draft-ietf-dnsext-rfc2671bis-edns0-00.txt   draft-ietf-dnsext-rfc2671bis-edns0-01.txt 
DNSEXT Working Group Paul Vixie, ISC DNSEXT Working Group Paul Vixie, ISC
INTERNET-DRAFT <draft-ietf-dnsext-rfc2671bis-edns0-00.txt> INTERNET-DRAFT
December 27, 2007 <draft-ietf-dnsext-rfc2671bis-edns0-01.txt> March 17, 2008
Intended Status: Standards Track
Obsoletes: 2671 (if approved)
Revised extension mechanisms for DNS (EDNS0) Revised extension mechanisms for DNS (EDNS0)
Status of this Memo Status of this Memo
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Copyright Notice Copyright Notice
Copyright (C) The IETF Trust (2007). Copyright (C) The IETF Trust (2007).
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 clients to advertise their capabilities to servers. This allow clients to advertise their capabilities to servers. This
document describes backward compatible mechanisms for allowing the document describes backward compatible mechanisms for allowing the
protocol to grow. protocol to grow.
1 - Introduction 1 - Introduction
1.1. DNS (see [RFC1035]) specifies a Message Format and within such 1.1. DNS (see [RFC1035]) specifies a Message Format and within such
messages there are standard formats for encoding options, errors, and messages there are standard formats for encoding options, errors, and
skipping to change at page 2, line 9 skipping to change at page 2, line 9
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 clients to advertise their capabilities to servers. This allow clients to advertise their capabilities to servers. This
document describes backward compatible mechanisms for allowing the document describes backward compatible mechanisms for allowing the
protocol to grow. protocol to grow.
1 - Introduction 1 - Introduction
1.1. DNS (see [RFC1035]) specifies a Message Format and within such 1.1. DNS (see [RFC1035]) specifies a Message Format and within such
messages there are standard formats for encoding options, errors, and messages there are standard formats for encoding options, errors, and
name compression. The maximum allowable size of a DNS Message is name compression. The maximum allowable size of a DNS Message is fixed.
fixed. Many of DNS's protocol limits are too small for uses which are Many of DNS's protocol limits are too small for uses which are or which
or which are desired to become common. There is no way for are desired to become common. There is no way for implementations to
implementations to advertise their capabilities. advertise their capabilities.
1.2. Unextended agents will not know how to interpret the protocol 1.2. Unextended agents will not know how to interpret the protocol
extensions detailed here. In practice, these clients will be upgraded extensions detailed here. In practice, these clients will be upgraded
when they have need of a new feature, and only new features will make when they have need of a new feature, and only new features will make
use of the extensions. Extended agents must be prepared for behaviour use of the extensions. Extended agents must be prepared for behaviour
of unextended clients in the face of new protocol elements, and fall of unextended clients in the face of new protocol elements, and fall
back gracefully to unextended DNS. back gracefully to unextended DNS. RFC 2671 originally has proposed
extensions to the basic DNS protocol to overcome these deficiencies.
This memo refines that specification and obsoletes RFC 2671.
1.3. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL 1.3. The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
this document are to be interpreted as described in RFC 2119 document are to be interpreted as described in RFC 2119 [RFC2119].
[RFC2119].
2 - Affected Protocol Elements 2 - Affected Protocol Elements
2.1. The DNS Message Header's (see [RFC1035 4.1.1]) second full 16-bit 2.1. The DNS Message Header's (see [RFC1035 4.1.1]) second full 16-bit
word is divided into a 4-bit OPCODE, a 4-bit RCODE, and a number of word is divided into a 4-bit OPCODE, a 4-bit RCODE, and a number of
1-bit flags. The original reserved Z bits have been allocated to 1-bit flags. The original reserved Z bits have been allocated to
various purposes, and most of the RCODE values are now in use. More various purposes, and most of the RCODE values are now in use. More
flags and more possible RCODEs are needed. flags and more possible RCODEs are needed. The OPT pseudo-RR specified
in Section 4 contains subfields that carry a bit field extension of the
RCODE field and additional flag bits, respectively; for details see
Section 4.6 below.
