DNSIND Working Group Paul Vixie INTERNET-DRAFT Vixie Enterprises March, 1998 Extended DNS Status of this Memo This document is an Internet-Draft. 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.'' To learn the current status of any Internet-Draft, please check the ``1id-abstracts.txt'' listing contained in the Internet-Drafts Shadow Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe), munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or ftp.isi.edu (US West Coast). Abstract The Domain Name System's wire protocol includes a number of fixed fields whose range has been or soon will be exhausted. This document describes backward compatible mechanisms for allowing the protocol to grow. 1 - Rationale and Scope 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. 1.2. Existing clients 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 Expires September 1998 [Page 1] INTERNET-DRAFT HTCP March 1998 use of the extensions. We must however take account of client behaviour in the face of extra fields, and design a fallback scheme for interoperability with these clients. 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 types and more possible RCODEs are needed. 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 are needed. 2.3. Compression pointers are 14 bits in size and are relative to the start of the DNS Message, which can be 64KB in length. 14 bits restrict pointers to the first 16KB of the message, which makes labels introduced in the last 48KB of the message unreachable by compression pointers. A longer pointer format is needed. 2.4. DNS Messages are limited to 512 octets in size when sent over UDP. While the minimum maximum reassembly buffer size is still 512 bytes, 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. 2.5. DNS Messages are limited to 65535 octets in size when sent over TCP. This acts as an effective maximum on RRset size and on RR size, since multiple TCP messages are only possible in the case of zone transfers. Some mechanism must be created to allow normal DNS responses (other than zone transfers) to span multiple DNS Messages when TCP is used. 2.6. Multiple queries in a question section have not been supported in DNS due the applicability of some DNS Message Header flags (such as AA) and of the RCODE field only to a single QNAME, QTYPE, and QCLASS. Multiple questions per request are desirable, and some way of asking them must be made available. Expires September 1998 [Page 2] INTERNET-DRAFT HTCP March 1998 3 - Extended Label Types 3.1. The ``1 0'' label type will now indicate an extended label type, whose value is encoded in the lower six bits of the first octet of a label. All subsequently developed label types should be encoded using an extended label type. The ``0 1'' label type should be permanently reserved. 3.2. The ``0 0 0 0 0 0'' extended label type will indicate an extended compression pointer, such that the following two octets comprise a 16-bit compression pointer in network byte order. Like the normal compression pointer, this pointer is relative to the start of the DNS Message. 3.3. XXX let's get any others in if we know of them. 4 - OPT pseudo-RR 4.1. The OPT pseudo-RR can be added to the additional data section of either a request or a response. 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 shall never be cached, forwarded, or stored in or loaded from master files. 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. 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 (XXX code) CLASS u_int16_t sender's UDP buffer 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: Expires September 1998 [Page 3] INTERNET-DRAFT HTCP March 1998 +0 (MSB) +1 (LSB) +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 0: | OPTION-LENGTH | OPTION-CODE | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 2: | | / OPTION-DATA / / / +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ OPTION-LENGTH Size (in octets) of OPTION-DATA. OPTION-CODE Assigned by the IANA. Value 255 is reserved for future expansion. OPTION-DATA Varies per OPTION-CODE. 4.5. The sender's UDP buffer size is the number of octets of the largest UDP payload that is reassemblable and deliverable in the sender's network stack. Note that path MTU, with or without fragmentation, may be smaller than this. Also note that a 512-octet UDP payload requires a 576-octet IP reassembly buffer. Choosing 1436 on an Ethernet connected requestor would be reasonable. 4.6. The extended RCODE and flags are structured as follows: +0 (MSB) +1 (LSB) +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 0: | EXTENDED-RCODE | VERSION | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ 2: |MD | Z | +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+ EXTENDED-RCODE Forms upper 8 bits of extended 12-bit RCODE. (Meaningless in requests.) VERSION Indicates the implementation level of whoever sets it. Full conformance with the draft standard version of this specification is version ``0.'' Z Set to zero by senders and ignored by receivers, unless modified in a subsequent specification. Expires September 1998 [Page 4] INTERNET-DRAFT HTCP March 1998 MD ``More data'' flag. Valid only in TCP streams where message ordering is reliable. This flag indicates that the current message is not the complete response, and should be aggregated with the following message(s) before being considered complete. Such responses are called ``segmented responses.'' It is an error for the RCODE (including the EXTENDED-RCODE), AA flag, or DNS Message ID to differ among messages in a segmented response. It is an error for TC to be set on any message in a segmented response. 5 - Multiple Questions 5.1. If QDCOUNT>1, multiple questions are present. All questions must be for the same QNAME and QCLASS; only the QTYPE is allowed to vary. It is an error for QDCOUNT>1 and any QTYPE=ANY. 5.2. RCODE and AA apply to all RRs in the answer section having the QNAME that is shared by all questions in the question section. 5.3. QCLASS=ANY is deprecated. 5.4. QTYPE=IQUERY is deprecated. 6 - Transport Considerations 6.1. The presence of an OPT pseudo-RR or any new label type, or QDCOUNT>1 in a request should be taken as an indication that the requestor fully implements this specification and can correctly understand any response that conforms to this specification. If a new label type or QDCOUNT>1 is used in a message that does not have an OPT RR, a VERSION of ``0'' shall be imputed. 6.2. 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. 6.2. Responders who do not understand these protocol extensions are expected to send a respose with RCODE=NOTIMPL or RCODE=FORMERR or even RCODE=SERVFAIL. Therefore use of these extensions should be ``probed'' such that a responder who isn't known to support them be allowed a retry with no extensions in use if it responds with one of the above mentioned RCODEs. Responder capability with respect to these extensions ought to be cached by requestors, but a new probe should be sent periodically to Expires September 1998 [Page 5] INTERNET-DRAFT HTCP March 1998 test for upgrades to responder capability. 7 - Security Considerations No new problems are added to DNS's security model by this specification. Correspondingly, no old problems in DNS security are resolved by this specification. 8 - Acknowledgements Paul Mockapetris, Mark Andrews, Robert Elz, Don Lewis, Bob Halley, and Donald Eastlake were each instrumental in creating this specification. 9 - References [RFC1035] P. Mockapetris, ``Domain Names - Implementation and Specification,'' RFC 1035, USC/Information Sciences Institute, November 1987. 10 - Author's Address Paul Vixie Vixie Enterprises 950 Charter Street Redwood City, CA 94063 +1 650 779 7001 Expires September 1998 [Page 6]