Linux: Find out who is listening on ports

 If you suspect the port you are trying to connect to is being blocked, you can check who is listening on that port using netstat -tnlp.

Better to connect as root for a more comprehensive listing. 

bash-3.2$ netstat -tnlp 


tcp     0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      545/sshd       
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN      705/cupsd      
tcp6       0      0 :::22                   :::*                    LISTEN      545/sshd       
tcp6       0      0 ::1:631                 :::*                    LISTEN      705/cupsd      

Now you have list of ports that are being used. If you want to find out more about the process that's running on 705, try running:

bash-3.2$ ps -aux | grep 705
 root       705  0.0  0.2   6792  2436 ?        Ss   09:33   0:00 /usr/sbin/cupsd -C /etc/cups/cupsd.conf