2.2. The first two bits of a wire format domain label are used to 2.2. The first two bits of a wire format domain label are used to denote
denote the type of the label. [RFC1035 4.1.4] allocates two of the the type of the label. [RFC1035 4.1.4] allocates two of the four
four possible types and reserves the other two. Proposals for use of possible types and reserves the other two. Proposals for use of the
the remaining types far outnumber those available. More label types remaining types far outnumber those available. More label types were
were needed, and an extension mechanism was proposed in RFC 2671 needed, and an extension mechanism was proposed in RFC 2671 [RFC2671
[RFC2671 Section 3]. Section 3]. Section 3 of this document reserves DNS labels with a first
octet in the range of 64-127 decimal (label type 01) for future
standardization of Extended DNS Labels.
2.3. DNS Messages are limited to 512 octets in size when sent over 2.3. DNS Messages are limited to 512 octets in size when sent over UDP.
UDP. While the minimum maximum reassembly buffer size still allows a While the minimum maximum reassembly buffer size still allows a limit of
limit of 512 octets of UDP payload, most of the hosts now connected to 512 octets of UDP payload, most of the hosts now connected to the
the Internet are able to reassemble larger datagrams. Some mechanism Internet are able to reassemble larger datagrams. Some mechanism must
must be created to allow requestors to advertise larger buffer sizes be created to allow requestors to advertise larger buffer sizes to
to responders. responders. To this end, the OPT pseudo-RR specified in Section 4
contains a maximum payload size field; for details see Section 4.5
below.
3 - Extended Label Types 3 - Extended Label Types
[RFC2671 Section 3] reserved label type "0 1" to indicate that an The first octet in the on-the-wire representation of a DNS label
extended label type followed in the next octet, but gave inadequate specifies the label type; the basic DNS specification [RFC1035]
guidance as to how EDNS, as a hop-by-hop signalling method, could be dedicates the two most significant bits of that octet for this purpose.
used to carry a new kind of DNS label. Extended label types might be
addressed in a future specification, perhaps requiring that the EDNS This document reserves DNS label type 0b01 for use as an indication for
VERSION be incremented. Extended Label Types. A specific extended label type is selected by the
6 least significant bits of the first octet. Thus, Extended Label Types
are indicated by the values 64-127 (0b01xxxxxx) in the first octet of
the label.
Allocations from this range are to be made for IETF documents fully
describing the syntax and semantics as well as the applicability of the
particular Extended Label Type.
This document does not describe any specific Extended Label Type.
4 - OPT pseudo-RR 4 - OPT pseudo-RR
4.1. One OPT pseudo-RR (RR type 41) MAY be added to the additional 4.1. One OPT pseudo-RR (RR type 41) MAY be added to the additional data
data section of a request, and to responses to such requests. An OPT section of a request, and to responses to such requests. An OPT is
is called a pseudo-RR because it pertains to a particular transport called a pseudo-RR because it pertains to a particular transport level
level message and not to any actual DNS data. OPT RRs MUST NOT be message and not to any actual DNS data. OPT RRs MUST NOT be cached,
cached, forwarded, or stored in or loaded from master files. The forwarded, or stored in or loaded from master files. The quantity of
quantity of OPT pseudo-RRs per message MUST be either zero or one, but OPT pseudo-RRs per message MUST be either zero or one, but not greater.
not greater.
4.2. An OPT RR has a fixed part and a variable set of options 4.2. An OPT RR has a fixed part and a variable set of options expressed
expressed as {attribute, value} pairs. The fixed part holds some DNS as {attribute, value} pairs. The fixed part holds some DNS meta data
meta data and also a small collection of new protocol elements which and also a small collection of new protocol elements which we expect to
we expect to be so popular that it would be a waste of wire space to be so popular that it would be a waste of wire space to encode them as
encode them as {attribute, value} pairs. {attribute, value} pairs.
4.3. The fixed part of an OPT RR is structured as follows: 4.3. The fixed part of an OPT RR is structured as follows:
Field Name Field Type Description Field Name Field Type Description
------------------------------------------------------ ------------------------------------------------------
NAME domain name empty (root domain) NAME domain name empty (root domain)
TYPE u_int16_t OPT TYPE u_int16_t OPT (41)
CLASS u_int16_t sender's UDP payload size CLASS u_int16_t sender's UDP payload size
TTL u_int32_t extended RCODE and flags TTL u_int32_t extended RCODE and flags
RDLEN u_int16_t describes RDATA RDLEN u_int16_t describes RDATA
RDATA octet stream {attribute,value} pairs RDATA octet stream {attribute,value} pairs
4.4. The variable part of an OPT RR is encoded in its RDATA and is 4.4. The variable part of an OPT RR is encoded in its RDATA and is
structured as zero or more of the following: structured as zero or more of the following:
+0 (MSB) +1 (LSB) : +0 (MSB) : +1 (LSB) :
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: | OPTION-CODE | 0: | OPTION-CODE |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | OPTION-LENGTH | 2: | OPTION-LENGTH |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
4: | | 4: | |
/ OPTION-DATA / / OPTION-DATA /
/ / / /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
OPTION-CODE (Assigned by IANA.) OPTION-CODE (Assigned by IANA.)
