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How do I find out running processes were associated with each open port on Linux? How do I find out what process has open TCP port # 111 or UDP port 7000 under Linux using the CLI?

A port is nothing but an endpoint of communication used in computer networks. You have physical or wireless connections at the hardware level. At software or operating system level a port act as a logical construct that acts as communication port of network service such as SSH, HTTPD and more. TCP and UDP are the most common port. TCP is an acronym for Transmission Control Protocol. UDP is an acronym for User Datagram Protocol. See the several difference between UDP and TCP internet protocols here. This page shows Linux commands to find out which process is listing upon a TCP or UDP 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:

Active Internet connections (only servers)
Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name
tcp        0      0 127.0.0.1:3306          0.0.0.0:*               LISTEN      1138/mysqld     
tcp        0      0 0.0.0.0:111             0.0.0.0:*               LISTEN      850/portmap     
tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN      1607/apache2    
tcp        0      0 0.0.0.0:55091           0.0.0.0:*               LISTEN      910/rpc.statd   
tcp        0      0 192.168.122.1:53        0.0.0.0:*               LISTEN      1467/dnsmasq    
tcp        0      0 0.0.0.0:22              0.0.0.0:*               LISTEN      992/sshd        
tcp        0      0 127.0.0.1:631           0.0.0.0:*               LISTEN      1565/cupsd      
tcp        0      0 0.0.0.0:7000            0.0.0.0:*               LISTEN      3813/transmission
tcp6       0      0 :::22                   :::*                    LISTEN      992/sshd        
tcp6       0      0 ::1:631                 :::*                    LISTEN      1565/cupsd      
tcp6       0      0 :::7000                 :::*                    LISTEN      3813/transmission
udp        0      0 0.0.0.0:111             0.0.0.0:*                           850/portmap     
udp        0      0 0.0.0.0:662             0.0.0.0:*                           910/rpc.statd   
udp        0      0 192.168.122.1:53        0.0.0.0:*                           1467/dnsmasq    
udp        0      0 0.0.0.0:67              0.0.0.0:*                           1467/dnsmasq    
udp        0      0 0.0.0.0:68              0.0.0.0:*                           3697/dhclient   
udp        0      0 0.0.0.0:7000            0.0.0.0:*                           3813/transmission
udp        0      0 0.0.0.0:54746           0.0.0.0:*                           910/rpc.statd   

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:

lrwxrwxrwx 1 root root 0 2010-10-29 10:20 /proc/1138/exe -> /usr/sbin/mysqld

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

tcp        0      0 0.0.0.0:80              0.0.0.0:*               LISTEN      1607/apache2

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

Video demo

fuser command

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

7000/tcp:             3813

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

lrwxrwxrwx 1 vivek vivek 0 2010-10-29 11:00 /proc/3813/exe -> /usr/bin/transmission

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

transmission (1)     - a bittorrent client

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:

lrwxrwxrwx 1 vivek vivek 0 2010-10-29 12:04 /proc/3813/cwd -> /home/vivek

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

3813: /home/vivek

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:

vivek     3813  1.9  0.3 188372 26628 ?        Sl   10:58   2:27 transmission

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

3813 vivek    vivek    transmission                   02:44:05 Fri Oct 29 10:58:40 2010

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):

lsof Command Example

Type the command as follows:

lsof -i :portNumber 
lsof -i tcp:portNumber 
lsof -i udp:portNumber 
lsof -i :80
lsof -i :80 | grep LISTEN

Sample outputs:

apache2   1607     root    3u  IPv4   6472      0t0  TCP *:www (LISTEN)
apache2   1616 www-data    3u  IPv4   6472      0t0  TCP *:www (LISTEN)
apache2   1617 www-data    3u  IPv4   6472      0t0  TCP *:www (LISTEN)
apache2   1618 www-data    3u  IPv4   6472      0t0  TCP *:www (LISTEN)
apache2   1619 www-data    3u  IPv4   6472      0t0  TCP *:www (LISTEN)
apache2   1620 www-data    3u  IPv4   6472      0t0  TCP *:www (LISTEN)

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 www-data www-data /usr/sbin/apache2 -k start     03:16:22 Fri Oct 29 10:20:17 2010

Where,

• 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:

https		443/tcp				# http protocol over TLS/SSL
https		443/udp

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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