As per Relevance of the word extended, we have this rfc below:
Network Working Group P.
Request for Comments: 2671
Category: Standards Track August 1999
Extension Mechanisms for DNS (EDNS0)
Status of this
This document specifies an Internet standards track protocol for
Internet community, and requests discussion and suggestions
improvements. Please refer to the current edition of the "
Official Protocol Standards" (STD 1) for the standardization
and status of this protocol. Distribution of this memo is unlimited
Copyright
Copyright (C) The Internet Society (1999). All Rights Reserved
The Domain Name System's wire protocol includes a number of
fields whose range has been or soon will be exhausted and does
allow clients to advertise their capabilities to servers.
document describes backward compatible mechanisms for allowing
protocol to grow
1 - Rationale and
1.1. DNS (see [RFC1035]) specifies a Message Format and within
messages there are standard formats for encoding options, errors
and name compression. The maximum allowable size of a DNS
is fixed. Many of DNS's protocol limits are too small for
which are or which are desired to become common. There is no
for implementations to advertise their capabilities
1.2. Existing clients will not know how to interpret the
extensions detailed here. In practice, these clients will
upgraded when they have need of a new feature, and only
features will make use of the extensions. We must however
account of client behaviour in the face of extra fields, and
a fallback scheme for interoperability with these clients
Vixie Standards Track [Page 1]
RFC 2671 Extension Mechanisms for DNS (EDNS0) August 1999
2 - Affected Protocol
2.1. The DNS Message Header's (see [RFC1035 4.1.1]) second full 16-
word is divided into a 4-bit OPCODE, a 4-bit RCODE, and a number
1-bit flags. The original reserved Z bits have been allocated
various purposes, and most of the RCODE values are now in use
More flags and more possible RCODEs are needed
2.2. The first two bits of a wire format domain label are used to
the type of the label. [RFC1035 4.1.4] allocates two of the
possible types and reserves the other two. Proposals for use
the remaining types far outnumber those available. More
types are needed
2.3. DNS Messages are limited to 512 octets in size when sent over UDP
While the minimum maximum reassembly buffer size still allows
limit of 512 octets of UDP payload, most of the hosts now
to the Internet are able to reassemble larger datagrams.
mechanism must be created to allow requestors to advertise
buffer sizes to responders
3 - Extended Label
3.1. The "0 1" label type will now indicate an extended label type
whose value is encoded in the lower six bits of the first octet
a label. All subsequently developed label types should be
using an extended label type
3.2. The "1 1 1 1 1 1" extended label type will be reserved for
expansion of the extended label type code space
4 - OPT pseudo-
4.1. One OPT pseudo-RR can be added to the additional data section
either a request or a response. An OPT is called a pseudo-
because it pertains to a particular transport level message and
to any actual DNS data. OPT RRs shall never be cached, forwarded
or stored in or loaded from master files. The quantity of
pseudo-RRs per message shall be either zero or one, but
greater
4.2. An OPT RR has a fixed part and a variable set of options
as {attribute, value} pairs. The fixed part holds some DNS
data and also a small collection of new protocol elements which
expect to be so popular that it would be a waste of wire space
encode them as {attribute, value} pairs
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RFC 2671 Extension Mechanisms for DNS (EDNS0) August 1999
4.3. The fixed part of an OPT RR is structured as follows
Field Name Field Type
------------------------------------------------------
NAME domain name empty (root domain
TYPE u_int16_t
CLASS u_int16_t sender's UDP payload
TTL u_int32_t extended RCODE and
RDLEN u_int16_t describes
RDATA octet stream {attribute,value}
4.4. The variable part of an OPT RR is encoded in its RDATA and
structured as zero or more of the following
+0 (MSB) +1 (LSB
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: | OPTION-CODE |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | OPTION-LENGTH |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
4: | |
/ OPTION-DATA /
/ /
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
OPTION-CODE (Assigned by IANA.)
OPTION-LENGTH Size (in octets) of OPTION-DATA
OPTION-DATA Varies per OPTION-CODE
4.5. The sender's UDP payload size (which OPT stores in the RR
field) is the number of octets of the largest UDP payload that
be reassembled and delivered in the sender's network stack.
that path MTU, with or without fragmentation, may be smaller
this
4.5.1. Note that a 512-octet UDP payload requires a 576-octet
reassembly buffer. Choosing 1280 on an Ethernet
requestor would be reasonable. The consequence of choosing
large a value may be an ICMP message from an
gateway, or even a silent drop of the response message
4.5.2. Both requestors and responders are advised to take account of
path's discovered MTU (if already known) when considering
sizes
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RFC 2671 Extension Mechanisms for DNS (EDNS0) August 1999
4.5.3. The requestor's maximum payload size can change over time,
should therefore not be cached for use beyond the transaction
which it is advertised
4.5.4. The responder's maximum payload size can change over time,
can be reasonably expected to remain constant between
sequential transactions; for example, a meaningless QUERY
discover a responder's maximum UDP payload size,
immediately by an UPDATE which takes advantage of this size
(This is considered preferrable to the outright use of TCP
oversized requests, if there is any reason to suspect that
responder implements EDNS, and if a request will not fit in
default 512 payload size limit.)
