Of RRsets

An RRset is the set of RRs associated with a name. In any zone there is at least one name which has several records associated with it. This is an extract from the penguin.bv zone as shown in Chapter 2, "DNS in Practice":

 @       3600    SOA     ns.penguin.bv.  hostmaster.penguin.bv. (                 2000042924      ; serial                 86400           ; refresh, 24h                 7200            ; retry, 2h                 3600000         ; expire, 1000h                 172800          ; minimum, 2 days                 )                 NS      ns                 NS      ns.herring.bv.                 MX      10 mail                 MX      20 mail.herring.bv. ... @               A       192.168.55.3 @               A       192.168.55.10 ... 

Associated with the name penguin.bv are one SOA record, two NS records, two MX records, and two A records. This makes one RRset. In RFC 2181, which updates RFC 1035, it is specified that all records in an RRset must have the same TTL. This RFC is listed as elective and this restriction is, in fact, not currently implemented in BIND 8. But acting like it is implemented in the case of dynamic zones might be a good idea, because in a dynamic zone the TTL of the records becomes very important. There is no mechanism to update cached RRs. They must be expired from the cache before any new value is requested from the zone servers.