The Allowed setting indicates that the host requires reverse host name lookup to work to determine that the connection comes from a recognizable server.

By default, the host resolves the IP address of the connecting server to a host name by performing a reverse lookup.

If the conversion of the IP address to host name fails, the connection fails.

Otherwise, it compares the host name to the list of known server host names. If the comparison fails, the host rejects the server and the connection fails.