scan the ports from outside

nmap -sT -O hostname -p 1234-3456
https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/3/html/Security_Guide/s1-server-ports.html
http://stackoverflow.com/questions/9609130/quick-way-to-find-if-a-port-is-open-on-linux
http://www.cyberciti.biz/faq/how-do-i-find-out-what-ports-are-listeningopen-on-my-linuxfreebsd-server/