EDNS0 (Extension Mechanism for DNS)
EDNS0 (RFC 6891) extends the original DNS message format with an optional pseudo-record, OPT, that lets a resolver advertise capabilities the 1987-era protocol never anticipated. Its most immediate job is signaling a larger UDP payload size than the original 512-byte limit, which is what lets DNS carry the bigger answers DNSSEC-signed zones produce without falling back to TCP for every query. EDNS0's extensible options field is also how later additions, like the DNSSEC OK (DO) bit and DNS Cookies (RFC 7873), attach to a query without changing the core wire format again. A resolver or server that doesn't understand EDNS0 simply ignores the OPT record and responds as if it weren't there, which is why support rolled out gradually without breaking older infrastructure. AdminKit's DNS lookup tool sends EDNS0-aware queries so it can retrieve full DNSSEC and large-answer responses correctly.
By AdminKit team, Netbase JSC · Updated