One document matched: draft-conroy-enum-edns0-01.txt
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ENUM L. Conroy
Internet-Draft RMRL
Expires: April 25, 2006 J. Reid
DNS-MODA
October 22, 2005
ENUM Requirement for EDNS0 Support
<draft-conroy-enum-edns0-01.txt>
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Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
This document mandates support for EDNS0 (Extension Mechanisms for
DNS) in DNS entities claiming to support ENUM query resolution (as
defined in RFC3761). This requirement is needed as DNS responses to
ENUM-related questions return larger sets of Resource Records than
typical DNS messages. Without EDNS0 support in all the involved
entities, a fallback to TCP transport for ENUM queries and responses
would typically occur. That has a severe impact on DNS Server load,
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and on latency of ENUM queries.
This document updates RFC3761 only in adding this requirement.
Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. DNS - Background . . . . . . . . . . . . . . . . . . . . . 4
3. Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
4. Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
4.1. Required Aspects of EDNS0 Support . . . . . . . . . . . . 7
4.1.1. TCP Requirement . . . . . . . . . . . . . . . . . . . 8
4.1.2. Fragmentation Requirement . . . . . . . . . . . . . . 8
4.1.3. Intermediary Node Requirement . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
8.1. Normative References . . . . . . . . . . . . . . . . . . . 13
8.2. Informative References . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 15
Intellectual Property and Copyright Statements . . . . . . . . . . 16
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1. Terminology
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 BCP 14, RFC2119 [8].
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2. Introduction
ENUM is defined in RFC3761[1]. It uses the underlying DNS protocol
to transfer its queries and to carry responses holding the NAPTR
resource records (defined in [2]) that are to be processed by the
ENUM client.
The DNS protocol is defined in RFC1034[3], RFC1035[4] and clarified
in RFC2181[5], whilst Requirements for Internet Hosts are specified
in RFC1123[6]. It is a simple and efficient protocol, and is
fundamental to the operation of Internet communications.
Entities involved in processing ENUM queries and responses have to
deal with messages that typically return large sets of resource
records (RRSets). These messages do not fit the profile for which
DNS was originally designed, and so it is necessary to implement the
standard Extension Mechanisms for DNS as described in RFC2671[7],
specifically the feature by which a DNS entity can indicate its
ability to process messages of a given size over UDP transport.
2.1. DNS - Background
DNS is based on a simple question and answer model. In the standard
approach described in RFC1035, a Resolver will construct and send a
question to a DNS Server using UDP transport.
For historical reasons, a size limit of 512 bytes is specified in
RFC1035 for all messages exchanged in DNS over UDP transport,
originally to avoid the risk of packet fragmentation over paths with
a small MTU, as handling such fragmentation was not well supported in
early communications software. Where an answer will not fit within
this limit, a basic DNS response will be truncated (indicated by the
"TC" flag being set to '1' in the response).
DNS queries and responses can also be carried over TCP transport. In
this case, the size limit is not applied, as TCP already has a robust
mechanism for fragmentation and reconstruction of packets, including
the selective re-transmission of fragments that have been corrupted
or discarded in transit. It does have performance implications,
increasing the overall time taken to complete the transaction, and
increasing the volume of network traffic. Thus it is not the default
choice of transport for the DNS protocol, but can be used if a
question sent using UDP fails due to truncation of the response.
Supporting UDP queries is mandatory, but support for TCP queries is
recommended also, and is (in effect) required as RFC1123 requires
that a DNS Resolver discard a truncated response and retry using
another transport protocol. In effect, Authoritative Name Servers
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that do not answer TCP queries after returning truncated responses
are misconfigured.
With the introduction of the Extension mechanisms described in
RFC2671, there is now a mechanism by which a DNS entity can indicate
that it is capable of handling messages larger than those implied in
the scheme described in RFC1035, so that it can use UDP transport but
still receive DNS messages up to the size it specifies in its request
or response.
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3. Problem
ENUM zones typically store large sets of Resource Records (RRSets),
and an answer returning such an RRSet may well exceed the capacity of
a DNS response meeting the size limit set in RFC1035 for messages
using UDP transport. RFC1035 (and RFC1123) outline a fallback
mechanism; the Server indicates that it cannot return the full answer
by setting the TC flag in its response, and, on receiving this
message, the Resolver will discard the partial result and use TCP
transport when re-trying its question.
This fallback does induce extra latency in resolving the question, as
the Resolver waits for the truncated response and then opens a TCP
connection over which to re-try its question. It generates extra
data traffic, as the initial truncated response is returned and
immediately discarded, the TCP transport connection creation and
release are both signalled, and the query is sent twice before the
final answer is returned.
