You may have come across an error in your server logs something along the lines of.
systemd-resolved[745]: Server returned error NXDOMAIN, mitigating potential DNS violation DVE-2018-0001, retrying transaction with reduced feature level UDP.
Having done a fair bit of research into to this I have yet to find a fix, however what I have found out is that this is not an error that should give you cause for concern and it is safe to ignore it.
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