Fun with email servers and FCrDNS

Er, yeah. I didn’t know what FCrDNS was either… Forward Confirmed Reverse DNS. [And if you don't know what DNS is then you might want to go read something else because this will bore you rigid]. In a nutshell…

  • DNS: Maps a domain name to an IP address e.g. commonagency.com -> 94.136.61.118
    This is fundamental to every day use of the web in that it ensures when someone enters a domain name they get routed to the server from which that domain is served.
  • Reverse DNS: Maps an IP address to a domain name e.g. 94.136.61.118 -> commonagency.com
    This is frequently used by email servers to validate that emails received from another email server IP address can be matched with a domain name that appears to correlate with the sender’s address [that's actually not exactly true but it's a simplistic explanation].
  • FCrDNS: RDNS lookup followed by a DNS lookup e.g. 94.136.61.118 -> commonagency.com -> 94.136.61.118
    This is apparently the best approach and is now used by many email servers.

Why care? Well hopefully you won’t ever need to, but when a client calls to say that they can’t get emails through to one of their customers, these are the kind of things you learn. Usually you’ll have set-up your DNS/RDNS in such a way that FCrDNS works by default, and if not you can usually do it quite quickly. There’s a great tool at http://ipadmin.junkemailfilter.com/rdns.php which helps with testing. Basically, if you know that your RDNS is set-up correctly and that you’ve got some appropriate SPF records set-up too but you’re still finding some emails get blocked, check out your FCrDNS. And if nothing else, it’s another acronym that you didn’t know you didn’t know, but now do.

8th December 2009

Ben Childs

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