OPTION-LENGTH Size (in octets) of OPTION-DATA. OPTION-LENGTH Size (in octets) of OPTION-DATA.
OPTION-DATA Varies per OPTION-CODE. OPTION-DATA Varies per OPTION-CODE.
4.4.1. Order of appearance of option tuples is never relevant. Any 4.4.1. Order of appearance of option tuples is never relevant. Any
option whose meaning is affected by other options is so affected no option whose meaning is affected by other options is so affected no
matter which one comes first in the OPT RDATA. matter which one comes first in the OPT RDATA.
4.4.2. Any OPTION-CODE values not understood by a responder or 4.4.2. Any OPTION-CODE values not understood by a responder or requestor
requestor MUST be ignored. So, specifications of such options might MUST be ignored. So, specifications of such options might wish to
wish to include some kind of signalled acknowledgement. For example, include some kind of signalled acknowledgement. For example, an option
an option specification might say that if a responder sees option XYZ, specification might say that if a responder sees option XYZ, it SHOULD
it SHOULD include option XYZ in its response. include option XYZ in its response.
4.5. The sender's UDP payload size (which OPT stores in the RR CLASS 4.5. The sender'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 sender's network stack. Note that reassembled and delivered in the sender's network stack. Note that path
path MTU, with or without fragmentation, may be smaller than this. MTU, with or without fragmentation, may be smaller than this. Values
lower than 512 are undefined, and may be treated as format errors, or
may be treated as equal to 512, at the implementor's discretion.
4.5.1. Note that a 512-octet UDP payload requires a 576-octet IP 4.5.1. Note that a 512-octet UDP payload requires a 576-octet IP
reassembly buffer. Choosing 1280 on an Ethernet connected requestor reassembly buffer. Choosing 1280 on an Ethernet connected requestor
would be reasonable. The consequence of choosing too large a value would be reasonable. The consequence of choosing too large a value may
may be an ICMP message from an intermediate gateway, or even a silent be an ICMP message from an intermediate gateway, or even a silent drop
drop of the response message. of the response message.
4.5.2. Both requestors and responders are advised to take account of 4.5.2. Both requestors and responders are advised to take account of the
the path's discovered MTU (if already known) when considering message path's discovered MTU (if already known) when considering message sizes.
sizes.
4.5.3. The requestor's maximum payload size can change over time, and 4.5.3. The requestor's maximum payload size can change over time, and
therefore MUST NOT be cached for use beyond the transaction in which therefore MUST NOT be cached for use beyond the transaction in which it
it is advertised. is advertised.
4.5.4. The responder's maximum payload size can change over time, but 4.5.4. The responder's maximum payload size can change over time, but
can be reasonably expected to remain constant between two sequential can be reasonably expected to remain constant between two sequential
transactions; for example, a meaningless QUERY to discover a transactions; for example, a meaningless QUERY to discover a responder's
responder's maximum UDP payload size, followed immediately by an maximum UDP payload size, followed immediately by an UPDATE which takes
UPDATE which takes advantage of this size. (This is considered advantage of this size. (This is considered preferrable to the outright
preferrable to the outright use of TCP for oversized requests, if use of TCP for oversized requests, if there is any reason to suspect
there is any reason to suspect that the responder implements EDNS, and that the responder implements EDNS, and if a request will not fit in the
if a request will not fit in the default 512 payload size limit.) default 512 payload size limit.)
4.5.5. Due to transaction overhead, it is unwise to advertise an 4.5.5. 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 spend
spend more than about 4KB of state memory per ongoing transaction. more than about 4KB of state memory per ongoing transaction.
4.6. The extended RCODE and flags (which OPT stores in the RR TTL 4.6. The extended RCODE and flags (which OPT stores in the RR TTL field)
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 Forms upper 8 bits of extended 12-bit RCODE. Note EXTENDED-RCODE Forms upper 8 bits of extended 12-bit RCODE. Note that
that EXTENDED-RCODE value "0" indicates that an EXTENDED-RCODE value zero (0) indicates that an
unextended RCODE is in use (values "0" through "15"). unextended RCODE is in use (values zero (0) through
fifteen (15)).
