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How can I check if a port is open or closed on a Linux server?

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11  
SSH is a protocol, not a TCP diagnostic tool. –  Shane Madden Sep 7 '11 at 17:28
    
Just to be clear. Are you asking how to with ssh from a remote system, or how to check while you are connected to a system via ssh? –  Zoredache Sep 7 '11 at 17:30
    
It's not quite clear what you're asking. What do you mean by "open"? Do you mean some server is listening on that port? Or do you mean it's allowed by the system firewall? Or what? –  David Schwartz Sep 7 '11 at 17:47
    
I think a port is being blocked on my server and want to unblock/open it again. –  James Anderson Sep 7 '11 at 17:49
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netstat -an | grep PORTNUMBER | grep -i listen If the output is empty, the port is not in use. –  automatix Sep 28 '13 at 13:12

4 Answers 4

up vote 32 down vote accepted

You can check if a process listens on a TCP or UDP port with netstat -tuplen.

To check whether some ports are accessible from the outside (this is probably what you want) you can use a port scanner like Nmap from another system. Running Nmap on the same host you want to check is quite useless for your purpose.

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There's no such thing as "tuplen" There is -t though. Also easier way than nmap is to just use Telnet. –  Dexter Apr 25 '13 at 17:20
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GNU netstat knows the parameters -t, -u, -p, -l, -e, and -n. Thanks to the options parser it can be expressed as -tuplen. linux.die.net/man/8/netstat –  joschi Apr 26 '13 at 9:53
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Also, the telnet command usually does only supports TCP, so you're out of luck if the service you want to check runs on another protocol. –  joschi Apr 26 '13 at 9:55
    
yeah, I was using windows hence the confusion. –  Dexter Apr 27 '13 at 15:42
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nc is (better) alternative to telnet. It supports UDP too. –  tsv.dimitrov Jul 10 '13 at 8:42

If you are connected to the system and can run a command as root then you can check the output of iptables

iptables -L -vn

this will list the firewall rules and which ports are open target ACCEPT and any explicitly closed ports target REJECT.

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OK, in summary, you have a server that you can log into. You want to see if something is listening on some port. As root, run:

netstat -nlp

this will show a listing of processes listening on TCP and UDP ports. You can scan (or grep) it for the process you're interest in,and/or the port numbers you expect to see.

If the process you expect isn't there, you should start up that process and check netstat again. If the process is there, but it's listening on a interface and port that you did not expect, then there's a configuration issue (e.g., it could be listening, but only on the loopback interface, so you would see 127.0.0.1:3306 and no other lines for port 3306, in the case of the default configuration for MySQL).

If the process is up, and it's listening on the port you expect, you can try running a "telnet" to that port from your Macbook in your office/home, e.g.,

 telnet xxxxxxxxxxxx.co.uk 443

That will test if (assuming standard ports) that there's a web server configured for SSL. Note that this test using telnet is only going to work if the process is listening on a TCP port. If it's a UDP port, you may as well try with whatever client you were going to use to connect to it. (I see that you used port 224. This is masqdialer, and I have no idea what that is).

If the service is there, but you can't get to it externally, then there's a firewall blocking you. In that case, run:

 iptables -L -n

This will show all the firewall rules as defined on your system. You can post that, but, generally, if you're not allowing everything on the INPUT chain, you probably will need to explicitly allow traffic on the port in question:

 iptables -I INPUT --dport 224 -j ACCEPT

or something along those lines. Do not run your firewall commands blindly based on what some stranger has told you on the Internet. Consider what you're doing.

If your firewall on the box is allowing the traffic you want, then your hosting company may be running a firewall (e.g., they're only allowing SSH (22/tcp), HTTP (80/tcp) and HTTPS (443/tcp) and denying all other incoming traffic). In this case, you will need to open a helpdesk ticket with them to resolve this issue, though I suppose there might be something in your cPanel that may allow it.

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Could you pls add how to undo the iptables -I command? Thanks!! –  Evgeny Sep 9 '13 at 22:52
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"iptables -D" followed by whatever else you had after the "-I" in the original command. Basically, look up the documentation. –  cjc Sep 10 '13 at 11:45

Quickest way to test if a TCP port is open (including any hardware firewalls you may have), is to type, from a remote computer (e.g. your desktop):

telnet myserver.com 80

Which will try to open a connection to port 80 on that server. If you get a time out or deny, the port is not open :)

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says that it doesn't recognise "telnet" as a command... –  James Anderson Sep 7 '11 at 17:37
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"yum install telnet" to install the telnet client package. –  cjc Sep 7 '11 at 17:41
    
Says: telnet: connect to address 82.165.148.224: Connection refused –  James Anderson Sep 7 '11 at 17:42
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Written above: if you get a time out or deny, the port is not open –  Industrial Sep 7 '11 at 18:09
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What if you don't have perms to install telnet? Is there another standard tool? –  KC Baltz Jan 14 '14 at 23:40

protected by Sven Jan 30 at 12:52

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