On the IETF list, Claus Färber wrote:
I think the only requirement that's really essential is that the serial
number changes whenever the data that would be returned by a zone
transfer changes (even if that breaks database consistency for the SOA
record's serial number).
It is also essential that two different authoritative servers that
show the same serial number actually serve the same data.
One strategy to implement this would be to keep an exact copy of a zone
obtained from other servers for outgoing transfers (so you'll always use
the original serial number), which is the BIND 9 strategy.
Another strategy would be to just increment the serial number of a zone
when it is actually changed by importing glue records from another zone.
This would solve the synchronisation problem, too: If the parent zone is
updated too early, the server might throw away the glue record. But it
will update the parent zone and increase its the serial number when it
gets an up-to-date copy of the child zone.
In principle, this approach could work on the primary master server
(at least assuming the zone serial number is being maintained by the
server rather than the administrator), but it won't work on a slave.
If a slave "imports" glue into a zone, it can't update the serial
number, and even if it could, that wouldn't make the zone contents
consistent with the master.
Followups to namedroppers, please.
--
Andreas Gustafsson, gson(_at_)nominum(_dot_)com