It also causes extra load on the DNS Server. The DNS Server has to
process the initial query and construct a truncated response, only to
receive the query again using TCP transport. Likewise, even after it
has returned the full answer over a TCP connection opened by the
Resolver, the Server must maintain a TCP control block for a certain
time after it has sent the answer and has requested closure of the
TCP connection. Thus answering a high volume of queries using TCP
connections causes issues with memory usage, involves the Server in
unnecessary processing and, due to the linger time required before
each connection can be released, limits the number of concurrent
connections that may be open.
While the percentage of queries processed that exceed the UDP size
limit specified in RFC1035 is relatively small, the impact on normal
query resolution of the fallback mechanism is minimal; it just does
not happen that often. However, where ENUM queries are concerned,
this design assumption no longer holds; this fallback procedure is no
longer the exception, and performance when handling ENUM queries
suffers as a result.
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4. Solution
In short, ENUM queries return responses containing large RRsets. If
ENUM clients use the query mechanism outlined in RFC1035, it will
create significant levels of truncated responses and query retries
over TCP. These can have a serious operational impact: poor
performance for clients and increased load on DNS servers. The
solution to this problem is for all entities that are involved in
ENUM resolution to use EDNS0. The ENUM client (and the DNS Resolver
it uses) will indicate its ability to accept large responses by
adding to its query an OPT pseudo-resource record as additional data,
showing the size of UDP packet it can process in response. This
allows the DNS Servers involved in the resolution to return answers
that fit within the limit set by the Resolver rather than that
specified in RFC1035, whilst still using UDP as the transport
protocol. For a description of other situations in which EDNS0 is
useful and for further motivations on its use, see RFC3225[9] and
RFC3226[10].
4.1. Required Aspects of EDNS0 Support
There are some subtleties with EDNS0 support within ENUM, so the full
implications of the requirement of EDNS0 support for ENUM resolution
are explained here.
The basic requirement for EDNS0 support in ENUM entities is in two
parts:
ALL entities involved in ENUM resolution MUST support EDNS0
Resolvers MUST use EDNS0 in their queries unless they have current
knowledge that EDNS0 support is not provided at the target of
their queries.
Note that ENUM-involved entities includes ALL the DNS Servers and
Resolvers used in ENUM resolution. The Resolvers involved include
both the Stub Resolver typically installed on a client node, plus the
Recursive Resolver to which it forwards requests and from which it
receives answers. The list of involved Servers includes any DNS
Servers authoritative for delegated ENUM domains, but also the DNS
Servers authoritative for .arpa, the ENUM Tier 0 DNS Servers
authoritative for .e164.arpa, and all ENUM Tier 1 DNS Servers with
zones delegated from those Tier 0 Servers.
Of course, support is one thing, but use is another. To clarify, the
mandate for Server support of EDNS0 when processing ENUM queries does
not imply spontaneous use. Such a Server operates in exactly the way
described in RFC2671. A DNS Server that is involved in ENUM
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resolution assumes nothing about Resolver support for EDNS0; the
Resolver will indicate this in its query. If a Server receives a
query indicating that the Resolver supports extended responses, then
it replies with an extended response; if it does not receive such an
indication, then it responds with a conventional RFC1035-style reply.
Equally, Resolvers involved in ENUM resolution MUST indicate their
ability to process extended responses when they send ENUM queries -
this is the only way that they will receive such responses.
There are three further aspects to EDNS0 support.
4.1.1. TCP Requirement
First, even with an EDNS0-enabled mechanism, it is still possible
that a query will not succeed. It is possible for a zone to include
a larger set of NAPTRs than will fit into the packet size the
Resolver has reported itself as supporting. Similarly, the ENUM
client may have requested all available resource records, rather than
just the NAPTRs. If for example the queried zone contains several
TXT records, the complete answer may exceed the reported packet size
supported even though a NAPTR-specific query would succeed. Also,
the EDNS0 query may fail for the reasons covered next. In all these
cases the fallback mechanism described in RFC2671 will be needed.
For that fallback process to work for large RRSets, entities will
need to support TCP transport even if EDNS0 is disabled for some
reason.
Thus:
Unless an entity involved knows that EDNS0 queries and responses
work in the current ENUM resolution chain, it SHOULD be willing to
support queries and responses using TCP transport.
4.1.2. Fragmentation Requirement
Second, a DNS Server may receive queries that indicate a given size
of response is acceptable. However, the Resolver may be connected
via a network with a lower MTU, in which case the response packet
will undergo fragmentation and reassembly in transit.
Thus, although obvious (and not directly related to its use in
processing ENUM requests), this means that:
A DNS Server responding to a query that includes the EDNS0 size
option MUST NOT set the DF (Don't Fragment) bit in the UDP packet
holding its answer.
