-A INPUT -m state --state NEW -m tcp -p tcp --dport 37 -j ACCEPTUsually You should use a similar row as a template. The port number 37 is the port to be opened.
$ telnet localhost 37 Trying ::1... Connected to localhost. Escape character is '^]'. 12ABConnection closed by foreign host. $ $ telnet 123.123.123.123 37 Trying 123.123.123.123... Connected to 123.123.123.123. Escape character is '^]'. 12EFConnection closed by foreign host.
# uname -a UNIX_System_V r4v44d 4.0 R40V4.4 m88k mc88110 # rdate timeserver Sun Oct 25 14:04:24 2015 # uname -a r3v8b r3v8b R3V8 ;50602 M68060 # /usr/local/bin/rdate timeserver timeserver Sun Oct 25 14:05:46 2015