I think that you are mistaken and this is working correctly... Your output of command 'netstat -tl' shows us that something that is listening on port 4201 is accepting connections only from 'localhost'.
For comparison I run two instances of shellinaboxd. One on port 4201 with '--localhost-only' switch, and one on port 4200 without the switch. This is the output: Proto Recv-Q Send-Q Local Address Foreign Address State tcp 0 0 *:4200 *:* LISTEN tcp 0 0 localhost:4201 *:* LISTEN Output '*:4200' tells us that port 4200 is accepting connections from anywhere. For other port we can see 'localhost:4201', just like you and this tells us that connections on port 4201 are accepted only from localhost. Regards, Luka Krajger