VERSION Indicates the implementation level of whoever sets it. VERSION Indicates the implementation level of whoever sets it.
Full conformance with this specification is indicated Full conformance with this specification is indicated by
by version ``0.'' Requestors are encouraged to set version zero (0). Requestors are encouraged to set this
this to the lowest implemented level capable of to the lowest implemented level capable of expressing a
expressing a transaction, to minimize the responder transaction, to minimize the responder and network load
and network load of discovering the greatest common of discovering the greatest common implementation level
implementation level between requestor and responder. between requestor and responder. A requestor's version
A requestor's version numbering strategy should numbering strategy should ideally be a run time
ideally be a run time configuration option. configuration option.
If a responder does not implement the VERSION level of If a responder does not implement the VERSION level of
the request, then it answers with RCODE=BADVERS. All the request, then it answers with RCODE=BADVERS. All
responses MUST be limited in format to the VERSION responses MUST be limited in format to the VERSION level
level of the request, but the VERSION of each response of the request, but the VERSION of each response MUST be
MUST be the highest implementation level of the the highest implementation level of the responder. In
responder. In this way a requestor will learn the this way a requestor will learn the implementation level
implementation level of a responder as a side effect of a responder as a side effect of every response,
of every response, including error responses, including error responses, including RCODE=BADVERS.
including RCODE=BADVERS.
DO DNSSEC OK bit [RFC3225]. DO DNSSEC OK bit [RFC3225].
Z Set to zero by senders and ignored by receivers, Z Set to zero by senders and ignored by receivers, unless
unless modified in a subsequent specification modified in a subsequent specification [IANAFLAGS].
[IANAFLAGS].
5 - Transport Considerations 5 - Transport Considerations
5.1. The presence of an OPT pseudo-RR in a request is an indication 5.1. The presence of an OPT pseudo-RR in a request is an indication that
that the requestor fully implements the given version of EDNS, and can the requestor fully implements the given version of EDNS, and can
correctly understand any response that conforms to that feature's correctly understand any response that conforms to that feature's
specification. specification.
5.2. Lack of use of these features in a request is an indication that 5.2. Lack of use of these features in a request is an indication that
the requestor does not implement any part of this specification and the requestor does not implement any part of this specification and that
that the responder SHOULD NOT use any protocol extension described the responder SHOULD NOT use any protocol extension described here in
here in its response. its response.
5.3. Responders who do not understand these protocol extensions are 5.3. Responders who do not understand these protocol extensions are
expected to send a response with RCODE NOTIMPL, FORMERR, or SERVFAIL, expected to send a response with RCODE NOTIMPL, FORMERR, or SERVFAIL, or
or to appear to "time out" due to inappropriate action by a "middle to appear to "time out" due to inappropriate action by a "middle box"
box" such as a NAT. Therefore use of extensions SHOULD be ``probed'' such as a NAT, or to ignore extensions and respond only to unextended
such that a responder who isn't known to support them be allowed a
retry with no extensions if it responds with such an RCODE, or does protocol elements. Therefore use of extensions SHOULD be "probed" such
not respond. If a responder's capability level is cached by a that a responder who isn't known to support them be allowed a retry with
requestor, a new probe SHOULD be sent periodically to test for changes no extensions if it responds with such an RCODE, or does not respond.
to responder capability. If a responder's capability level is cached by a requestor, a new probe
SHOULD be sent periodically to test for changes to responder capability.
5.4. If EDNS is used in a request, and the response arrives with TC set
and with no EDNS OPT RR, a requestor should assume that truncation
prevented the OPT RR from being appended by the responder, and further,
that EDNS is not used in the response. Correspondingly, an EDNS
responder who cannot fit all necessary elements (including an OPT RR)
into a response, should respond with a normal (unextended) DNS response,
possibly setting TC if the response will not fit in the unextended
response message's 512-octet size.
6 - Security Considerations 6 - Security Considerations
Requestor-side specification of the maximum buffer size may open a new 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
messages which are too large for intermediate gateways to forward, which are too large for intermediate gateways to forward, thus leading
thus leading to potential ICMP storms between gateways and responders. to potential ICMP storms between gateways and responders.
