draft-ietf-dnsext-rfc2671bis-edns0-03.txt   draft-ietf-dnsext-rfc2671bis-edns0-04.txt 
DNSEXT Working Group M. Graff DNSEXT Working Group J. Damas
Internet-Draft P. Vixie Internet-Draft M. Graff
Obsoletes: 2671, 2673 Internet Systems Consortium Obsoletes: 2671, 2673 P. Vixie
(if approved) March 25, 2010 (if approved) Internet Systems Consortium
Intended status: Standards Track Intended status: Standards Track November 8, 2010
Expires: September 26, 2010 Expires: May 12, 2011
Extension Mechanisms for DNS (EDNS0) Extension Mechanisms for DNS (EDNS0)
draft-ietf-dnsext-rfc2671bis-edns0-03 draft-ietf-dnsext-rfc2671bis-edns0-04
Abstract Abstract
The Domain Name System's wire protocol includes a number of fixed The Domain Name System's wire protocol includes a number of fixed
fields whose range has been or soon will be exhausted and does not fields whose range has been or soon will be exhausted and does not
allow requestors to advertise their capabilities to responders. This allow requestors to advertise their capabilities to responders. This
document describes backward compatible mechanisms for allowing the document describes backward compatible mechanisms for allowing the
protocol to grow. protocol to grow.
This document updates the EDNS0 specification (RFC2671) based on 10 This document updates the EDNS0 specification (RFC2671) based on 10
years of deployment experience. years of deployment experience.
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 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). Note that other groups may also distribute
other groups may also distribute working documents as Internet- working documents as Internet-Drafts. The list of current Internet-
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Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
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 This Internet-Draft will expire on May 12, 2011.
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This Internet-Draft will expire on September 26, 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
carefully, as they describe your rights and restrictions with respect carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must to this document. Code Components extracted from this document must
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the Trust Legal Provisions and are provided without warranty as the Trust Legal Provisions and are provided without warranty as
described in the BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. EDNS Support Requirement . . . . . . . . . . . . . . . . . . . 3 3. EDNS Support Requirement . . . . . . . . . . . . . . . . . . . 3
4. Affected Protocol Elements . . . . . . . . . . . . . . . . . . 4 4. Affected Protocol Elements . . . . . . . . . . . . . . . . . . 4
4.1. Message Header . . . . . . . . . . . . . . . . . . . . . . 4 4.1. Message Header . . . . . . . . . . . . . . . . . . . . . . 4
4.2. Label Types . . . . . . . . . . . . . . . . . . . . . . . 4 4.2. Label Types . . . . . . . . . . . . . . . . . . . . . . . 4
4.3. UDP Message Size . . . . . . . . . . . . . . . . . . . . . 4 4.3. UDP Message Size . . . . . . . . . . . . . . . . . . . . . 4
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"Requestor" is the side which sends a request. "Responder" is an "Requestor" is the side which sends a request. "Responder" is an
authoritative, recursive resolver, or other DNS component which authoritative, recursive resolver, or other DNS component which
responds to questions. responds to questions.
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 RFC 2119 [RFC2119]. document are to be interpreted as described in RFC 2119 [RFC2119].
3. EDNS Support Requirement 3. EDNS Support Requirement
EDNS support is mandatory in a modern world. DNSSEC requires EDNS EDNS support is practically mandatory in a modern world. DNSSEC
support, and many other Features are made possible only by EDNS requires EDNS support, and many other Features are made possible only
support to request or advertise them. Many organizations are by EDNS support to request or advertise them. Many organizations are
beginning to require DNSSEC. Without common interoperability, DNSSEC beginning to require DNSSEC. Without common interoperability, DNSSEC
cannot be as easily deployed. cannot be as easily deployed.
DNS publishers are wanting to put more data in answers. DNSSEC DNS publishers are wanting to put more data in answers. DNSSEC
DNSKEY records, negative answers, and many other DNSSEC queries cause DNSKEY records, negative answers, and many other DNSSEC queries cause
larger answers to be returned. In order to support this, DNS larger answers to be returned. In order to support this, DNS
servers, middleware, and stub resolvers MUST support larger packet servers, middleware, and stub resolvers MUST support larger packet
sizes advertised via EDNS0. sizes advertised via EDNS0.
