DNSEXT Working Group Paul Vixie, ISC INTERNET-DRAFT March 17, 2008 Intended Status: Standards Track Obsoletes: 2671 (if approved) Revised extension mechanisms for DNS (EDNS0) Status of this Memo By submitting this Internet-Draft, each author represents that any applicable patent or other IPR claims of which he or she is aware have been or will be disclosed, and any of which he or she becomes aware will be disclosed, in accordance with Section 6 of BCP 79. 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. Copyright Notice Copyright (C) The IETF Trust (2007). Abstract 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 allow clients to advertise their capabilities to servers. This document describes backward compatible mechanisms for allowing the protocol to grow. Expires September 2008 [Page 1] INTERNET-DRAFT EDNS0 March 2008 1 - Introduction 1.1. DNS (see [RFC1035]) specifies a Message Format and within such messages there are standard formats for encoding options, errors, and name compression. The maximum allowable size of a DNS Message is fixed. Many of DNS's protocol limits are too small for uses which are or which are desired to become common. There is no way for implementations to advertise their capabilities. 1.2. Unextended agents will not know how to interpret the protocol extensions detailed here. In practice, these clients will be upgraded 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 of unextended clients in the face of new protocol elements, and fall 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 NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119]. 2 - Affected Protocol Elements 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 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 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 denote the type of the label. [RFC1035 4.1.4] allocates two of the four possible types and reserves the other two. Proposals for use of the remaining types far outnumber those available. More label types were needed, and an extension mechanism was proposed in RFC 2671 [RFC2671 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. Expires September 2008 [Page 2] INTERNET-DRAFT EDNS0 March 2008 2.3. DNS Messages are limited to 512 octets in size when sent over UDP. While the minimum maximum reassembly buffer size still allows a limit of 512 octets of UDP payload, most of the hosts now connected to the Internet are able to reassemble larger datagrams. Some mechanism must be created to allow requestors to advertise larger buffer sizes to 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 The first octet in the on-the-wire representation of a DNS label specifies the label type; the basic DNS specification [RFC1035] dedicates the two most significant bits of that octet for this purpose. This document reserves DNS label type 0b01 for use as an indication for 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 4.1. One OPT pseudo-RR (RR type 41) MAY be added to the additional data section of a request, and to responses to such requests. An OPT is called a pseudo-RR because it pertains to a particular transport level message and not to any actual DNS data. OPT RRs MUST NOT be cached, forwarded, or stored in or loaded from master files. The quantity of OPT pseudo-RRs per message MUST be either zero or one, but not greater. 4.2. An OPT RR has a fixed part and a variable set of options expressed as {attribute, value} pairs. The fixed part holds some DNS meta data and also a small collection of new protocol elements which we expect to be so popular that it would be a waste of wire space to encode them as {attribute, value} pairs. Expires September 2008 [Page 3] INTERNET-DRAFT EDNS0 March 2008 4.3. The fixed part of an OPT RR is structured as follows: Field Name Field Type Description ------------------------------------------------------ NAME domain name empty (root domain) TYPE u_int16_t OPT (41) CLASS u_int16_t sender's UDP payload size TTL u_int32_t extended RCODE and flags RDLEN u_int16_t describes RDATA RDATA octet stream {attribute,value} pairs 4.4. The variable part of an OPT RR is encoded in its RDATA and is 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. 4.4.1. Order of appearance of option tuples is never relevant. Any option whose meaning is affected by other options is so affected no matter which one comes first in the OPT RDATA. 4.4.2. Any OPTION-CODE values not understood by a responder or requestor MUST be ignored. So, specifications of such options might wish to include some kind of signalled acknowledgement. For example, an option specification might say that if a responder sees option XYZ, it SHOULD include option XYZ in its response. Expires September 2008 [Page 4] INTERNET-DRAFT EDNS0 March 2008 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 reassembled and delivered in the sender's network stack. Note that path 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 reassembly buffer. Choosing 1280 on an Ethernet connected requestor would be reasonable. The consequence of choosing too large a value may be an ICMP message from an intermediate gateway, or even a silent drop of the response message. 4.5.2. Both requestors and responders are advised to take account of the path's discovered MTU (if already known) when considering message sizes. 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 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: | DO| Z | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ Expires September 2008 [Page 5] INTERNET-DRAFT EDNS0 March 2008 EXTENDED-RCODE Forms upper 8 bits of extended 12-bit RCODE. Note that EXTENDED-RCODE value zero (0) indicates that an unextended RCODE is in use (values zero (0) through fifteen (15)). VERSION Indicates the implementation level of whoever sets it. Full conformance with this specification is indicated by version zero (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 MUST be limited in format to the VERSION level of the request, but the VERSION of each response MUST 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. DO DNSSEC OK bit [RFC3225]. Z Set to zero by senders and ignored by receivers, unless modified in a subsequent specification [IANAFLAGS]. 5 - Transport Considerations 5.1. The presence of an OPT pseudo-RR in a request is 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 is an indication that the requestor does not implement any part of this specification and that the responder SHOULD NOT use 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, or to appear to "time out" due to inappropriate action by a "middle box" such as a NAT, or to ignore extensions and respond only to unextended Expires September 2008 [Page 6] INTERNET-DRAFT EDNS0 March 2008 protocol elements. 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, or does not respond. 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 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 IANA has allocated RR type code 41 for OPT. This document controls the following IANA sub-registries in registry "DOMAIN NAME SYSTEM PARAMETERS": "EDNS Extended Label Type" "EDNS Option Codes" "EDNS Version Numbers" "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 Type." We request that IANA record this assignment. This document assigns option code 65535 to "Reserved for future expansion." This document assigns EDNS Extended RCODE "16" to "BADVERS". Expires September 2008 [Page 7] INTERNET-DRAFT EDNS0 March 2008 IESG approval is 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) is 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, Thomas Narten, Alfred Hoenes and Markku Savela were each instrumental in creating and refining this specification. 9 - References [RFC1035] P. Mockapetris, "Domain Names - Implementation and Specification," RFC 1035, USC/Information Sciences Institute, November 1987. [RFC2119] S. Bradner, "Key words for use in RFCs to Indicate Requirement Levels," RFC 2119, Harvard University, March 1997. [RFC2671] P. Vixie, "Extension mechanisms for DNS (EDNS0)," RFC 2671, Internet Software Consortium, August 1999. [RFC3225] D. Conrad, "Indicating Resolver Support of DNSSEC," RFC 3225, Nominum Inc., December 2001. [IANAFLAGS] IANA, "DNS Header Flags and EDNS Header Flags," web site http://www.iana.org/assignments/dns-header-flags, as of June 2005 or later. 10 - Author's Address Paul Vixie Internet Systems Consortium 950 Charter Street Redwood City, CA 94063 +1 650 423 1301 EMail: vixie@isc.org Expires September 2008 [Page 8] INTERNET-DRAFT EDNS0 March 2008 Full Copyright Statement Copyright (C) IETF Trust (2007). This document is subject to the rights, licenses and restrictions contained in BCP 78, and except as set forth therein, the authors retain all their rights. 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