RFC 2671 (rfc2671) - Page 2 of 7
Extension Mechanisms for DNS (EDNS0)
Alternative Format: Original Text Document
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