Except for algorithm number 253 where it is null, the actual signature portion of the SIG RR binds the other RDATA fields to all of the "type covered" RRs with that owner name and class. These covered RRs are thereby authenticated. To accomplish this, a data sequence is constructed as follows:
data = RDATA | RR(s)...
where "|" is concatenation, RDATA is all the RDATA fields in the SIG RR itself before and not including the signature, and RR(s) are all the RR(s) of the type covered with the same owner name and class as the SIG RR in canonical form and order. How this data sequence is processed into the signature is algorithm dependent.
For purposes of DNS security, the canonical form for an RR is the RR with domain names (1) fully expanded (no name compression via pointers), (2) all domain name letters set to lower case, and (3) the original TTL substituted for the current TTL.
For purposes of DNS security, the canonical order for RRs is to sort them in ascending order by name, considering labels as a left justified unsigned octet sequence in network (transmission) order where a missing octet sorts before a zero octet. (See also ordering discussion in Section 5.1.) Within any particular name they are similarly sorted by type and then RDATA as a left justified unsigned octet sequence. EXCEPT that the type SIG RR(s) covering any particular type appear immediately after the other RRs of that type. (This special consideration for SIG RR(s) in ordering really only applies to calculating the AXFR SIG RR as explained in section 4.1.3 below.) Thus if at name a.b there are two A RRs and one KEY RR, their order with SIGs for concatenating the "data" to be signed would be as follows:
a.b. A ....
a.b. A ....
a.b. SIG A ...
a.b. KEY ...
a.b. SIG KEY ...
SIGs covering type ANY should not be included in a zone.