Courier-MTA reacts badly if own DNS server goes down [Update]

August 18, 2009

Today my DNS registrar and in my case also the provider of the DNS servers for my domains had a total blackout. He was down for about an hour, none of the 3 DNS servers was reachable. Ok, this was bad, no system could get the IPs for domain names – nothing I can to there except using a 4th DNS server operated by me in the future. But this was not the biggest problem, some DNS servers had the IP for my mail servers stored and tried to connect via SMTP to my courier-mta. The problem now is that courier didn’t accept the connections it tried to lookup its own name via the DNS servers in /etc/resolve.conf and ignored the settings in /etc/hosts (basically its own name). I thought surely I’ve a configuration error until I found following in the courier-mta FAQ.

NOTE: The Courier mail server does not read the hosts file. It needs a DNS server (although it is possible to have a working the Courier mail server configuration in a completely DNS-free environment, this excersize requires changing many configuration files, and perhaps will be its own FAQ entry some day).

What the fuck? Courier stops working if the DNS server for the own domain/hostname is not reachable? That must be a bug and not a feature. I will talk to the author and report back what he says.

[update] I talked with the author and other courier experts. The way courier is written it uses one internal function to all his DNS stuff and as it needs MX,TXT queries it uses a glibc function in it with does ignore the hosts file. There is currently no way around it, and so I’ll need to deploy an DNS server on my mail server to provide a DNS which can always resolve it’s own hostname. [/update]

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