When this is set to Yes (as it is by default), Virtualmin will check the system's DNS client configuration file `/etc/resolv.conf` to ensure that it is setup to use the local DNS server for name resolution. This is necessary, as without it and DNS domains you create may not be resolvable by the system running Virtualmin until they are fully registered by a DNS registrar.

If you are certain that this is not needed (perhaps because you have an automatic secondary DNS server setup), this can be changed to No to prevent spurious configuration check errors.