4. Affected Protocol Elements 4. Affected Protocol Elements
4.1. Message Header 4.1. Message Header
The DNS Message Header's second full 16-bit word is divided into a The DNS Message Header's second full 16-bit word is divided into a
4-bit OPCODE, a 4-bit RCODE, and a number of 1-bit flags (see , 4-bit OPCODE, a 4-bit RCODE, and a number of 1-bit flags (see ,
section 4.1.1 [RFC1035]). Some of these were marked for future use, section 4.1.1 [RFC1035]). Some of these were marked for future use,
and most these have since been allocated. Also, most of the RCODE and most these have since been allocated. Also, most of the RCODE
values are now in use. The OPT pseudo-RR specified below contains values are now in use. The OPT pseudo-RR specified below contains
subfields that carry a bit field extension of the RCODE field and extensions to the RCODE bit field as well as additional flag bits.
additional flag bits, respectively.
4.2. Label Types 4.2. Label Types
The first two bits of a wire format domain label are used to denote The first two bits of a wire format domain label are used to denote
the type of the label. [RFC1035] allocates two of the four possible the type of the label. [RFC1035] allocates two of the four possible
types and reserves the other two. More label types were defined in types and reserves the other two. More label types were defined in
[RFC2671]. [RFC2671]. This document obsoletes the use of the 2-bit combination
defined by [RFC2671] to identify extended label types.
4.3. UDP Message Size 4.3. UDP Message Size
Traditional DNS Messages are limited to 512 octets in size when sent Traditional DNS Messages are limited to 512 octets in size when sent
over UDP ([RFC1035]). Today, many organizations wish to return many over UDP ([RFC1035]). Today, many organizations wish to return many
records in a single reply, and special tricks are needed to make the records in a single reply, and special tricks are needed to make the
responses fit in this 512-byte limit. Additionally, DNSSEC responses fit in this 512-byte limit. Additionally, DNSSEC
signatures can easily generate a much larger response than a 512 byte signatures can easily generate a much larger response than a 512 byte
message can hold. message can hold.
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6.3. Caching behavior 6.3. Caching behavior
The OPT record must not be cached. The OPT record must not be cached.
6.4. Fallback 6.4. Fallback
If a requestor detects that the remote end does not support EDNS0, it If a requestor detects that the remote end does not support EDNS0, it
MAY issue queries without an OPT record. It MAY cache this knowledge MAY issue queries without an OPT record. It MAY cache this knowledge
for a brief time in order to avoid fallback delays in the future. for a brief time in order to avoid fallback delays in the future.
However, if DNSSEC is required, no fallback should be performed as However, if DNSSEC or any future option using EDNS is required, no
DNSSEC is only signaled through EDNS0. fallback should be performed as they are only signaled through EDNS0.
6.5. Requestor's Payload Size 6.5. Requestor's Payload Size
The requestor's UDP payload size (which OPT stores in the RR CLASS The requestor's UDP payload size (which OPT stores in the RR CLASS
field) is the number of octets of the largest UDP payload that can be field) is the number of octets of the largest UDP payload that can be
reassembled and delivered in the requestor's network stack. Note reassembled and delivered in the requestor's network stack. Note
that path MTU, with or without fragmentation, may be smaller than that path MTU, with or without fragmentation, may be smaller than
this. Values lower than 512 MUST be treated as equal to 512. this. Values lower than 512 MUST be treated as equal to 512.
Requestors SHOULD place a value in this field that it can actually Requestors SHOULD place a value in this field that it can actually
receive. For example, if a requestor sits behind a firewall which receive. For example, if a requestor sits behind a firewall which
will block fragmented IP packets, a requestor SHOULD not choose a will block fragmented IP packets, a requestor SHOULD not choose a
value which will cause fragmentation. Doing so will prevent large value which will cause fragmentation. Doing so will prevent large
responses from being received, and can cause fallback to occur. responses from being received, and can cause fallback to occur.
