Hei,
I'm about implementing mail infrastructure for receiving mails from the
outside ("MX servers", they will run postfix). But now I have a question
which more or less a generic one. I would like to minimalize the amount of
information needed for a DNS zone to set up mail receiving through these
servers (for customers it's more easy to say only one RR to set, if they are
the administrators of their own zones), so I'm thinking to have only a
single MX record. However that MX record would point to a name which can be
resolved to multiple A records to have some kind of DNS based "load
balancing". Now the problem is here: what will happen if one of the MX
servers dies. If I would have multiple MX records, according to RFCs, MTAs
should try each destination ordered by the priority field inside the MX DNS
RR. But if I have a single MX record which points to name having multiple A
records then what is the standard behaviour? Can I trust in the theory,
that in case of a dead MX records, remote MTAs will probe the others A
records for the same name? Or is there any standard about this case?
An example, because my English is a bit bad for expressing my ideas:
customerdomain1.tld. MX 10 isp-mx-server.ispdomain.tld.
customerdomain2.tld. MX 10 isp-mx-server.ispdomain.tld.
[... etc ...]
at the ISP side (us):
isp-mx-server.ispdomain.tld. A 192.168.0.1
isp-mx-server.ispdomain.tld. A 192.168.0.2
isp-mx-server.ispdomain.tld. A 192.168.0.3
isp-mx-server.ispdomain.tld. A 192.168.0.4
isp-mx-server.ispdomain.tld. A 192.168.0.5
isp-mx-server.ispdomain.tld. A 192.168.0.6
If - let's say - 192.168.0.2 dies (of course I know it's a private IP space,
it's just an example) but isp-mx-server.ispdomain.tld resolve to that
address at the side of the sender MTA then what will happen? No other
addresses will be tried? Should I avoid this theory at all?
Thanks in advance!
--
- Gábor
