RFC 2671 (rfc2671) - Page 2 of 7

Extension Mechanisms for DNS (EDNS0)

RFC 2671          Extension Mechanisms for DNS (EDNS0)       August 1999


2 - Affected Protocol Elements

2.1. The DNS Message Header's (see [RFC 1035 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.

2.2. The first two bits of a wire format domain label are used to denote
     the type of the label.  [RFC 1035 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. 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.

3 - Extended Label Types

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 of
     a label.  All subsequently developed label types should be encoded
     using an extended label type.

3.2. The "1 1 1 1 1 1" extended label type will be reserved for future
     expansion of the extended label type code space.

4 - OPT pseudo-RR

4.1. One 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.  The quantity of OPT
     pseudo-RRs per message shall 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.





Vixie                       Standards Track