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4.1.3. Intermediary Node Requirement
The final point concerns intermediate nodes. It has been noticed
that some intermediate nodes exhibit overly aggressive behaviour.
Specifically:
Intermediate nodes MUST NOT block valid ENUM queries and responses
that indicate EDNS0 support, even if these are larger that the
size given in RFC1035. In particular, intermediate packet filters
MUST NOT assume that large DNS queries and responses are invalid;
they are not if they indicate EDNS0 support correctly. Such
packet discard strategies are in error.
Intermediate nodes MUST NOT block valid ENUM queries and responses
sent over TCP transport.
This last requirement means that intermediary packet filters MUST NOT
simply block all TCP traffic; it is perfectly reasonable for DNS
queries to be sent over TCP transport, and a more selective strategy
will need to be chosen.
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5. Security Considerations
This document does appear to introduce any extra security issues over
and above those mentioned in RFC3761 and in RFC2671, as well as those
listed in the thorough analysis of the threats to DNS in RFC3833
[11].
It should be noted that mandating the use of EDNS0 by ENUM-related
entities also facilitates the deployment of Secure DNS, DNSSEC,
currently defined in RFC4033 [12], RFC4034 [13] and RFC4035 [14].
Secure DNS will be necessary to verify the integrity of ENUM
responses. RFC3225 [9] states that clients signal their ability to
handle signed responses via the DO (DNSSEC OK) bit in the EDNS0
header and a name server will not return these unless this bit is
set. So unless EDNS0 is used, ENUM-related entities will be unable
to verify DNSSEC-signed responses from the DNS. Signed replies from
the DNS are also much larger than unsigned ones, which provides an
added incentive to use larger UDP payloads.
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6. IANA Considerations
This document has no IANA requirements.
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7. Acknowledgements
We would like to thank the working group members active on the ENUM
mailing list who engaged in this topic, the development and
operational teams that collected data confirming the need for this
mandate, and Alexander Mayrhofer for his detailed review and helpful
comments.
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8. References
8.1. Normative References
[1] Faltstrom, P. and M. Mealling, "The E.164 to Uniform Resource
Identifiers (URI) Dynamic Delegation Discovery System (DDDS)
Application (ENUM)", RFC 3761, April 2004.
[2] Mealling, M., "Dynamic Delegation Discovery System (DDDS) Part
Three: The Domain Name System (DNS) Database", RFC 3403,
October 2002.
[3] Mockapetris, P., "DOMAIN NAMES - CONCEPTS AND FACILITIES",
RFC 1034, November 1987.
[4] Mockapetris, P., "Domain names - implementation and
specification", STD 13, RFC 1035, November 1987.
[5] Elz, R. and R. Bush, "Clarifications to the DNS Specification",
RFC 2181, July 1997.
[6] Braden, R., "Requirements for Internet Hosts -- Application and
Support", RFC 1123, October 1989.
[7] Vixie, P., "Extension Mechanisms for DNS (EDNS0)", RFC 2671,
August 1999.
8.2. Informative References
[8] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", RFC 2119, BCP 14, March 1997.
[9] Conrad, D., "Indicating Resolver Support of DNSSEC", RFC 3225,
December 2001.
[10] Gudmundsson, O., "DNSSEC and IPv6 A6 Requirements", RFC 3226,
December 2001.
[11] Atkins, D. and R. Austein, "Threat Analysis of the Domain Name
System (DNS)", RFC 3833, August 2004.
[12] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"DNS Security Introduction and Requirements", RFC 4033,
March 2005.
[13] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Resource Records for the DNS Security Extensions", RFC 4034,
March 2005.
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[14] Arends, R., Austein, R., Larson, M., Massey, D., and S. Rose,
"Protocol Modifications for the DNS Security Extensions",
RFC 4035, March 2005.
[15] Bradner, S., "The Internet Standards Process -- Revision 3",
RFC 2026, BCP 9, October 1996.
[16] Bradner, S., "IETF Rights in Contributions", BCP 78, RFC 3978,
March 2005.
[17] Bradner, S., "Intellectual Property Rights in IETF Technology",
BCP 79, RFC 3979, March 2005.
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Authors' Addresses
Lawrence Conroy
Roke Manor Research
Roke Manor
Old Salisbury Lane
Romsey
United Kingdom
Phone: +44-1794-833666
Email: lconroy@insensate.co.uk
URI: http://www.sienum.co.uk
Jim Reid
DNS-MODA
DNS-MODA
6 Langside Court
Bothwell, SCOTLAND
United Kingdom
Phone: +44 1698 852881
Email: jim@dns-moda.org
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