that it should retry over TCP instead. indication that it should retry over TCP instead.
RFC 1123 also says: RFC 1123 also says:
... it is also clear that some new DNS record types defined in the ... it is also clear that some new DNS record types defined in the
future will contain information exceeding the 512 byte limit that future will contain information exceeding the 512 byte limit that
applies to UDP, and hence will require TCP. Thus, resolvers and applies to UDP, and hence will require TCP. Thus, resolvers and
name servers should implement TCP services as a backup to UDP name servers should implement TCP services as a backup to UDP
today, with the knowledge that they will require the TCP service today, with the knowledge that they will require the TCP service
in the future. in the future.
Existing deployments of DNSSEC [RFC4033] have shown that truncation Existing deployments of DNSSEC [RFC4033] have shown that truncation
at the 512 byte boundary is now commonplace. For example an NXDOMAIN at the 512 byte boundary is now commonplace. For example an NXDOMAIN
(RCODE == 3) response from a DNSSEC signed zone using NSEC3 [RFC5155] (RCODE == 3) response from a DNSSEC signed zone using NSEC3 [RFC5155]
is almost invariably longer than 512 bytes. is almost invariably larger than 512 bytes.
Since the original core specifications for DNS were written, the Since the original core specifications for DNS were written, the
Extension Mechanisms for DNS (EDNS0 [RFC2671]) have been introduced. Extension Mechanisms for DNS (EDNS0 [RFC2671]) have been introduced.
These extensions can be used to indicate that the client is prepared These extensions can be used to indicate that the client is prepared
to receive UDP responses longer than 512 bytes. An EDNS0 compatible to receive UDP responses larger than 512 bytes. An EDNS0 compatible
server receiving a request from an EDNS0 compatible client may send server receiving a request from an EDNS0 compatible client may send
UDP packets up to that client's announced buffer size without UDP packets up to that client's announced buffer size without
truncation. truncation.
However, transport of UDP packets that exceed the size of the path However, transport of UDP packets that exceed the size of the path
MTU causes IP packet fragmentation, which has been found to be MTU causes IP packet fragmentation, which has been found to be
unreliable in some circumstances. Many firewalls routinely block unreliable in some circumstances. Many firewalls routinely block
fragmented IP packets, and some implementations lack the software fragmented IP packets, and some do not implement the algorithms
logic necessary to reassemble a fragmented datagram. Worse still, necessary to reassemble fragmented packets. Worse still, some
some devices deliberately refuse to handle DNS packets containing network devices deliberately refuse to handle DNS packets containing
EDNS0 options. Other issues relating to UDP transport and packet EDNS0 options. Other issues relating to UDP transport and packet
size are discussed in [RFC5625]. size are discussed in [RFC5625].
The MTU most commonly found in the core of the Internet is around The MTU most commonly found in the core of the Internet is around
1500 bytes, and even that limit is routinely exceeded by DNSSEC 1500 bytes, and even that limit is routinely exceeded by DNSSEC
signed responses. signed responses.
The future that was anticipated in RFC 1123 has arrived, and the only The future that was anticipated in RFC 1123 has arrived, and the only
standardised UDP-based mechanism which may have resolved the packet standardised UDP-based mechanism which may have resolved the packet
size issue has been found inadequate. size issue has been found inadequate.
4. Transport Protocol Selection 4. Transport Protocol Selection
All DNS implementations MUST support both UDP and TCP transport. All general purpose DNS implementations MUST support both UDP and TCP
transport.
o Authoritative resolver implementations MUST support TCP so that o Authoritative server implementations MUST support TCP so that they
they may serve any long responses that they are configured to do not limit the size of responses.
serve.
o A recursive resolver or forwarder MUST support TCP so that it does o Recursive resolver (or forwarder) implementations MUST support TCP
not prevent long responses from a TCP-capable server from reaching so that the do not prevent large responses from a TCP-capable
its TCP-capable clients. server from reaching its TCP-capable clients.
o A general purpose stub resolver implementation (e.g. an operating o Stub resolver implementations (e.g. an operating system's DNS
system's DNS resolution library) MUST support TCP since to do resolution library) MUST support TCP since to do otherwise would
otherwise would limit its interoperability with its own clients limit their interoperability with their own clients and with
and with upstream servers. upstream servers.
