John Doe via Postfix-users:
> Hi,
>
> I was hoping for real MX record round-robin but it does not work on one of
> my servers.
>
> Somehow, postfix is prioritising one of the MX more than others.
By default, Postfix looks up SMTP servers in DNS, and randomizes
the order of equal-preference records. After 25 years this is one
of the more bizarre problem reports.
> Always the same: nlp3.loc-prd.net
Then you aren't using the default settings for server lookup, or
Postfix is using a different resolver than your manual tests (with
dig), or Postfix has to ignore hosts that speak only IPv6 while
Postfix speaks only IPv4, or your resolver is doing nasty things.
Postfix has no built-in DNS protocol support and relies on system
infrastructure. Historically that was a local bind or unbound
server, but more recently, systemd has also invaded into this
territory.
> All MX servers, are in local network to this client mailserver.
Postfix can handle that.
> We have relayhost in main.cf:
>
> relayhost = mxmail.adatum.net
Good. You have MX lookups turned on.
> We have MX record for this relayhost with equal weights of 10 for each, DNS
> TTL is setup to 300s, dns system is BIND, not Windows.
> dig mx mxmail.adatum.net
Can you demonstrate that this uses the exact same resolv.conf file
as Postfix?
> mxmail.adatum.net. 146 IN MX 10 nlp1.loc-prd.net.
> mxmail.adatum.net. 146 IN MX 10 nlp2.loc-prd.net.
> mxmail.adatum.net. 146 IN MX 10 nlp3.loc-prd.net.
> mxmail.adatum.net. 146 IN MX 10 nlp6.loc-prd.net.
> mxmail.adatum.net. 146 IN MX 10 nlp5.loc-prd.net.
> mxmail.adatum.net. 146 IN MX 10 nlp4.loc-prd.net.
What are the IP addresses for these hosts? Postfix will
use only hosts that it gets an IP address for.
> Some setup parametres for better view:
This is not useful, because you select only the things that YOU
thnk are useful. Send "postconf -n" output (as an attachment if
your mail software does not respect line endomgs and mashes everything
up.
There is no need for smtpd_xxx settings.
But I do need to see things like any non-default inet_protocol settings.
> We have another 2 servers with the same setup, and they relay quite equally.
Equally as in that they also "prefer" the same MX host? Or a different one?
> Anything that will help to sove this issue would be much appreciated.
Please provide evidence in the form of logs that show the preference.
grep 'postfix/smtp.*relay=' /the/maillog/file | grep -v nlp3.loc-prd.net
You can anonymize user names and domain names, and you can omit
mail that wasn't using the $relayhost setting.
Wietse
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