Mechanism
Since no new flags could be added in the DNS header, EDNS adds information to DNS messages in the form of pseudo-resource-records included in the "additional data" section of a DNS message. Note that this section exists in both requests and responses.
The pseudo-RR EDNS introduces for this are of type OPT.
As pseudo-RRs, OPT type RRs never appear in any zone file; they exist only in messages, fabricated by the DNS participants.
The mechanism is backward compatible, because older DNS responders ignore any RR of the unknown OPT type in a request and a newer DNS responder never includes an OPT in a response unless there was one in the request. The presence of the OPT in the request signifies a newer requester that knows what to do with an OPT in the response.
The OPT pseudo-record provides space for up to 16 flags and it extends the space for the response code. The overall size of the UDP packet and the version number (at present 0) are contained in the OPT record. A variable length data field allows further information to be registered in future versions of the protocol. The original DNS protocol provided two label types, which are defined by the first two bits in DNS packets (RFC 1035): 00 (standard label) and 11 (compressed label). EDNS introduces the label type 01 as extended label. The lower 6 bits of the first byte may be used to define up to 63 new extended labels.
Read more about this topic: Extension Mechanisms For DNS
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