Tag Archives: dns

Postfix configuration: dealing with spam

Recently had an issue where everytime I tried to send an email, it was getting rejected by my smtp server. At the same time, it seemed like I was getting a barrage of spam hitting my inboxes, so I assumed I had been hacked.

To cut a long story short, turned out to be that my server had lost its configuration for DNS lookup, and the anti-spam rule reject_unknown_sender_domain was then being triggered! 3 fucking days to work that one out…the following two files should have contained some DNS info:

  • /var/spool/postfix/etc/resolv.conf
  • /etc/resolv.conf

I used dig to prove that I did not even have DNS lookup abilities. dig is a command-line tool for querying DNS name servers for information about host addresses, mail exchanges, name servers, and related information.

  • # get a list of yahoo's mail servers
    dig yahoo.com MX +noall +answer

In the end, I defined Google’s open DNS servers in the interfaces configuration file at /etc/network/interfaces: dns-nameservers (ref : http://wiki.debian.org/NetworkConfiguration#Definingthe.28DNS.29Nameservers)

In the process, I learnt a lot about Postfix anti-spam config: