what is the best shell command for port state?

Question

Let's say, i'd like to know whether certain port (e.g. 21) is open or closed.

What is the best shell command for that?

bodhi.zazen · Answer 1 · 2011-12-15 16:16:26Z

up vote 2 down vote

My favorites are:

From a local machine

sudo lsof -i -n -P 

netstat -an | grep LISTEN | grep -v ^unix

netstat -ntulp

From a remote box

nmap -v -A ip_address

edited Dec 15 '11 at 16:16

answered Dec 14 '11 at 22:02

bodhi.zazen
23.1k135113

	Why bash -c in second command? – enzotib Dec 14 '11 at 22:52
	because you often can not pipe output of sudo commands directly. – bodhi.zazen Dec 14 '11 at 23:13
	Mmh, I don't understand when you cannot. Surely in your example you can (a so you should). Can you show an example when you cannot? – enzotib Dec 15 '11 at 8:34
	Good point, thank you. I have edited my post. – bodhi.zazen Dec 15 '11 at 16:17

Caesium · Answer 2 · 2011-12-14 21:59:30Z

up vote 1 down vote

netstat will tell you this. To check for all open (listening) ports:

netstat -l

You can also show all current connections:

netstat -an

See the manpage for loads more, netstat is a very powerful tool :)

answered Dec 14 '11 at 21:59

Caesium
4,0761830

pacofvf · Answer 3 · 2011-12-14 22:01:22Z

up vote 0 down vote

nmap -A -T4 localhost -p 80

It even shows you OS version and the service on that port

answered Dec 14 '11 at 22:01

pacofvf
19318

	You should note that nmap isn't part of base and needs installing with apt first – Caesium Dec 14 '11 at 22:26
	@Caesium you're right `sudo apt-get install nmap` and also nmap is used in the matrix movie – pacofvf Dec 14 '11 at 22:27

Florian Diesch · Answer 4 · 2011-12-14 22:04:36Z

You can use telnet to try to connect to a TCP port on a give host:

telnet somehost.example.com 21

tries to connect to port 21 on somehost.example.com

If the port is closed you get a message like

telnet: Unable to connect to remote host: Connection refused

otherwise you'll see whatever the program listening on that port tells you on connect.

4 Answers