Linux show ports and process

Linux List The Open Ports And The Process That Owns Them

So how do you list the network open ports on your Linux server and the process that owns them? The answer is simple. Use the following command (must be run as the root user):

sudo lsof -i
sudo netstat -lptu
sudo netstat -tulpn

Sample outputs (see video demo):

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Comments on this entry are closed.

Shameless plug to a really good article I wrote about lsof and some of the more nifty tricks:

We recently did patching in our linux servers and want to know which process is using which patch. Do we have any linux command to know that.

Thanks in advance
Ankita

Thanks Vikrant for the useful tip…I’ve used lsof for ages but sort of taken it for granted,didn’t check the man pages, and wasn’t aware of this option.

Great (. ) site, by the way, I’ll start contributing my own tips as well. Thanks for giving me lots of ideas-projects to increase my understanding of Linux/BSD.

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Which Linux process is using a particular network port?

Most network related services have to open up a network socket, so they can start listening for incoming network requests. It is common to find the TCP or UDP being used as the main communication protocol. In this article, we will check what ports are used by which Linux process.

Table of Contents

Auditing processes and network services

Find out what process is listening to a port

Only one process can actively listen to a TCP or UDP port. We usually only discover this when another process is already running on a specific port, while we try to start another service:

[emerg] 9400#0: bind() to 0.0.0.0:80 failed (98: Address already in use)

or something like:

Time to tackle which process is keeping these addresses or ports in use!

Using lsof to show open files and network ports

Let’s start with a powerful utility named lsof. It is not always installed by default, but still a very common utility. Its name is derived from listing open files. In Linux, even a network socket is a file. So this is definitely the right utility to retrieve some useful information. LSOF will include network-related data like port numbers and process names.

To find open ports and the related processes, ask lsof to see the related details. We filter out all UDP ports and only want to see TCP ports that are listening to data.

This output might look something like this:

LSOF displaying UDP ports and TCP ports in LISTEN state

This commands includes all UDP ports and for TCP only the ports which are actually in “LISTEN” state. Perfect to determine which process is listening to what port (or ports).

Note: Although egrep is getting deprecated, you can still use it. Or use ‘grep -E’ instead.

If you are interested in a particular port, lsof can filter by protocol and port number.

Using netstat to show ports and applications

The netstat utility is most likely the easiest way to determine what processes are running and what port they listen to. It is available on most systems by default.

The result might be looking something like this:

Netstat showing all running processes and ports they listen to

In this output, we see the following details:

• Column 1: Proto
• Column 2: Recv-Q
• Column 3: Send-Q
• Column 4: Local Address
• Column 5: Foreign Address
• Column 6: State
• Column 7: PID/Program name

The protocol (Proto) defines the data transfer protocol. This is typically tcp or udp for IPv4, tcp or udp6 for IPv6. The Recv-Q and Send-Q are the queues to for receiving or sending data. In the output they are usually zero unless a transfer occurs at that moment. The Local Address field specifies the IP address with the related port number. Can listen to a particular IP address (like localhost or 127.0.0.1) or on all interfaces (0.0.0.0 for IPv4). This is usually the interesting part to filter out (tip: use grep). The State column is usually showing LISTEN for TCP. Then finally the PID/Program name column shows the actual process that is listening to a particular network port.

Using the ss command

f you don’t have the netstat utility available, it might have been replaced with a newer toolkit. In that case, you usually have the ss utility available (iproute2 package).

To use the ss tool to see on Linux which ports are used by a particular process:

This will show a similar output. It shows all the listening ports, limited to UDP/TCP only, not translated to hostnames to speed up the results.

Detecting network ports for new processes

Sometimes you might be running a new process and not aware of any network ports being opened. This might occur when it just quickly happens, or due to a port listening conflict. Also when running services in containers, it might be harder to know what ports need to be opened, to get it fully functioning.

In these cases, the strace utility is of great help. It can track a running or new process and alert for any events of your interest. To track the open request of a network port, we can monitor the related system call (syscall), which is ‘bind’.

With this command we will see quickly after execution the following line:

This means it tried to open a network socket with port 80. Unfortunately, it does not show if it opens a TCP or UDP port. If we broaden the system calls a little bit, this information becomes available:

strace -f -e trace=network nc -lu 80
socket(PF_INET, SOCK_DGRAM, IPPROTO_UDP) = 3
setsockopt(3, SOL_SOCKET, SO_REUSEADDR, [1], 4) = 0
bind(3, , 16) = 0

Conclusion

That’s it for today. Linux systems are very versatile, yet sometimes need the right tool to dig into the details you want to know. With tools like lsof, netstat, ss, and strace, we can find exactly the network information we are looking for.

