INTERNET-DRAFT M. Sawyer A. Gustafsson M. Graff Nominum 15 November 2000 Handling of unknown EDNS0 attributes Status of this Memo This document is an Internet-Draft and is in full conformance with all provisions of Section 10 of RFC2026. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF), its areas, and its working groups. Note that other groups may also distribute working documents as Internet- Drafts. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as ``work in progress.'' The list of current Internet-Drafts can be accessed at http://www.ietf.org/ietf/1id-abstracts.txt The list of Internet-Draft Shadow Directories can be accessed at http://www.ietf.org/shadow.html This draft expires on May 15, 2001. Abstract This document provides a clarification of the EDNS0 protocol, specifying the behavior of servers when they receive unknown EDNS options. 1.1 - Introduction Familiarity with DNS [RFC1034, RFC1035] and DNS Extension Mechanisms [RFC2671] is helpful. EDNS0 [RFC2671] specifies a general framework for extending the packet format used by the Domain Name System protocol. The framework provides for a series of additional options, identified by a 16 bit attribute ID and arbitrary sized payload. Any number of these additional options can be specified in the DNS packet. As specified, the current scheme has drawbacks: Expires May 2001 [Page 1] INTERNET-DRAFT Handling of unknown EDNS attributes October 2000 - It provides no way for implementers to deploy test systems with experimental features prior to approval of the feature and assignment of an attribute ID. - It provides no specification on what an application should do when receiving unrecognized options. This draft proposes to clarify the original EDNS0 draft [RFC2671], addressing these drawbacks. 1.2 - Requirements 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 [RFC2119]. 2 - Protocol changes: This document updates [RFC2671]. Conformance to this specification is claimed by specifying EDNS version 1. 2.1 - Advisory and Required Options: Some potential uses of EDNS options are advisory in nature, For example, a hypothetical option indicating that "I understand frobozz compression in responses" can be safely ignored by the recipient, which will then simply not use frobozz compression. Others uses of options, such as a hypothetical "send only cryptographically verified data in responses" option, cannot be safely ignored, and should cause the request to fail if not understood by the receiver. This suggests that we need two types of options, "advisory" options (that can be ignored) and "required" options (that can not). Because a server needs to classify options as advisory or required even if they were not yet defined when the server was implemented, the type of an option must be evident without knowing its internal structure. This is achieved by splitting the option type codes into two ranges: options with type code 0x0000-0x7FFF are considered "advisory", and options with type code 0x8000-0xFFFF are considered "required". 2.2 - Handling of Unknown and Unsupported EDNS Option Types When a server receives an unknown or unsupported advisory option in a request or response message, it MUST ignore the unknown option and process the message as if the option was not present. In the reply, it SHOULD include an advisory UNSUPPORTED option (TBD). Expires May 2001 [Page 2] INTERNET-DRAFT Handling of unknown EDNS attributes October 2000 When a server receives an unknown or unsupported required option in a request message, it MUST NOT act on the request, and it MUST return an error response with the extended result code BADOPT (TBD). In the reply, it SHOULD include an advisory UNSUPPORTED option. When a server receives an unknown or unsupported required option in a response message, it MUST ignore the response. The server SHOULD continue to parse options after the unknown option, including a list of all unsupported options in the UNSUPPORTED option in the reply. Servers MAY include SUPPORTED options in replies to messages, listing option codes which they understand. This message SHOULD contain all option codes the server understands. This facility MAY NOT be used in place of the UNSUPPORTED option to identify unsupported options in replies. Clients MAY include SUPPORTED or UNSUPPORTED options in queries. UNSUPPORTED options SHOULD only list those option codes which the client has received in previous replies from the server, not an inclusive list of all unsupported option codes. 2.3 - Use of Reserved Options for Development Option codes in the range of 0x7FF0 to 0x7FFF and 0xFFF0 to 0xFFFF are considered "reserved" and shall not be assigned to new protocols. Software vendors SHOULD use these options for testing of protocols under development, provided the following conditions are met: - Vendors MUST NOT ship any versions of the software which use option codes in this range. They MUST delay shipping software with the new options until IANA has assigned permanent option codes. - Vendors MUST NOT place development servers on the live internet which send options in this range to remote servers or which understand options in this range as having any meaning. 3.1 - SUPPORTED and UNSUPPORTED options The SUPPORTED and UNSUPPORTED options contain a list of option codes which the server or client does or doesn't support. The list contains, in network byte order, the supported or unsupported 16 bit option codes: 1 1 1 1 1 1 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ | SUPPORTED or UNSUPPORTED (TBD) | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ Expires May 2001 [Page 3] INTERNET-DRAFT Handling of unknown EDNS attributes October 2000 | LENGTH (number of options * 2) | +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ / OPTION CODE(s) / / / +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+ Sending a SUPPORTED option with a zero-length payload indicates that the server or client supports no EDNS options, so none should be used. UNSUPPORTED options with zero-length payloads SHOULD NOT be sent, as such a message is meaningless. 4 - IANA considerations When allocating EDNS Option Codes, the IANA shall henceforth require the RFC defining the new option to specify whether the option is an "advisory" or a "required" option. The option code for an advisory option shall be allocated from the range 0x0000-0x7FEF, and the code for a required option shall be allocated from the range 0x8000-0xFFEF. Option codes in the ranges of 0x7FF0 to 0x7FFF and 0xFFF0 to 0xFFFF are considered "reserved." The IANA is hereby requested to assign EDNS Version Number 1 to this specification, to assign a new extended RCODE value for BADOPT, and to assign advisory option codes for UNSUPPORTED and SUPPORTED. 5 - Security considerations This document provides a mechanism for users to override the default behavior of the server when accessing data from its internal zone databases. Software developers and administrators should use some care when enabling these options, as they may provide outside users the ability to retrieve information otherwise not available. 6 - Acknowledgments The authors would like to thank Olafur Gudmundsson for his input on this draft. 7 - References [RFC2119] S. Brander, ``Key words for use in RFCs to Indicate Requirement Levels,'' RFC 2119, ISI, November 1997. [RFC2671] P. Vixie, ``Extension Mechanisms for DNS (EDNS0),'' RFC 2671, ISI, August, 1999 Expires May 2001 [Page 4] INTERNET-DRAFT Handling of unknown EDNS attributes October 2000 Author's Address Michael Sawyer Nominum, Inc. 950 Charter St. Redwood City, CA 94063 Phone: +1-650-779-6021 michael.sawyer@nominum.com Andreas Gustafsson Nominum, Inc. 950 Charter St. Redwood City, CA 94063 Phone: +1-650-779-6004 andreas.gustafsson@nominum.com Michael Graff Nominum, Inc. 950 Charter St. Redwood City, CA 94063 Phone: +1-650-779-6034 michael.graff@nominum.com Expires May 2001 [Page 5]