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Ubuntu/Linux: systemd-resolved[2344]: Server returned error NXDOMAIN, mitigating potential DNS violation DVE-2018-0001, retrying transaction with reduced feature level UDP.

In one of my systems the system log was reporting every 2-3 minutes the following error message:

Sep  3 13:43:57 tux1 systemd-resolved[2344]: Server returned error NXDOMAIN, mitigating potential DNS violation DVE-2018-0001, retrying transaction with reduced feature level UDP.
Sep  3 13:45:34 tux1 systemd-resolved[2344]: Server returned error NXDOMAIN, mitigating potential DNS violation DVE-2018-0001, retrying transaction with reduced feature level UDP.
Sep  3 13:48:58 tux1 systemd-resolved[2344]: Server returned error NXDOMAIN, mitigating potential DNS violation DVE-2018-0001, retrying transaction with reduced feature level UDP.
Sep  3 13:50:34 tux1 systemd-resolved[2344]: Server returned error NXDOMAIN, mitigating potential DNS violation DVE-2018-0001, retrying transaction with reduced feature level UDP.
Sep  3 13:53:56 tux1 systemd-resolved[2344]: Server returned error NXDOMAIN, mitigating potential DNS violation DVE-2018-0001, retrying transaction with reduced feature level UDP.

This was caused by a mismatch between the systemd configuration and /etc/resolv.conf.

/etc/resolv.conf should be a symbolic link pointing to the systemd DNS configuration in /run/systemd/resolve/resolv.conf

You can check if this is in place just by listing the file.

$ ls -l /etc/resolv.conf

If it isn’t pointing to the right file (and you are using systemd) you can fix it:

# rm /etc/resolv.conf
# ln -s /etc/resolv.conf  /run/systemd/resolve/resolv.conf

The errors stopped after this fix.

$ cat /var/log/syslog | grep -i error | grep -i dns