How about this: if svdrphosts.conf contains only one single IP number, then open the port for only that IP number. Otherwise i needs to be opened generally,
This only works if the single line in svdrphosts.conf is 127.0.0.1.
But how about this much simpler solution: if svdrphosts.conf is missing or empty, bind to localhost only. (And perhaps supply a sample svdrphosts.conf with all lines commented out.)
Olaf