Last week I changed the main record of my domain from A-Records with fix IPs to CNAME-Records pointing to another domain. So instead of having something like
$ dig chrisschell.de chrisschell.de. 1800 IN A 188.8.131.52
now dig returned:
$ dig chrisschell.de chrisschell.de. 1800 IN CNAME mywebspace.example.com mywebspace.example.com. 3600 IN A 184.108.40.206
I had successfully verified this change in advance on a test domain and have therefor been pretty confident. A request for
chrisschell.de returned a CNAME to
mywebspace.example.com which returned the correct ip – everything's fine!
But little did I know *diabolic laughter*.
After getting unusually few mails it dawned me that there might be something off. And indeed, requesting my domain's MX record resulted in an empty answer:
$ dig mx chrisschell.de chrisschell.de. 1800 IN CNAME mywebspace.example.com mywebspace.example.com. IN MX
If someone tried to send me an email, his server would try to forward the mail to the mailserver listed for
chrisschell.de. Since the destination domain has no MX-record set, mailservers got an empty answer and I didn't get any mails.
After switching back to status quo I did some digging and found the RFC for "Common DNS Operational and Configuration Errors", which had the following to say to me:
A CNAME record is not allowed to coexist with any other data.
This is often attempted by inexperienced administrators as an obvious way to allow your domain name to also be a host.