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Old 04-30-2015, 01:37 AM   #1
ulto
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Sendmail name lookup issue


I am having some strange issues with sendmail at the moment.

When a mail comes into my sendmail server, where it is then relayed onwards, the sendmail server for whatever reason tries to forward it a hostname that it shouldn't.

Have no idea where it gets this hostname from in the first place as it should be sending it to a mail.full.domain.name server.

So my question is, does anyone exactly know how sendmail performs a hostname or DNS lookup and where it's cached. The strange thing is though that a restart of the sendmail service fixes this, yet it reappears sometimes, usually when we have some network issue in the back-end.

One would think that sendmail would clear its cache every few minutes. Or does it leverage something else to perform this lookup.
 
Old 04-30-2015, 12:18 PM   #2
MensaWater
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Sendmail gets the MX (Mail Exchange) record for the target domain (assuming you don't have a relay host defined). Caching if any is done by the resolver (e.g. Windows DNS or BIND on Linux/UNIX) not by sendmail itself so far as I know.

Typically you have an /etc/nsswitch.conf that says how to lookup names which usually will have something like "host: files dns" in it that means to first search files (/etc/hosts in this case) and if not found look in dns. There are other possible entries such nis+ that could be there.

For DNS you also usually have an /etc/resolv.conf that tells it which DNS servers to query for the answer.
 
  


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