EDNS0
Extension Mechanisms for DNS: lets DNS messages exceed the 512-byte limit and carry extra options like DNSSEC, client subnet, and cookies.
EDNS0 (Extension Mechanisms for DNS, version 0) is the workaround that lets modern DNS escape the constraints baked into the 1987 spec. By adding an OPT pseudo-record to a query, a client can advertise a larger UDP buffer size (so responses bigger than 512 bytes are not truncated), request DNSSEC validation data, send EDNS Client Subnet for CDN geo-targeting, or include DNS cookies to mitigate spoofing. Almost every DNS feature added in the last 20 years rides on top of EDNS0. Resolvers and authoritative servers that fail to handle EDNS0 cleanly cause many obscure DNS failures.