This section contains a set of RRset prerequisites which must be
satisfied at the time the UPDATE packet is received by the primary
master server. The format of this section is as specified by
[RFC1035 4.1.3]. There are five possible sets of semantics that can
be expressed here, summarized as follows and then explained below.
RRset exists (value independent). At least one RR with a
specified NAME and TYPE (in the zone and class specified by
the Zone Section) must exist.
RRset exists (value dependent). A set of RRs with a
specified NAME and TYPE exists and has the same members
with the same RDATAs as the RRset specified here in this
Section.
RRset does not exist. No RRs with a specified NAME and TYPE
(in the zone and class denoted by the Zone Section) can exist.
Name is in use. At least one RR with a specified NAME (in
the zone and class specified by the Zone Section) must exist.
Note that this prerequisite is NOT satisfied by empty
nonterminals.
Name is not in use. No RR of any type is owned by a
specified NAME. Note that this prerequisite IS satisfied by
empty nonterminals.