Note that a 512-octet UDP payload requires a 576-octet IP reassembly Note that a 512-octet UDP payload requires a 576-octet IP reassembly
buffer. Choosing 1280 for IPv4 over Ethernet would be reasonable. buffer. Choosing between 1280 and 1410 bytes for IP (v4 or v6) over
Choosing a very large value will guarantee fragmentation at the IP Ethernet would be reasonable. Choosing a very large value will
layer, and may prevent answers from being received due to a single guarantee fragmentation at the IP layer, and may prevent answers from
fragment loss or misconfigured firewalls. being received due to a single fragment loss or misconfigured
firewalls.
The requestor's maximum payload size can change over time. It MUST The requestor's maximum payload size can change over time. It MUST
not be cached for use beyond the transaction in which it is not be cached for use beyond the transaction in which it is
advertised. advertised.
6.6. Responder's Payload Size 6.6. Responder's Payload Size
The responder's maximum payload size can change over time, but can be The responder's maximum payload size can change over time, but can be
reasonably expected to remain constant between two sequential reasonably expected to remain constant between two closely spaced
transactions; for example, a meaningless QUERY to discover a sequential transactions; for example, a meaningless QUERY to discover
responder's maximum UDP payload size, followed immediately by an a responder's maximum UDP payload size, followed immediately by an
UPDATE which takes advantage of this size. This is considered UPDATE which takes advantage of this size. This is considered
preferable to the outright use of TCP for oversized requests, if preferable to the outright use of TCP for oversized requests, if
there is any reason to suspect that the responder implements EDNS, there is any reason to suspect that the responder implements EDNS,
and if a request will not fit in the default 512 payload size limit. and if a request will not fit in the default 512 payload size limit.
6.7. Payload Size Selection 6.7. Payload Size Selection
Due to transaction overhead, it is unwise to advertise an Due to transaction overhead, it is unwise to advertise an
architectural limit as a maximum UDP payload size. Just because your architectural limit as a maximum UDP payload size. Just because your
stack can reassemble 64KB datagrams, don't assume that you want to stack can reassemble 64KB datagrams, don't assume that you want to
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A requestor MAY choose to implement a fallback to smaller advertised A requestor MAY choose to implement a fallback to smaller advertised
sizes to work around firewall or other network limitations. A sizes to work around firewall or other network limitations. A
requestor SHOULD choose to use a fallback mechanism which begins with requestor SHOULD choose to use a fallback mechanism which begins with
a large size, such as 4096. If that fails, a fallback around the a large size, such as 4096. If that fails, a fallback around the
1280 byte range SHOULD be tried, as it has a reasonable chance to fit 1280 byte range SHOULD be tried, as it has a reasonable chance to fit
within a single Ethernet frame. Failing that, a requestor MAY choose within a single Ethernet frame. Failing that, a requestor MAY choose
a 512 byte packet, which with large answers may cause a TCP retry. a 512 byte packet, which with large answers may cause a TCP retry.
6.8. Middleware Boxes 6.8. Middleware Boxes
Middleware boxes MUST NOT limit DNS messages over UDP to 512 bytes. Middleware boxes (e.g. firewalls, SOHO routers, load balancers, etc)
MUST NOT limit DNS messages over UDP to 512 bytes.
Middleware boxes which simply forward requests to a recursive Middleware boxes which simply forward requests to a recursive
resolver MUST NOT modify the OPT record contents in either direction. resolver MUST NOT modify and MUST NOT delete the OPT record contents
in either direction.
Middleware boxes which have additional functionality, such as Middleware boxes which have additional functionality, such as
answering certain queries or acting like an intelligent forwarder, answering certain queries or acting like an intelligent forwarder,
MUST understand the OPT record. These boxes MUST consider the MUST understand the OPT record. These boxes MUST consider the
incoming request and any outgoing requests as separate transactions incoming request and any outgoing requests as separate transactions
if the characteristics of the messages are different. if the characteristics of the messages are different.
6.9. OPT Record TTL Field Use 6.9. OPT Record TTL Field Use
The extended RCODE and flags (which OPT stores in the RR TTL field) The extended RCODE and flags (which OPT stores in the RR TTL field)
are structured as follows: are structured as follows:
+0 (MSB) +1 (LSB) +0 (MSB) +1 (LSB)
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: | EXTENDED-RCODE | VERSION | 0: | EXTENDED-RCODE | VERSION |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | DO| Z | 2: | DO| Z |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
EXTENDED-RCODE EXTENDED-RCODE
Forms upper 8 bits of extended 12-bit RCODE. Note that Forms upper 8 bits of extended 12-bit RCODE (together with the
EXTENDED-RCODE value 0 indicates that an unextended RCODE is in 4 bits defined in [RFC1035]. Note that EXTENDED-RCODE value 0
use (values 0 through 15). indicates that an unextended RCODE is in use (values 0 through
15).