Did this article help you or have a good one-liner for other readers? Share your comments to make this article even better!

Keep learning

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Run automated security scans and increase your defenses. Lynis is an open source security tool to perform in-depth audits. It helps with system hardening, vulnerability discovery, and compliance.

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Linux Find Out Which Process Is Listening Upon a Port

Linux Find Out Which Process Is Listening Upon a Port

You can the following programs to find out about port numbers and its associated process:

1. netstat command or ss command – a command-line tool that displays network connections, routing tables, and a number of network interface statistics.
2. fuser command – a command line tool to identify processes using files or sockets.
3. lsof command – a command line tool to list open files under Linux / UNIX to report a list of all open files and the processes that opened them.
4. /proc/$pid/ file system – Under Linux /proc includes a directory for each running process (including kernel processes) at /proc/PID, containing information about that process, notably including the processes name that opened port.

You must run above command(s) as the root user.

Linux netstat command find out which process is listing upon a port

Type the following command:
# netstat -tulpn
Sample outputs:

TCP port 3306 was opened by mysqld process having PID # 1138. You can verify this using /proc, enter:
# ls -l /proc/1138/exe
Sample outputs:

You can use grep command or egrep command to filter out information:
# netstat -tulpn | grep :80
Sample outputs:

A note about ss command

Some Linux distro considered the nestat command as deprecated and therefore should be phased out in favor of more modern replacements such as ss command. The syntax is:
$ sudo ss -tulpn
$ sudo ss -tulpn | grep :3306

Click to enlarge image

Video demo

fuser command

Find out the processes PID that opened tcp port 7000, enter:
# fuser 7000/tcp
Sample outputs:

Finally, find out process name associated with PID # 3813, enter:
# ls -l /proc/3813/exe
Sample outputs:

/usr/bin/transmission is a bittorrent client, enter:
# man transmission
OR
# whatis transmission
Sample outputs:

Find Out Current Working Directory Of a Process

To find out current working directory of a process called bittorrent or pid 3813, enter:
# ls -l /proc/3813/cwd
Sample outputs:

OR use pwdx command, enter:
# pwdx 3813
Sample outputs:

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Find Out Owner Of a Process on Linux

Use the following command to find out the owner of a process PID called 3813:
# ps aux | grep 3813
OR
# ps aux | grep ‘[3]813’
Sample outputs:

OR try the following ps command:
# ps -eo pid,user,group,args,etime,lstart | grep ‘[3]813’
Sample outputs:

Another option is /proc/$PID/environ, enter:
# cat /proc/3813/environ
OR
# grep —color -w -a USER /proc/3813/environ
Sample outputs (note –colour option):

Fig.01: grep output

lsof Command Example

Type the command as follows:

Now, you get more information about pid # 1607 or 1616 and so on:
# ps aux | grep ‘[1]616’
Sample outputs:
www-data 1616 0.0 0.0 35816 3880 ? S 10:20 0:00 /usr/sbin/apache2 -k start
I recommend the following command to grab info about pid # 1616:
# ps -eo pid,user,group,args,etime,lstart | grep ‘[1]616’
Sample outputs:

• 1616 : PID
• www-date : User name (owner – EUID)
• www-date : Group name (group – EGID)
• /usr/sbin/apache2 -k start : The command name and its args
• 03:16:22 : Elapsed time since the process was started, in the form [[dd-]hh:]mm:ss.
• Fri Oct 29 10:20:17 2010 : Time the command started.

Help: I Discover an Open Port Which I Don’t Recognize At All

The file /etc/services is used to map port numbers and protocols to service names. Try matching port numbers:
$ grep port /etc/services
$ grep 443 /etc/services
Sample outputs:

Check For rootkit

I strongly recommend that you find out which processes are really running, especially servers connected to the high speed Internet access. You can look for rootkit which is a program designed to take fundamental control (in Linux / UNIX terms “root” access, in Windows terms “Administrator” access) of a computer system, without authorization by the system’s owners and legitimate managers. See how to detecting / checking rootkits under Linux.

Keep an Eye On Your Bandwidth Graphs

Usually, rooted servers are used to send a large number of spam or malware or DoS style attacks on other computers.

Conlcusion

You learned various Linux commands to find information about running process and their ports. See the following man pages for more information:
$ man ps
$ man grep
$ man lsof
$ man netstat
$ man fuser

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