Extension Mechanisms for DNS (EDNS) is a specification for expanding the size of several parameters of the Domain Name System (DNS) protocol which had size restrictions that the Internet engineering community deemed too limited for increasing functionality of the protocol.
The restrictions in the size of several flags fields, return codes and label types available in the basic DNS protocol prevented the support of some desirable features.
Moreover, DNS messages carried by UDP were restricted to 512 bytes, not considering the Internet Protocol (IP) and transport layer headers.
[3] Resorting to a virtual circuit transport, using the Transmission Control Protocol (TCP), would greatly increase overhead.
As pseudo-RRs, OPT type RRs never appear in any zone file; they exist only in messages, fabricated by the DNS participants.
An example of an OPT pseudo-record, as displayed by the dig command: The result of "EDNS: version: 0" indicates full conformance with EDNS0.