An exception may be made for proprietary stub resolver An exception may be made for proprietary stub resolver
implementations. These MAY omit support for TCP if operating in an implementations. These MAY omit support for TCP if operating in an
environment where truncation can never occur, or where DNS lookup environment where truncation can never occur, or where DNS lookup
failure is acceptable should truncation occur. failure is acceptable should truncation occur.
Regarding the choice of when to use UDP or TCP, RFC 1123 says: Regarding the choice of when to use UDP or TCP, RFC 1123 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.
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reason to expect the response would be truncated if it were sent over reason to expect the response would be truncated if it were sent over
UDP (with or without EDNS0) or for other operational reasons, in UDP (with or without EDNS0) or for other operational reasons, in
particular if it already has an open TCP connection to the server. particular if it already has an open TCP connection to the server.
5. Connection Handling 5. Connection Handling
Section 4.2.2 of [RFC1035] says: Section 4.2.2 of [RFC1035] says:
If the server needs to close a dormant connection to reclaim If the server needs to close a dormant connection to reclaim
resources, it should wait until the connection has been idle for a resources, it should wait until the connection has been idle for a
period on the order of two minutes. period on the order of two minutes. In particular, the server
should allow the SOA and AXFR request sequence (which begins a
refresh operation) to be made on a single connection. Since the
server would be unable to answer queries anyway, a unilateral
close or reset may be used instead of a graceful close.
Other more modern protocols (e.g. HTTP [RFC2616]) have support for Other more modern protocols (e.g. HTTP [RFC2616]) have support for
persistent TCP connections and operational experience has shown that persistent TCP connections and operational experience has shown that
long timeouts can easily cause resource exhaustion and poor response long timeouts can easily cause resource exhaustion and poor response
under heavy load. Intentionally opening many connections and leaving under heavy load. Intentionally opening many connections and leaving
them dormant can trivially create a "denial of service" attack. them dormant can trivially create a "denial of service" attack.
This document therefore RECOMMENDS that the application-level idle This document therefore RECOMMENDS that the default application-level
period should be of the order of TBD seconds. idle period should be of the order of seconds, but does not specify
any particular value. In practise the idle period may vary
Servers MAY allow dormant connections to remain open for longer dynamically, and servers MAY allow dormant connections to remain open
periods, but for the avoidance of doubt persistent DNS connections for longer periods as resources permit.
should generally be considered to be as much for the server's benefit
as for the client's. Therefore if the server needs to unilaterally
close a dormant TCP connection it MUST be free to do so whenever
required.
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. made to any individual server. Similarly servers MAY impose limits
on the number of concurrent TCP connections being handled for any
particular client.
Further recommendations for the tuning of TCP parameters to allow Further recommendations for the tuning of TCP stacks to allow higher
higher throughput or improved resiliency against denial of service throughput or improved resiliency against denial of service attacks
attacks are outside the scope of this document. are outside the scope of this document.
6. Response re-ordering 6. Response re-ordering
RFC 1035 is ambiguous on the question of whether TCP queries may be RFC 1035 is ambiguous on the question of whether TCP queries may be
re-ordered - the only relevant text is in Section 4.2.1 which relates re-ordered - the only relevant text is in Section 4.2.1 which relates
to UDP: to UDP:
Queries or their responses may be reordered by the network, or by Queries or their responses may be reordered by the network, or by
processing in name servers, so resolvers should not depend on them processing in name servers, so resolvers should not depend on them
being returned in order. being returned in order.
For the avoidance of future doubt, this requirement is clarified. For the avoidance of future doubt, this requirement is clarified.
Client resolvers MUST be able to process responses which arrive in a Client resolvers MUST be able to process responses which arrive in a
different order to that in which the requests were sent, regardless different order to that in which the requests were sent, regardless
of the transport protocol in use. of the transport protocol in use.