VERSION VERSION
Indicates the implementation level of whoever sets it. Full Indicates the implementation level of whoever sets it. Full
conformance with this specification is indicated by version conformance with this specification is indicated by version
``0.'' Requestors are encouraged to set this to the lowest ``0.'' Requestors are encouraged to set this to the lowest
implemented level capable of expressing a transaction, to implemented level capable of expressing a transaction, to
minimize the responder and network load of discovering the minimize the responder and network load of discovering the
greatest common implementation level between requestor and greatest common implementation level between requestor and
responder. A requestor's version numbering strategy MAY responder. A requestor's version numbering strategy MAY
ideally be a run time configuration option. ideally be a run time configuration option.
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distinguish between servers which do not implement EDNS and format distinguish between servers which do not implement EDNS and format
errors within EDNS. errors within EDNS.
The minimal response must be the DNS header, question section, and an The minimal response must be the DNS header, question section, and an
OPT record. This must also occur when an truncated response (using OPT record. This must also occur when an truncated response (using
the DNS header's TC bit) is returned. the DNS header's TC bit) is returned.
8. Security Considerations 8. Security Considerations
Requestor-side specification of the maximum buffer size may open a Requestor-side specification of the maximum buffer size may open a
new DNS denial of service attack if responders can be made to send DNS denial of service attack if responders can be made to send
messages which are too large for intermediate gateways to forward, messages which are too large for intermediate gateways to forward,
thus leading to potential ICMP storms between gateways and thus leading to potential ICMP storms between gateways and
responders. responders.
Announcing very large UDP buffer sizes may result in dropping by Announcing very large UDP buffer sizes may result in dropping by
firewalls. This could cause retransmissions with no hope of success. middleboxes (see Section 6.8). This could cause retransmissions with
Some devices reject fragmented UDP packets. no hope of success. Some devices have been found to reject
fragmented UDP packets.
Announcing too small UDP buffer sizes may result in fallback to TCP. Announcing too small UDP buffer sizes may result in fallback to TCP
This is especially important with DNSSEC, where answers are much with a corresponding load impact on DNS servers. This is especially
larger. important with DNSSEC, where answers are much larger.
9. IANA Considerations 9. IANA Considerations
The IANA has assigned RR type code 41 for OPT. The IANA has assigned RR type code 41 for OPT.
[RFC2671] specified a number of IANA sub-registries within "DOMAIN [RFC2671] specified a number of IANA sub-registries within "DOMAIN
NAME SYSTEM PARAMETERS:" NAME SYSTEM PARAMETERS:"
o EDNS Extended Label Type o EDNS Extended Label Type
skipping to change at page 12, line 25 skipping to change at page 12, line 25
[RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC", [RFC3225] Conrad, D., "Indicating Resolver Support of DNSSEC",
RFC 3225, December 2001. RFC 3225, December 2001.
10.2. Informative References 10.2. Informative References
[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.
Authors' Addresses Authors' Addresses
Joao Damas
Internet Systems Consortium
950 Charter Street
Redwood City, California 94063
US
Phone: +1 650.423.1312
Email: joao@isc.org
Michael Graff Michael Graff
Internet Systems Consortium Internet Systems Consortium
950 Charter Street 950 Charter Street
Redwood City, California 94063 Redwood City, California 94063
US US
Phone: +1 650.423.1304 Phone: +1 650.423.1304
Email: mgraff@isc.org Email: mgraff@isc.org
Paul Vixie Paul Vixie
Internet Systems Consortium Internet Systems Consortium
950 Charter Street 950 Charter Street
Redwood City, California 94063 Redwood City, California 94063
US US
Phone: +1 650.423.1301 Phone: +1 650.423.1301
Email: vixie@isc.org Email: vixie@isc.org
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