3. EDNS Support Requirement

3. EDNS Support Requirement

EDNS provides a mechanism to improve the scalability of DNS as its uses get more diverse on the Internet. It does this by enabling the use of UDP transport for DNS messages with sizes beyond the limits specified in RFC 1035 as well as providing extra data space for additional flags and return codes (RCODEs). However, implementation experience indicates that adding new RCODEs should be avoided due to the difficulty in upgrading the installed base. Flags SHOULD be used only when necessary for DNS resolution to function.

For many uses, an EDNS Option Code may be preferred.

Over time, some applications of DNS have made EDNS a requirement for their deployment. For instance, DNSSEC uses the additional flag space introduced in EDNS to signal the request to include DNSSEC data in a DNS response.

Given the increase in DNS response sizes when including larger data items such as AAAA records, DNSSEC information (signatures, keys, etc.), or other DNS data, the additional UDP payload capabilities provided by EDNS can help improve the scalability of the DNS by avoiding widespread use of TCP for DNS transport.