We still need to specify what an SMTP server should do if it
sees more than one.
I'd say that senders MUST NOT publish more than one SRV record per
name. Recipient SMTP servers SHOULD treat an RRset with more than one
SRV record as an error and act as though it found no RR's at all, but
they MAY be coded by amateurs and pick one record at one random and
pretend it's the only one.
By the way, the comment about BIND doing round robin is pretty
persuasive. Now that I think about it, if you look at the semantics
of all of the other RR types, if there are a whole bunch of records
and you only get some of them, that's OK because they're telling you
about multiple alternate places to get equivalent services, and any
will do. But now we have a design that depends on getting all of the
RRs in a set.
So let's say I have a pool of 50 hosts that HELO as, say, blurch.com,
so I set up a SRV record that refers to _csv.blurch.com, and then add
50 A records for _csv.blurch.com for all those hosts. Will DNS servers
return all 50 of them, or will they pick a dozen and figure that's all
I need. Has anyone done experiments, particularly with bind 4.x?
This problem affects SPF, too, but I don't think it has occurred to them.
R's,
John