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.








