DNSEXT R. Bellis
Internet-Draft Nominet UK
-Updates: 1035, 1123 October 26, 2009
+Updates: 1035, 1123 January 6, 2010
(if approved)
Intended status: Standards Track
-Expires: April 29, 2010
+Expires: July 10, 2010
- DNS Transport over TCP
- draft-ietf-dnsext-dns-tcp-requirements-01
+ DNS Transport over TCP - Implementation Requirements
+ draft-ietf-dnsext-dns-tcp-requirements-02
+
+Abstract
+
+ This document updates the requirements for the support of TCP as a
+ transport protocol for DNS implementations.
Status of this Memo
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
- This Internet-Draft will expire on April 29, 2010.
+ This Internet-Draft will expire on July 10, 2010.
Copyright Notice
- Copyright (c) 2009 IETF Trust and the persons identified as the
+ Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
- Provisions Relating to IETF Documents in effect on the date of
- publication of this document (http://trustee.ietf.org/license-info).
- Please review these documents carefully, as they describe your rights
- and restrictions with respect to this document.
-
-Abstract
+ 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
- This document updates the requirements for the support of the TCP
-
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- protocol for the transport of DNS traffic.
+ 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 BSD License.
Table of Contents
4. Transport Protocol Selection . . . . . . . . . . . . . . . . . 4
- 5. Dormant Connection Handling . . . . . . . . . . . . . . . . . . 5
+ 5. Connection Handling . . . . . . . . . . . . . . . . . . . . . . 5
6. Response re-ordering . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
- 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
+ 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
- 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
- 9.1. Normative References . . . . . . . . . . . . . . . . . . . 6
+ 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
+ 9.1. Normative References . . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . . 7
- Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . . 7
-
- Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7
-
-
-
+ Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . . 8
+ Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
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1. Introduction
- Most DNS [RFC1035] transactions take place over the UDP [RFC0792]
- protocol. The TCP [RFC0793] protocol is used for zone transfers and
- is supported by many implementations for the transfer of other
- packets which exceed the protocol's original 512 byte packet-size
- limit.
+ Most DNS [RFC1035] transactions take place over UDP [RFC0792]. The
+ TCP [RFC0793] is used for zone transfers and for the transfer of
+ other packets which exceed the protocol's original 512 byte packet-
+ size limit.
Section 6.1.3.2 of [RFC1123] states:
support TCP, for sending (non-zone-transfer) queries.
However, some implementors have taken the text quoted above to mean
- that TCP support is truly optional for typical DNS operation.
+ that TCP support is an optional feature of the DNS protocol.
- This document normatively updates the core DNS protocol
- specifications such that (except in very limited circumstances)
- support for the TCP protocol is henceforth REQUIRED.
+ 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 failure to support TCP restricts interoperability and limits
+ 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.
+
+ Whilst this document makes no specific recommendations to operators
+ 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
+ resolution failure and application-level timeouts.
2. Terminology used in this document
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
- limit is for the server to truncate the response at the 512 byte
- limit and set the TC flag in the response header. When the client
- receives such a response it takes the TC flag as notice that it
- should retry over TCP instead.
+ 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.
+ When the client receives such a response it takes the TC flag as an
+ indication that it should retry over TCP instead.
RFC 1123 also says:
+
+
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... 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
Existing deployments of DNSSEC [RFC4033] have shown that truncation
at the 512 byte boundary is now commonplace. For example an NXDOMAIN
-
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(RCODE == 3) response from a DNSSEC signed zone using NSEC3 [RFC5155]
is almost invariably longer than 512 bytes.
UDP packets up to that client's announced buffer size without
truncation.
- However, transport of UDP packets which exceed the size of the path
- MTU has been found to be unreliable in some circumstances because of
- IP packet fragmentation. Many firewalls routinely block fragmented
- IP packets, and some implementations lack the software logic
- necessary to reassemble a fragmented datagram. Worse still, some
- devices deliberately refuse to handle DNS packets containing EDNS0
- options. Other issues relating to UDP transport and packet size are
- discussed in [RFC5625].
+ 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 some circumstances. Many firewalls routinely block
+ fragmented IP packets, and some implementations lack the software
+ logic necessary to reassemble a fragmented datagram. Worse still,
+ some 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 mechanism which may have resolved the packet size issue
- has been found inadequate.
+ standardised UDP-based mechanism which may have resolved the packet
+ size issue has been found inadequate.