7 - IANA Considerations 7 - IANA Considerations
IANA has allocated RR type code 41 for OPT. IANA has allocated RR type code 41 for OPT.
This document controls the following IANA sub-registries in registry This document controls the following IANA sub-registries in registry
"DOMAIN NAME SYSTEM PARAMETERS": "DOMAIN NAME SYSTEM PARAMETERS":
"EDNS Extended Label Type" "EDNS Extended Label Type"
"EDNS Option Codes" "EDNS Option Codes"
"EDNS Version Numbers" "EDNS Version Numbers"
"Domain System Response Code" "Domain System Response Code"
IANA is advised to re-parent these subregistries to this document.
This document assigns label type 0b01xxxxxx as "EDNS Extended Label This document assigns label type 0b01xxxxxx as "EDNS Extended Label
Type." We request that IANA record this assignment. Type." We request that IANA record this assignment.
This document assigns option code 65535 to "Reserved for future This document assigns option code 65535 to "Reserved for future
expansion." expansion."
This document assigns EDNS Extended RCODE "16" to "BADVERS". This document assigns EDNS Extended RCODE "16" to "BADVERS".
IESG approval is required to create new entries in the EDNS Extended IESG approval is required to create new entries in the EDNS Extended
Label Type or EDNS Version Number registries, while any published RFC Label Type or EDNS Version Number registries, while any published RFC
(including Informational, Experimental, or BCP) is grounds for (including Informational, Experimental, or BCP) is grounds for
allocation of an EDNS Option Code. allocation of an EDNS Option Code.
8 - Acknowledgements 8 - Acknowledgements
Paul Mockapetris, Mark Andrews, Robert Elz, Don Lewis, Bob Halley, Paul Mockapetris, Mark Andrews, Robert Elz, Don Lewis, Bob Halley,
Donald Eastlake, Rob Austein, Matt Crawford, Randy Bush, and Thomas Donald Eastlake, Rob Austein, Matt Crawford, Randy Bush, Thomas Narten,
Narten were each instrumental in creating and refining this Alfred Hoenes and Markku Savela were each instrumental in creating and
specification. refining this specification.
9 - References 9 - References
[RFC1035] P. Mockapetris, ``Domain Names - Implementation and [RFC1035] P. Mockapetris, "Domain Names - Implementation and
Specification,'' RFC 1035, USC/Information Sciences Specification," RFC 1035, USC/Information Sciences
Institute, November 1987. Institute, November 1987.
[RFC2119] S. Bradner, ``Key words for use in RFCs to Indicate [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels,'' RFC 2119, Harvard University, March Requirement Levels," RFC 2119, Harvard University, March
1997. 1997.
[RFC2671] P. Vixie, ``Extension mechanisms for DNS (EDNS0),'' RFC [RFC2671] P. Vixie, "Extension mechanisms for DNS (EDNS0)," RFC 2671,
2671, Internet Software Consortium, August 1999. Internet Software Consortium, August 1999.
[RFC3225] D. Conrad, ``Indicating Resolver Support of DNSSEC,'' RFC [RFC3225] D. Conrad, "Indicating Resolver Support of DNSSEC," RFC
3225, Nominum Inc., December 2001. 3225, Nominum Inc., December 2001.
[IANAFLAGS] IANA, ``DNS Header Flags and EDNS Header Flags,'' web [IANAFLAGS] IANA, "DNS Header Flags and EDNS Header Flags," web site
site http://www.iana.org/assignments/dns-header-flags, as http://www.iana.org/assignments/dns-header-flags, as of
of June 2005 or later. June 2005 or later.
10 - Author's Address 10 - Author's Address
Paul Vixie Paul Vixie
Internet Systems Consortium Internet Systems Consortium
950 Charter Street 950 Charter Street
Redwood City, CA 94063 Redwood City, CA 94063
+1 650 423 1301 +1 650 423 1301
<vixie@isc.org> EMail: vixie@isc.org
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rights that may cover technology that may be required to implement that may cover technology that may be required to implement this
this standard. Please address the information to the IETF at standard. Please address the information to the IETF at
ietf-ipr@ietf.org. ietf-ipr@ietf.org.
Acknowledgement Acknowledgement
Funding for the RFC Editor function is provided by the IETF Funding for the RFC Editor function is provided by the IETF
Administrative Support Activity (IASA). Administrative Support Activity (IASA).
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