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I am presented a mail server setup, that is working well. There's only a small issue, which makes me think, there's something wrong in the DNS setup. It is a small setup with a Debian mail server (postfix/dovecot) behind a router. The router has port forwarding configured and is named "router.intern" (IP 10.0.0.1). So incoming mail gets delivered to the mail server and outgoing mails are also fine. Every time a mail gets delivered, there is a reverse DNS check for the delivering server IP to block spammers. But there's also a reverse DNS check on the router's name:

postfix/smtps/smtpd[8088]: warning: hostname router.intern does not resolve to address 123.123.123.123

(123.123.123.123 is a placeholder for its WAN IP.)

/etc/resolv.conf:

domain intern
nameserver 10.0.0.1

Why does the mail server try do resolve the router's DNS name over WAN? I'm at loss where to search as everything else is working fine.

  • Sounds like your router is doing either application level proxying or SNAT. You might want to check the configuration settings for its port forwarding. – roaima Jun 10 '15 at 16:47
  • No, checked that. None of both is set. – awado Jun 10 '15 at 17:18
  • Is there any relevance to the address you've written as 123.123.123.123? Is this a literal part of the error or a placeholder you've invented for this question? If it's a placeholder please identify what it corresponds to. – roaima Jun 10 '15 at 17:42
  • Sorry, it's a placeholder. I just edited my post to reflect that. – awado Jun 10 '15 at 17:57
  • Try giving a look here: postfix.org/BASIC_CONFIGURATION_README.html with particolar reference to paragraph "Proxy/NAT external network addresses" – Damiano Verzulli Jun 10 '15 at 19:09

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