draft-ietf-dnsext-dns-tcp-requirements-01.txt   draft-ietf-dnsext-dns-tcp-requirements-02.txt 
DNSEXT R. Bellis DNSEXT R. Bellis
Internet-Draft Nominet UK Internet-Draft Nominet UK
Updates: 1035, 1123 October 26, 2009 Updates: 1035, 1123 January 6, 2010
(if approved) (if approved)
Intended status: Standards Track Intended status: Standards Track
Expires: April 29, 2010 Expires: July 10, 2010
DNS Transport over TCP DNS Transport over TCP - Implementation Requirements
draft-ietf-dnsext-dns-tcp-requirements-01 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 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.
Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet- other groups may also distribute working documents as Internet-
Drafts. Drafts.
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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
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This Internet-Draft will expire on April 29, 2010. This Internet-Draft will expire on July 10, 2010.
Copyright Notice 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. 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 in effect on the date of Provisions Relating to IETF Documents
publication of this document (http://trustee.ietf.org/license-info). (http://trustee.ietf.org/license-info) in effect on the date of
Please review these documents carefully, as they describe your rights publication of this document. Please review these documents
and restrictions with respect to this document. carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
Abstract include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
This document updates the requirements for the support of the TCP described in the BSD License.
protocol for the transport of DNS traffic.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology used in this document . . . . . . . . . . . . . . . 3 2. Terminology used in this document . . . . . . . . . . . . . . . 3
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Transport Protocol Selection . . . . . . . . . . . . . . . . . 4 4. Transport Protocol Selection . . . . . . . . . . . . . . . . . 4
5. Dormant 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 . . . . . . . . . . . . . . . . . . . . . . 6 8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 6 9.1. Normative References . . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . . 7 9.2. Informative References . . . . . . . . . . . . . . . . . . 7
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . . 7 Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 7 Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
1. Introduction 1. Introduction
Most DNS [RFC1035] transactions take place over the UDP [RFC0792] Most DNS [RFC1035] transactions take place over UDP [RFC0792]. The
protocol. The TCP [RFC0793] protocol is used for zone transfers and TCP [RFC0793] is used for zone transfers and for the transfer of
is supported by many implementations for the transfer of other other packets which exceed the protocol's original 512 byte packet-
packets which exceed the protocol's original 512 byte packet-size 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 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 The majority of DNS server operators already support TCP and the
specifications such that (except in very limited circumstances) default configuration for most software implementations is to support
support for the TCP protocol is henceforth REQUIRED. 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 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 exceeds that 512 byte
limit is for the server to truncate the response at the 512 byte limit is for the server to truncate the response so that it fits
limit and set the TC flag in the response header. When the client within the 512 byte limit and set the TC flag in the response header.
receives such a response it takes the TC flag as notice that it When the client receives such a response it takes the TC flag as an
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.
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is almost invariably longer than 512 bytes. is almost invariably longer 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 longer 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 which exceed the size of the path However, transport of UDP packets that exceed the size of the path
MTU has been found to be unreliable in some circumstances because of MTU causes IP packet fragmentation, which has been found to be
IP packet fragmentation. Many firewalls routinely block fragmented unreliable in some circumstances. Many firewalls routinely block
IP packets, and some implementations lack the software logic fragmented IP packets, and some implementations lack the software
necessary to reassemble a fragmented datagram. Worse still, some logic necessary to reassemble a fragmented datagram. Worse still,
devices deliberately refuse to handle DNS packets containing EDNS0 some devices deliberately refuse to handle DNS packets containing
options. Other issues relating to UDP transport and packet size are EDNS0 options. Other issues relating to UDP transport and packet
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 mechanism which may have resolved the packet size issue standardised UDP-based mechanism which may have resolved the packet
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 DNS implementations MUST support both UDP and TCP transport.
protocols, except as set out below.
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 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 o Authoritative resolver implementations MUST support TCP so that
system's DNS resolution library) MUST support TCP since to do they may serve any long responses that they are configured to
otherwise would limit its interoperability with its own clients and serve.
with upstream servers.
A proprietary stub resolver implementation MAY omit support for TCP o A recursive resolver or forwarder MUST support TCP so that it does
if it is operating in an environment where truncation can never not prevent long responses from a TCP-capable server from reaching
occur, or if it is prepared to accept a DNS lookup failure should its TCP-capable clients.
truncation occur. 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 An exception may be made for proprietary stub resolver
not prevent long responses from a TCP-capable server from reaching implementations. These MAY omit support for TCP if operating in an
its TCP-capable clients. 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: 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.
That requirement is hereby relaxed. A resolver SHOULD send a UDP 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 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 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: 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.
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 idle period should be of This document therefore RECOMMENDS that the application-level idle
the order of TBD seconds. period should be of the order of TBD seconds.
Servers MAY allow dormant connections to remain open for longer Servers MAY allow dormant connections to remain open for longer
periods, but for the avoidance of doubt persistent DNS connections periods, but for the avoidance of doubt persistent DNS connections
should generally be considered to be as much for the server's benefit 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 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 close a dormant TCP connection it MUST be free to do so whenever
required. required.
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.
Further recommendations for the tuning of TCP parameters to allow Further recommendations for the tuning of TCP parameters to allow
higher throughput or improved resiliency against denial of service higher throughput or improved resiliency against denial of service
attacks are (currently) outside the scope of this document. attacks 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"
attacks. (DoS) attacks.
Many large authoritative DNS operators including all but one of the Whilst there is a theoretically higher risk of such attacks against
root servers and the vast majority of TLDs already support TCP and TCP-enabled servers, techniques for the mitigation of DoS attacks at
attacks against them are infrequent and very rarely successful. 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 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 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
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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-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 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
draft-ietf-dnsext-dns-tcp-requirements-00 draft-ietf-dnsext-dns-tcp-requirements-00
Initial draft Initial draft
Author's Address Author's Address
Ray Bellis Ray Bellis
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