Extension Mechanisms for DNS (EDNS0)
RFC 2671
Document | Type |
RFC
- Proposed Standard
(August 1999)
Obsoleted by RFC 6891
|
|
---|---|---|---|
Author | Paul A. Vixie | ||
Last updated | 2013-03-02 | ||
RFC stream | Internet Engineering Task Force (IETF) | ||
Formats | |||
Additional resources | Mailing list discussion | ||
IESG | Responsible AD | (None) | |
Send notices to | (None) |
RFC 2671
RFC 2671 Extension Mechanisms for DNS (EDNS0) August 1999 4.5.3. The requestor's maximum payload size can change over time, and should therefore not be cached for use beyond the transaction in which it is advertised. 4.5.4. The responder's maximum payload size can change over time, but can be reasonably expected to remain constant between two sequential transactions; for example, a meaningless QUERY to discover a responder's maximum UDP payload size, followed immediately by an UPDATE which takes advantage of this size. (This is considered preferrable to the outright use of TCP for oversized requests, if 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.) 4.5.5. Due to transaction overhead, it is unwise to advertise an architectural limit as a maximum UDP payload size. Just because your stack can reassemble 64KB datagrams, don't assume that you want to spend more than about 4KB of state memory per ongoing 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. Note that EXTENDED-RCODE value "0" indicates that an unextended RCODE is in use (values "0" through "15"). VERSION Indicates the implementation level of whoever sets it. Full conformance with this specification is indicated by version "0." Requestors are encouraged to set this to the lowest implemented level capable of expressing a transaction, to minimize the responder and network load of discovering the greatest common implementation level between requestor and responder. A requestor's version numbering strategy should ideally be a run time configuration option. If a responder does not implement the VERSION level of the request, then it answers with RCODE=BADVERS. All responses will be limited in format to the Vixie Standards Track [Page 4] RFC 2671 Extension Mechanisms for DNS (EDNS0) August 1999 VERSION level of the request, but the VERSION of each response will be the highest implementation level of the responder. In this way a requestor will learn the implementation level of a responder as a side 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 Considerations 5.1. The presence of an OPT pseudo-RR in a request should be taken as an indication that the requestor fully implements the given version of EDNS, and can correctly understand any response that conforms to that feature's specification. 5.2. Lack of use of these features in a request must be taken as an indication that the requestor does not implement any part of this specification and that the responder may make no use of any protocol extension described here in its response. 5.3. Responders who do not understand these protocol extensions are expected to send a response with RCODE NOTIMPL, FORMERR, or SERVFAIL. Therefore use of extensions should be "probed" 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. 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. 6 - Security Considerations Requestor-side specification of the maximum buffer size may open a new DNS denial of service attack if responders can be made to send messages which are too large for intermediate gateways to forward, thus leading to potential ICMP storms between gateways and responders. 7 - IANA Considerations The IANA has assigned RR type code 41 for OPT. It is the recommendation of this document and its working group that IANA create a registry for EDNS Extended Label Types, for EDNS Option Codes, and for EDNS Version Numbers. This document assigns label type 0b01xxxxxx as "EDNS Extended Label 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 "Reserved for future extended label types." We request that IANA record this assignment. This document assigns option code 65535 to "Reserved for future expansion." This document expands the RCODE space from 4 bits to 12 bits. This will allow IANA to assign more than the 16 distinct RCODE values allowed in [RFC1035]. This document assigns EDNS Extended RCODE "16" to "BADVERS". IESG approval should be required to create new entries in the EDNS Extended Label Type or EDNS Version Number registries, while any published RFC (including Informational, Experimental, or BCP) should be grounds for allocation of an EDNS Option Code. 8 - Acknowledgements Paul Mockapetris, Mark Andrews, Robert Elz, Don Lewis, Bob Halley, Donald Eastlake, Rob Austein, Matt Crawford, Randy Bush, and Thomas Narten were each instrumental in creating and refining this specification. 9 - References [RFC1035] Mockapetris, P., "Domain Names - Implementation and Specification", STD 13, RFC 1035, November 1987. 10 - Author's Address Paul Vixie Internet Software Consortium 950 Charter Street Redwood City, CA 94063 Phone: +1 650 779 7001 EMail: vixie@isc.org Vixie Standards Track [Page 6] RFC 2671 Extension Mechanisms for DNS (EDNS0) August 1999 11 - Full Copyright Statement Copyright (C) The Internet Society (1999). All Rights Reserved. This document and translations of it may be copied and furnished to others, and derivative works that comment on or otherwise explain it or assist in its implementation may be prepared, copied, published and distributed, in whole or in part, without restriction of any kind, provided that the above copyright notice and this paragraph are included on all such copies and derivative works. However, this document itself may not be modified in any way, such as by removing the copyright notice or references to the Internet Society or other Internet organizations, except as needed for the purpose of developing Internet standards in which case the procedures for copyrights defined in the Internet Standards process must be followed, or as required to translate it into languages other than English. The limited permissions granted above are perpetual and will not be revoked by the Internet Society or its successors or assigns. This document and the information contained herein is provided on an "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. Acknowledgement Funding for the RFC Editor function is currently provided by the Internet Society. Vixie Standards Track [Page 7]