Merge pull request #1663 from yubiuser/CNAME_limits

Add hint to the web interface about the limits of CNAME records

cname_records.php

@@ -39,6 +39,11 @@
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+              <strong>Note:</strong>
+              <p>The target of a <code>CNAME</code> must be a domain that the Pi-hole already has in its cache or is authoritative for. This is an universal limitation of <code>CNAME</code> records.</p>
+              <p>The reason for this is that Pi-hole will not send additional queries upstream when serving <code>CNAME</code> replies. As consequence, if you set a target that isn't already known, the reply to the client may be incomplete. Pi-hole just returns the information it knows at the time of the query. This results in certain limitations for <code>CNAME</code> targets,
+                for instance, only <i>active</i> DHCP leases work as targets - mere DHCP <i>leases</i> aren't sufficient as they aren't (yet) valid DNS records.</p>
+                <p>Additionally, you can't <code>CNAME</code> external domains (<code>bing.com</code> to <code>google.com</code>) successfully as this could result in invalid SSL certificate errors when the target server does not serve content for the requested domain.</p>
                 <button type="button" id="btnAdd" class="btn btn-primary pull-right">Add</button>
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