Lsof is an extremely powerful unix diagnostic tool. Its name stands for LiSt Open Files, and it does just that. It lists information about any files that are open by processes currently running on the system. It easily pinpoints which process is using each network connection / open port. FAQ available here.
You can download it from the following link: https://packetstormsecurity.com/files/download/25693/lsof_4.61.tar.gz
Source: https://packetstormsecurity.com/files/25693/lsof_4.61.tar.gz.html