How to know which ports are open on linux ubuntu

Question

I want to know which ports are open on my ubuntu machine.

I just want to initially block all ports in iptables

and then slowly and slowly will open only those ports which i need. Currenly i have no idea how many services have opened their ports on my system

any idea to know

Answer 1

Two ways I do it are:

• lsof -i (listed open files, a network socket counts as a file and that's what the -i flag specifies)
• netstat -antp

Answer 2

netstat -l

will list all open ports as well as the address it's listening on. You may have to resolve names to port numbers. The list of associations is found in /etc/services.

You might also want to check out the man page on netstat. There may be other options you want to add to the command.

man netstat

Answer 3

netstat -tunpl will tell you the numbers of tcp and udp ports that are being listened on.and what process is listening on each one (so you can tell whether you want it to do so or not).

The manpage has further options that may be of interest.