Are you behind NAT? If so DMZ that box. Keep the firewall up. Also assuming the networking and firewall is together, using the
-nomaster parameter on the startup would make people not see it on the Master Server but still be able to connect.
Having these CVARS too low will do it too.
Code:
//max_queries_sec 16
//max_queries_sec_global 64
//max_queries_window 256