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