upstream/bind9
1
0
Fork 0
mirror of https://github.com/isc-projects/bind9.git synced 2026-03-07 16:00:34 -05:00
Code Issues Projects Releases Packages Wiki Activity Actions

sync

Browse source
This commit is contained in:
Automatic Updater 2010-03-24 01:23:39 +00:00
parent b448c168e6
commit 125a6afaec
2 changed files with 0 additions and 2240 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

1792
doc/draft/draft-ietf-behave-dns64-07.txt
View file

File diff suppressed because it is too large Load diff

448
doc/draft/draft-ietf-dnsext-dns-tcp-requirements-02.txt
View file

@ -1,448 +0,0 @@
DNSEXT R. Bellis
Internet-Draft Nominet UK
Updates: 1035, 1123 January 6, 2010
(if approved)
Intended status: Standards Track
Expires: July 10, 2010
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
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on July 10, 2010.
Copyright Notice
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
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
Bellis Expires July 10, 2010 [Page 1]
Internet-Draft DNS over TCP January 2010
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
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology used in this document . . . . . . . . . . . . . . . 3
3. Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . 3
4. Transport Protocol Selection . . . . . . . . . . . . . . . . . 4
5. Connection Handling . . . . . . . . . . . . . . . . . . . . . . 5
6. Response re-ordering . . . . . . . . . . . . . . . . . . . . . 6
7. Security Considerations . . . . . . . . . . . . . . . . . . . . 6
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 7
9.2. Informative References . . . . . . . . . . . . . . . . . . 7
Appendix A. Change Log . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 8
Bellis Expires July 10, 2010 [Page 2]
Internet-Draft DNS over TCP January 2010
1. Introduction
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:
DNS resolvers and recursive servers MUST support UDP, and SHOULD
support TCP, for sending (non-zone-transfer) queries.
However, some implementors have taken the text quoted above to mean
that TCP support is an optional feature of the DNS protocol.
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
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
3. Discussion
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 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:
Bellis Expires July 10, 2010 [Page 3]
Internet-Draft DNS over TCP January 2010
... 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
name servers should implement TCP services as a backup to UDP
today, with the knowledge that they will require the TCP service
in the future.
Existing deployments of DNSSEC [RFC4033] have shown that truncation
at the 512 byte boundary is now commonplace. For example an NXDOMAIN
(RCODE == 3) response from a DNSSEC signed zone using NSEC3 [RFC5155]
is almost invariably longer than 512 bytes.
Since the original core specifications for DNS were written, the
Extension Mechanisms for DNS (EDNS0 [RFC2671]) have been introduced.
These extensions can be used to indicate that the client is prepared
to receive UDP responses longer than 512 bytes. An EDNS0 compatible
server receiving a request from an EDNS0 compatible client may send
UDP packets up to that client's announced buffer size without
truncation.
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 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.
o Authoritative resolver implementations MUST support TCP so that
they may serve any long responses that they are configured to
serve.
Bellis Expires July 10, 2010 [Page 4]
Internet-Draft DNS over TCP January 2010
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.
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:
... a DNS resolver or server that is sending a non-zone-transfer
query MUST send a UDP query first.
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, in
particular if it already has an open TCP connection to the server.
5. Connection Handling
Section 4.2.2 of [RFC1035] says:
If the server needs to close a dormant connection to reclaim
resources, it should wait until the connection has been idle for a
period on the order of two minutes.
Other more modern protocols (e.g. HTTP [RFC2616]) have support for
persistent TCP connections and operational experience has shown that
long timeouts can easily cause resource exhaustion and poor response
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 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
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.
Bellis Expires July 10, 2010 [Page 5]
Internet-Draft DNS over TCP January 2010
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
higher throughput or improved resiliency against denial of service
attacks are outside the scope of this document.
6. Response re-ordering
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
to UDP:
Queries or their responses may be reordered by the network, or by
processing in name servers, so resolvers should not depend on them
being returned in order.
For the avoidance of future doubt, this requirement is clarified.
Client resolvers MUST be able to process responses which arrive in a
different order to that in which the requests were sent, regardless
of the transport protocol in use.
7. Security Considerations
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"
(DoS) attacks.
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
unknown sources. In the case of UDP traffic this will protect
against reflector attacks [RFC5358] and in the case of TCP traffic it
will prevent an unknown client from exhausting the server's limits on
the number of concurrent connections.
Bellis Expires July 10, 2010 [Page 6]
Internet-Draft DNS over TCP January 2010
8. IANA Considerations
This document requests no IANA actions.
9. References
9.1. Normative References
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC0793] Postel, J., "Transmission Control Protocol", STD 7,
RFC 793, September 1981.
[RFC1035] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[RFC1123] Braden, R., "Requirements for Internet Hosts - Application
and Support", STD 3, RFC 1123, October 1989.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC2671] Vixie, P., "Extension Mechanisms for DNS (EDNS0)",
RFC 2671, August 1999.
9.2. Informative References
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC4033] Arends, R., Austein, R., Larson, M., Massey, D., and S.
Rose, "DNS Security Introduction and Requirements",
RFC 4033, March 2005.
[RFC5155] Laurie, B., Sisson, G., Arends, R., and D. Blacka, "DNS
Security (DNSSEC) Hashed Authenticated Denial of
Existence", RFC 5155, March 2008.
[RFC5358] Damas, J. and F. Neves, "Preventing Use of Recursive
Nameservers in Reflector Attacks", BCP 140, RFC 5358,
October 2008.
[RFC5625] Bellis, R., "DNS Proxy Implementation Guidelines",
BCP 152, RFC 5625, August 2009.
Bellis Expires July 10, 2010 [Page 7]
Internet-Draft DNS over TCP January 2010
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
draft-ietf-dnsext-dns-tcp-requirements-00
Initial draft
Author's Address
Ray Bellis
Nominet UK
Edmund Halley Road
Oxford OX4 4DQ
United Kingdom
Phone: +44 1865 332211
Email: ray.bellis@nominet.org.uk
URI: http://www.nominet.org.uk/
Bellis Expires July 10, 2010 [Page 8]