[CentOS] DNS lookup delay with centos & postfix

> dig uses resolv.conf and no timeouts are configured there.  I don't know
> there the OS would have a default configured or what it is.  Another
> reply indicated there would be a 5 second delay.  That seems a bit high
> to me.
>
> I used dig from the email svr command line with the primary DNS svr up
> and (naturally) it pulled from there as normal.  Then I downed the
> primary DNS svr, saw the nagios check fail and tried again.  The same
> dig lookup was actually faster and pulled from the secondary DNS svr
> just fine.  And, again, the nagios alert cleared as soon as the primary
> DNS svr was back online.
>
> For both tests I used:  dig mx google.com

i would always have a timeout in your resolv.conf rather than relying
on the OS default.

Set that to 1 second and test again to see if there is any difference.