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 first set of extensions was published in 1999 by the Internet Engineering Task Force as RFC 2671, also known as EDNS0 which was updated by RFC 6891 in 2013 changing abbreviation slightly to EDNS(0)