4.3. UDP Message Size

4.3. UDP Message Size

Traditional DNS messages are limited to 512 octets in size when sent over UDP [RFC1035]. Fitting the increasing amounts of data that can be transported in DNS in this 512-byte limit is becoming more difficult. For instance, inclusion of DNSSEC records frequently requires a much larger response than a 512-byte message can hold.

EDNS(0) specifies a way to advertise additional features such as larger response size capability, which is intended to help avoid truncated UDP responses, which in turn cause retry over TCP. It therefore provides support for transporting these larger packet sizes without needing to resort to TCP for transport.