Adding mail on a domain that has a CNAME record silently breaks its zonefile

[espressoelf]

Let's say I had a typical configuration for domain example.net with MX being box.example.org and www.example.net and sub.example.net being a CNAME for example.net.

Later on, I added a new email account noreply@sub.example.net which automatically created a MX record for sub.example.net.

This leads to the updated zonefile not being loaded by nsd:

example.net.signed:183: CNAME and other data at the same name
...

An error or at least a warning would've been useful, i.e. a check if there's a CNAME on the subdomain I want to create a first account for. A general info on the account/alias creation page could be useful, too.

See also #1916

[myfirstnameispaul]

I agree. CNAME can create a lot of issues for those not aware of limitations.

One simple thing could be to add an explanation on the Custom DNS page for CNAME type.

mailinabox/management/templates/custom-dns.html

Line 37 in 05c2f3c

<option value="CNAME" data-hint="Enter another domain name followed by a period at the end (e.g. mypage.github.io.).">CNAME (DNS forwarding)</option>

[espressoelf]

One simple thing could be to add an explanation on the Custom DNS page for CNAME type.
Generally a good idea, though I was more looking for something like a reminder. I've created that CNAME years ago and generally know about that limitation, but just didn't think about it when I created the new email account. Even worse, everything seemed working, until some coincidence made me investigate why the zone is outdated even with the zonefile being up to date.

[myfirstnameispaul]

You could also add a list item to the Users page:

mailinabox/management/templates/users.html

Lines 37 to 43 in 05c2f3c

	<ul style="margin-top: 1em; padding-left: 1.5em; font-size: 90%;">
	<li>Passwords must be at least eight characters consisting of English letters and numbers only. For best results, <a href="#" onclick="return generate_random_password()">generate a random password</a>.</li>
	<li>Use <a href="#aliases">aliases</a> to create email addresses that forward to existing accounts.</li>
	<li>Administrators get access to this control panel.</li>
	<li>User accounts cannot contain any international (non-ASCII) characters, but <a href="#aliases">aliases</a> can.</li>
	<li>Quotas may not contain any spaces, commas or decimal points. Suffixes of G (gigabytes) and M (megabytes) are allowed. For unlimited storage enter 0 (zero)</li>
	</ul>

[myfirstnameispaul]

My reason for linking to the files was hoping you would submit a PR, because I think it is a good idea, and I suspect a printed text line will be merged since it doesn't affect any functionality.