7. Security Considerations 7. Security Considerations
Some DNS server operators have expressed concern that wider use of Some DNS server operators have expressed concern that wider use of
DNS over TCP will expose them to a higher risk of "denial of service" DNS over TCP will expose them to a higher risk of denial of service
(DoS) attacks. (DoS) attacks.
Whilst there is a theoretically higher risk of such attacks against Although there is a higher risk of such attacks against TCP-enabled
TCP-enabled servers, techniques for the mitigation of DoS attacks at servers, techniques for the mitigation of DoS attacks at the network
the network level have improved substantially since DNS was first level have improved substantially since DNS was first designed.
designed.
The vast majority of TLD authority servers and all but one of the At the time of writing the vast majority of TLD authority servers and
root name servers already support TCP and the author knows of no all of the root name servers support TCP and the author knows of no
evidence to suggest that TCP-based DoS attacks against existing DNS evidence to suggest that TCP-based DoS attacks against existing DNS
infrastructure are commonplace. infrastructure are commonplace.
That notwithstanding, readers are advised to familiarise themselves
with [CPNI-TCP].
Operators of recursive servers should ensure that they only accept Operators of recursive servers should ensure that they only accept
connections from expected clients, and do not accept them from connections from expected clients, and do not accept them from
unknown sources. In the case of UDP traffic this will protect unknown sources. In the case of UDP traffic this will help protect
against reflector attacks [RFC5358] and in the case of TCP traffic it against reflector attacks [RFC5358] and in the case of TCP traffic it
will prevent an unknown client from exhausting the server's limits on will prevent an unknown client from exhausting the server's limits on
the number of concurrent connections. the number of concurrent connections.
8. IANA Considerations 8. IANA Considerations
This document requests no IANA actions. This document requests no IANA actions.
9. References 9. Acknowledgements
9.1. Normative References The author would like to thank the document reviewers from the DNSEXT
Working Group, and in particular George Barwood, Alex Bligh, Alfred
Hoenes, Fernando Gont, Jim Reid, Paul Vixie and Nicholas Weaver.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5, 10. References
RFC 792, September 1981.
10.1. Normative References
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7, [RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981. RFC 793, September 1981.
[RFC1035] Mockapetris, P., "Domain names - implementation and [RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987. specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application [RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989. and Support", STD 3, RFC 1123, October 1989.
[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, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", [RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999. RFC 2671, August 1999.
9.2. Informative References 10.2. Informative References
[CPNI-TCP]
CPNI, "Security Assessment of the Transmission Control
Protocol (TCP)", 2009, <http://www.cpni.gov.uk/Docs/
tn-03-09-security-assessment-TCP.pdf>.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H., [RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999. Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[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", Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005. RFC 4033, March 2005.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS [RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
skipping to change at page 8, line 9 skipping to change at page 8, line 25
Nameservers in Reflector Attacks", BCP 140, RFC 5358, Nameservers in Reflector Attacks", BCP 140, RFC 5358,
October 2008. October 2008.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines", [RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
BCP 152, RFC 5625, August 2009. BCP 152, RFC 5625, August 2009.
Appendix A. Change Log Appendix A. Change Log
NB: to be removed by the RFC Editor before publication. NB: to be removed by the RFC Editor before publication.
draft-ietf-dnsext-dns-tcp-requirements-03
Editorial nits from WGLC
Clarification on "general purpose"
Fixed ref to UDP (RFC 768)
Included more S.4.2.2 text from RFC 1035 and removed some from
this draft relating to connection resets.
s/long/large/ for packet sizes
draft-ietf-dnsext-dns-tcp-requirements-02 draft-ietf-dnsext-dns-tcp-requirements-02
Change of title - more focus on implementation and not operation Change of title - more focus on implementation and not operation
Re-write of some of the security section Re-write of some of the security section
Added recommendation for minimal concurrent connections Added recommendation for minimal concurrent connections
Minor editorial nits from Alfred Hoenes Minor editorial nits from Alfred Hoenes
draft-ietf-dnsext-dns-tcp-requirements-01 draft-ietf-dnsext-dns-tcp-requirements-01
Addition of response ordering section Addition of response ordering section
Various minor editorial changes from WG reviewers Various minor editorial changes from WG reviewers
 End of changes. 30 change blocks. 
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