RFC 6891 / START HERE
EDNS, before the details
EDNS: Extension Mechanisms for DNS
EDNS adds room for capabilities and options to the Domain Name System without replacing its basic message structure.
The idea in one picture
A resolver advertises capabilities
DNS carries an extension record
The server can use those capabilities
A simplified picture of the idea; the chapters explain the actual mechanisms.
Why it exists
The original DNS format has limits. Extensions let a requester advertise capabilities, including larger UDP response sizes.
A familiar example
A resolver signals that it can receive a DNS reply larger than the original 512-byte UDP limit.
Helpful background
You can start here without reading another RFC. If these ideas are unfamiliar, these short introductions may help.
What you’ll learn next
Begin with chapter one, or choose a topic.
- Attach message metadata using OPTOPT is a pseudo-record in the Additional section, not ordinary zone data.→
- Advertise a UDP receive limitOPT’s CLASS field signals the maximum UDP payload the requester can reassemble and receive.→
- Separate EDNS version and response codesOPT reuses its TTL-shaped field for extended response code, EDNS version, and flags.→
- Parse extensible option tuplesVariable OPT data consists of option-code, option-length, and option-data tuples.→
- Keep OPT out of the record cacheOPT describes one DNS message and is not cacheable resource data.→
This introduction explains RFC 6891 (2013). Read the original RFC ↗