How to know what program is listening on a given port?

Open your terminal and type as

lsof -i :8000

that command will list you the application used by that port with PID. (If no results run via sudo since your might have no permission to certain processes.)

For example, with port 8000 (python3 -m http.server):

$ lsof -i :8000
COMMAND  PID USER   FD   TYPE  DEVICE SIZE/OFF NODE NAME
python3 3269 user    3u  IPv4 1783216      0t0  TCP *:8000 (LISTEN)

And port 22 (SSH):

$ sudo lsof -i :22
COMMAND  PID USER   FD   TYPE  DEVICE SIZE/OFF NODE NAME
sshd     998 root    3u  IPv4 1442116      0t0  TCP *:ssh (LISTEN)
sshd     998 root    4u  IPv6 1442118      0t0  TCP *:ssh (LISTEN)

Hope that helps.

You can use netstat to see which process is listening on which port.

You can use this command to have a full detail :

sudo netstat -peanut

if you need to know exactly which one is listening on port 8000 you can use this :

sudo netstat -peanut | grep ":8000 "

There is no process that can hide from netstat.

To expound on the answer by @33833 you can get some very detailed info, for example:

$ lsof -i :8000
COMMAND  PID  USER   FD   TYPE   DEVICE SIZE/OFF NODE NAME
squid3  1289 proxy   15u  IPv6 14810490      0t0  TCP *:8000 (LISTEN)

$ ps -fp 1289
UID        PID  PPID  C STIME TTY          TIME CMD
proxy     1289     1  0 09:48 ?        00:00:00 /usr/sbin/squid3 -N -f /etc/squid-deb-proxy/squid-deb-proxy.conf

I can see right there that squid is the process, but it is actualy my squid-deb-proxy that is taking up the port.

Another good example of a java app:

$ lsof -i :4242
COMMAND  PID USER   FD   TYPE   DEVICE SIZE/OFF NODE NAME
java    3075 root   86u  IPv4    12019      0t0  TCP *:4242 (LISTEN)

$ ps -fp 3075
UID        PID  PPID  C STIME TTY          TIME CMD
root      3075     1 15 May24 ?        3-16:07:25 /usr/local/crashplan/jre/bin/java -Dfile.encoding=UTF-8 -Dapp=CrashPlanService -DappBaseName=CrashPl

You can see in lsof (LiSt Open Files) that it is java, which is less than helpful. Running the ps command with the PID we can see right away that it is CrashPlan.

Try ss from iproute2 package:

ss -nlp | grep 8000

Another way using socklist from procinfo package:

man socklist

DESCRIPTION
socklist is a Perl script that gives you a list of all open sockets, enumerating types, port, inode, uid, pid, fd and the program to which it belongs.

sudo socklist

type  port      inode     uid    pid   fd  name
tcp     53      28749       0   1721    5  dnsmasq
tcp    631      29190       0   2433   11  cupsd
tcp  42376      82230    1000   1345   84  firefox
tcp  49048      71686    1000   1345   67  firefox
tcp  41248      81672    1000   1345  119  firefox
tcp  54676      84558    1000   1345   73  firefox
udp  56107      66258       0   3268   20  dhclient
udp  40804      17857     107    679   14  avahi-daemon
udp     53      28748       0   1721    4  dnsmasq
udp     68      67427       0   3268    6  dhclient
udp    631      19692       0    765    8  cups-browsed
udp   5353      17855     107    679   12  avahi-daemon

Due to low point in community, I can't comment here. Without sudo all above commands will not provide you Process Id / Program Name.

So, you need type following things:

sudo lsof -i :8000

or

sudo netstat -lntup

you can check those command details via man, help or my favorite helper

You can use nmap.

It is really important to know which ports are open in your PC, this is not only useful for Linux, but also for other operating systems, Linux has a lot of tools to check which ports are open, the most common is nmap which is a command line tool, but also exist a Graphical frontEnd for it if you prefer that way.¹

to install it, just press Ctrl+Alt+T on your keyboard to open Terminal. When it opens, run the command below:

sudo apt-get install nmap

For more information about nmap, and other utilities, go Here

^{1Source:garron.me}

Here is an easy to remember command

ss -townlup 

Netid               State                 Recv-Q                Send-Q                                    Local Address:Port                               Peer Address:Port
icmp6               UNCONN                0                     0                                                *%eth0:58                                            *:*
udp                 UNCONN                0                     0                                         127.0.0.53%lo:53                                      0.0.0.0:*
udp                 UNCONN                0                     0                                     172.31.26.71%eth0:68                                      0.0.0.0:*
tcp                 LISTEN                0                     128                                             0.0.0.0:22                                      0.0.0.0:*
tcp                 LISTEN                0                     128                                             0.0.0.0:443                                     0.0.0.0:*
tcp                 LISTEN                0                     128                                       127.0.0.53%lo:53                                      0.0.0.0:*
tcp                 LISTEN                0                     128                                                [::]:22                                         [::]:*
tcp                 LISTEN                0                     128                                                [::]:443                                        [::]:*
tcp                 LISTEN                0                     100                                                   *:9090                                          *:*                    users:(("java",pid=13008,fd=16))
tcp                 LISTEN                0                     100                                                   *:8080                                          *:*                    users:(("java",pid=11556,fd=12))