1.1. Comparison Rules

1.1.1. Two RRs are considered equal if their NAME, CLASS, TYPE, RDLENGTH and RDATA fields are equal. Note that the time-to-live (TTL) field is explicitly excluded from the comparison.

1.1.2. The rules for comparison of character strings in names are specified in [RFC1035 2.3.3].

1.1.3. Wildcarding is disabled. That is, a wildcard ("*") in an update only matches a wildcard ("*") in the zone, and vice versa.

1.1.4. Aliasing is disabled: A CNAME in the zone matches a CNAME in the update, and will not otherwise be followed. All UPDATE operations are done on the basis of canonical names.

1.1.5. The following RR types cannot be appended to an RRset. If the following comparison rules are met, then an attempt to add the new RR will result in the replacement of the previous RR:

SOA

compare only NAME, CLASS and TYPE -- it is not possible to have more than one SOA per zone, even if any of the data fields differ.

WKS

compare only NAME, CLASS, TYPE, ADDRESS, and PROTOCOL -- only one WKS RR is possible for this tuple, even if the services masks differ.

CNAME

compare only NAME, CLASS, and TYPE -- it is not possible to have more than one CNAME RR, even if their data fields differ.