EDNS

networking

Stands for: Extension Mechanisms for DNS

An extension that adds capabilities to the original DNS message format.

Extension Mechanisms for DNS (RFC 6891) let DNS carry larger responses, signal DNSSEC support, and pass options, all via the OPT pseudo-record, without breaking the legacy message format.

EDNS extends DNS beyond the constraints of the original message format, carrying larger message sizes, additional flags and option codes that later features depend on. Without it, DNSSEC and several other extensions would not fit.

It is transported in the OPT pseudo-record, which exists only on the wire and never in a zone file. The operational trouble is middleboxes: firewalls that reject DNS messages containing OPT records or exceeding 512 bytes cause failures that look like intermittent resolution problems, and the workaround of falling back to TCP is slower and sometimes also blocked. A resolver that works for simple names and fails for DNSSEC-signed ones is usually meeting one of these.

Also known as: edns, edns0

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