< 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.
	skipping to change at page 1, line 39 ¶		skipping to change at page 1, line 39 ¶
	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
	skipping to change at page 2, line 28 ¶		skipping to change at page 2, line 28 ¶
	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
	skipping to change at page 3, line 33 ¶		skipping to change at page 3, line 33 ¶
	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.
	skipping to change at page 5, line 35 ¶		skipping to change at page 5, line 35 ¶
	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.
	58 lines changed or deleted		83 lines changed or added
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