4. Transport Protocol Selection
- All DNS implementations MUST support both UDP and TCP transport
- protocols, except as set out below.
+ All DNS implementations MUST support both UDP and TCP transport.
- On a case by case basis, authoritative DNS server operators MAY elect
- to disable DNS transport over TCP if all of the following conditions
- are satisfied:
+ o Authoritative resolver implementations MUST support TCP so that
+ they may serve any long responses that they are configured to
+ serve.
- o the server is authoritative only
- o the server does not support AXFR
- o all requests and responses are guaranteed to be <= 512 bytes
- A general purpose stub resolver implementation (e.g. an operating
- system's DNS resolution library) MUST support TCP since to do
- otherwise would limit its interoperability with its own clients and
- with upstream servers.
- A proprietary stub resolver implementation MAY omit support for TCP
-
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- if it is operating in an environment where truncation can never
- occur, or if it is prepared to accept a DNS lookup failure should
- truncation occur.
+ o A recursive resolver or forwarder MUST support TCP so that it does
+ not prevent long responses from a TCP-capable server from reaching
+ its TCP-capable clients.
+ o A general purpose stub resolver implementation (e.g. an operating
+ system's DNS resolution library) MUST support TCP since to do
+ otherwise would limit its interoperability with its own clients
+ and with upstream servers.
- A recursive resolver or forwarder MUST support TCP so that it does
- not prevent long responses from a TCP-capable server from reaching
- its TCP-capable clients.
+ An exception may be made for proprietary stub resolver
+ implementations. These MAY omit support for TCP if operating in an
+ environment where truncation can never occur, or where DNS lookup
+ failure is acceptable should truncation occur.
Regarding the choice of when to use UDP or TCP, RFC 1123 says:
That requirement is hereby relaxed. A resolver SHOULD send a UDP
query first, but MAY elect to send a TCP query instead if it has good
reason to expect the response would be truncated if it were sent over
- UDP (with or without EDNS0) or for other operational reasons.
+ UDP (with or without EDNS0) or for other operational reasons, in
+ particular if it already has an open TCP connection to the server.
-5. Dormant Connection Handling
+5. Connection Handling
Section 4.2.2 of [RFC1035] says:
under heavy load. Intentionally opening many connections and leaving
them dormant can trivially create a "denial of service" attack.
- This document therefore RECOMMENDS that the idle period should be of
- the order of TBD seconds.
+ This document therefore RECOMMENDS that the application-level idle
+ period should be of the order of TBD seconds.
Servers MAY allow dormant connections to remain open for longer
periods, but for the avoidance of doubt persistent DNS connections
close a dormant TCP connection it MUST be free to do so whenever
required.
- Further recommendations for the tuning of TCP parameters to allow
- higher throughput or improved resiliency against denial of service
- attacks are (currently) outside the scope of this document.
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+ 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.
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+ Further recommendations for the tuning of TCP parameters to allow
+ higher throughput or improved resiliency against denial of service
+ attacks are outside the scope of this document.
6. Response re-ordering
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"
- attacks.
+ (DoS) attacks.
- Many large authoritative DNS operators including all but one of the
- root servers and the vast majority of TLDs already support TCP and
- attacks against them are infrequent and very rarely successful.
+ Whilst there is a theoretically higher risk of such attacks against
+ TCP-enabled servers, techniques for the mitigation of DoS attacks at
+ the network level have improved substantially since DNS was first
+ designed.
+
+ The vast majority of TLD authority servers and all but one of the
+ root name servers already support TCP and the author knows of no
+ evidence to suggest that TCP-based DoS attacks against existing DNS
+ infrastructure are commonplace.
Operators of recursive servers should ensure that they only accept
connections from expected clients, and do not accept them from
the number of concurrent connections.
+
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8. IANA Considerations
This document requests no IANA actions.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
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[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
BCP 152, RFC 5625, August 2009.
+
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Appendix A. Change Log
NB: to be removed by the RFC Editor before publication.
+ draft-ietf-dnsext-dns-tcp-requirements-02
+ Change of title - more focus on implementation and not operation
+ Re-write of some of the security section
+ Added recommendation for minimal concurrent connections
+ Minor editorial nits from Alfred Hoenes
+
draft-ietf-dnsext-dns-tcp-requirements-01
Addition of response ordering section
Various minor editorial changes from WG reviewers
Initial draft
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Author's Address
Ray Bellis
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