4.5.5. Due to transaction overhead, it is unwise to advertise
architectural limit as a maximum UDP payload size. Just
your stack can reassemble 64KB datagrams, don't assume that
want to spend more than about 4KB of state memory per
transaction
4.6. The extended RCODE and flags (which OPT stores in the RR TTL field
are structured as follows
+0 (MSB) +1 (LSB
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
0: | EXTENDED-RCODE | VERSION |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
2: | Z |
+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
EXTENDED-RCODE Forms upper 8 bits of extended 12-bit RCODE.
that EXTENDED-RCODE value "0" indicates that
unextended RCODE is in use (values "0" through "15").
VERSION Indicates the implementation level of whoever
it. Full conformance with this specification
indicated by version "0." Requestors are
to set this to the lowest implemented level
of expressing a transaction, to minimize
responder and network load of discovering
greatest common implementation level
requestor and responder. A requestor's
numbering strategy should ideally be a run
configuration option
If a responder does not implement the VERSION
of the request, then it answers with RCODE=BADVERS
All responses will be limited in format to
Vixie Standards Track [Page 4]
RFC 2671 Extension Mechanisms for DNS (EDNS0) August 1999
VERSION level of the request, but the VERSION of
response will be the highest implementation level
the responder. In this way a requestor will
the implementation level of a responder as a
effect of every response, including error responses
including RCODE=BADVERS
Z Set to zero by senders and ignored by receivers
unless modified in a subsequent specification
5 - Transport
5.1. The presence of an OPT pseudo-RR in a request should be taken as
indication that the requestor fully implements the given version
EDNS, and can correctly understand any response that conforms
that feature's specification
5.2. Lack of use of these features in a request must be taken as
indication that the requestor does not implement any part of
specification and that the responder may make no use of
protocol extension described here in its response
5.3. Responders who do not understand these protocol extensions
expected to send a response with RCODE NOTIMPL, FORMERR,
SERVFAIL. Therefore use of extensions should be "probed" such
a responder who isn't known to support them be allowed a retry
no extensions if it responds with such an RCODE. If a responder'
capability level is cached by a requestor, a new probe should
sent periodically to test for changes to responder capability
6 - Security
Requestor-side specification of the maximum buffer size may open
new DNS denial of service attack if responders can be made to
messages which are too large for intermediate gateways to forward
thus leading to potential ICMP storms between gateways
responders
7 - IANA
The IANA has assigned RR type code 41 for OPT
It is the recommendation of this document and its working
that IANA create a registry for EDNS Extended Label Types, for
Option Codes, and for EDNS Version Numbers
This document assigns label type 0b01xxxxxx as "EDNS Extended
Type." We request that IANA record this assignment
Vixie Standards Track [Page 5]
RFC 2671 Extension Mechanisms for DNS (EDNS0) August 1999
This document assigns extended label type 0bxx111111 as "
for future extended label types." We request that IANA record
assignment
This document assigns option code 65535 to "Reserved for
expansion."
This document expands the RCODE space from 4 bits to 12 bits.
will allow IANA to assign more than the 16 distinct RCODE
allowed in [RFC1035].
This document assigns EDNS Extended RCODE "16" to "BADVERS".
IESG approval should be required to create new entries in the
Extended Label Type or EDNS Version Number registries, while
published RFC (including Informational, Experimental, or BCP
should be grounds for allocation of an EDNS Option Code
8 -
Paul Mockapetris, Mark Andrews, Robert Elz, Don Lewis, Bob Halley
Donald Eastlake, Rob Austein, Matt Crawford, Randy Bush, and
Narten were each instrumental in creating and refining
specification
9 -
[RFC1035] Mockapetris, P., "Domain Names - Implementation
Specification", STD 13, RFC 1035, November 1987.
10 - Author's
Paul
Internet Software
950 Charter
Redwood City, CA 94063
Phone: +1 650 779 7001
EMail: vixie@isc.
Vixie Standards Track [Page 6]
RFC 2671 Extension Mechanisms for DNS (EDNS0) August 1999
11 - Full Copyright
Copyright (C) The Internet Society (1999). All Rights Reserved
This document and translations of it may be copied and furnished
others, and derivative works that comment on or otherwise explain
or assist in its implementation may be prepared, copied,
and distributed, in whole or in part, without restriction of
kind, provided that the above copyright notice and this paragraph
included on all such copies and derivative works. However,
document itself may not be modified in any way, such as by
the copyright notice or references to the Internet Society or
Internet organizations, except as needed for the purpose
developing Internet standards in which case the procedures
copyrights defined in the Internet Standards process must
followed, or as required to translate it into languages other
English
The limited permissions granted above are perpetual and will not
revoked by the Internet Society or its successors or assigns
This document and the information contained herein is provided on
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET
TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED,
BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE
Funding for the RFC Editor function is currently provided by
Internet Society
Vixie Standards Track [Page 7]
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