How to properly close a port? (2024)

Sylwia Vargas

Posted on Mar 30, 2020 • Updated on Mar 31, 2020

#beginners #codenewbie #todayisearched

This is a thing I need to google every now and then so here's a simple recipe for closing neglected ports on MacOS, Windows and Linux.

Here are the steps:

1. Find the process ID (PID) of the port (replace the 'portNumber' with the number)

sudo lsof -i :portNumber
This will give you a response as follows — copy the PID number for the next step:

2. Kill the process

First, try this (replace PID with the number you copied above):

kill PID

Now, test if it's closed by connecting to the port (replace portNumber with the actual port number):

nc localhost portNumber

If it returns immediately with no output, the port isn't open. However, if it returns some input, try to kill it with:

kill -9 PID

Again, try to connect. If it's still running, try this:

sudo kill -9 PID

Here are the steps for Windows:

1. Find the process ID (PID) of the port (replace the 'portNumber' with the number)

netstat -ano | findstr :portNumber

Copy the PID number for the next step.

2. Kill the process

First, try this (replace typeyourPIDhere with the number you copied above):

taskkill /PID typeyourPIDhere /F

Run the first command again to see if it's closed.

Here are the steps for Linux (courtesy of mayankjoshi)

1. Get a list of all open processes

$top

2. Kill a process

kill pid kills the process by process id
killall pname kills the process by name
-9 for forceful killing in both kill and killall
Use sudo if it's a root process.

Top comments (23)

CapSel

Nothing worth mentioning upfront.

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Do I need a title? at I don't want to give a company name
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• Mar 31 '20

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Do not use lsof on live Linux (and possibly FreeBSD) servers. In very rare conditions it can cause entire server to hang - hardware reboot needed.

Instead use ss (fast!) and if you really have to then netstat (sloooww, cpu hog).

Sylwia Vargas

I'm a tech writer and educator advocating for code newbies ✨ I'm also a Developer Relations Lead + front-end dev at @inngest

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• Mar 31 '20

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Thank you! I don't think I propose it in the post 🤔
What do you think about the steps proposed by mayankjoshi that I integrated into the blog post:

Get a list of all open processes$top
Kill a processkill pid kills the process by process idkillall pname kills the process by name-9 for forceful killing in both kill and killallUse sudo if it's a root process.

Would you add anything?

CapSel

Nothing worth mentioning upfront.

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• Mar 31 '20

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ss -tnlp and netstat -tnlp shows pid of processes, their names and their open/listening (aka server) ports. There are tons of tutorials @google about these two commands. top on the other hand does not show open ports. Depending on the OS it can have some shortcuts to kill.

As to root and sudo I would be very careful. You may end up with a surprise ;) You can kill some system service like X server, print spooler, OS upgrade process.

kill (pid) and killall (matching a name) sends signals to processes. Without a name of a signal given default one is used - SIGTERM. This is just "asking" process to exit - similar to alt+f4. The -9 signal is a SIGKILL signal - usually just called kill. It cannot be ignored by processes.

After killing process that had opened TCP port it make take a while before this port is closed. It hangs in OS in special state - only thing you can do is wait or reboot.

Sooner or later you're going to need kill some process to free some port. It's a good idea to glance some docs/manuals (man ss, man netstat) to have some vague memories about what each of these commands can do. Every command is useful. Everyone has their favourite set. Do an experiment - but before you do save your files.

Tisha Murvihill

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Nov 5, 2020

• Oct 22 '22

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just wanted to say thanks for the info :)

Richard Herbert

In Linux

$top to see the list of open process

Killing a process

kill pid

kills the process by process id

killall pname

kills the process by name

-9 for forceful killing in both kill and killall

Use sudo if it's a root process.

Sylwia Vargas

I'm a tech writer and educator advocating for code newbies ✨ I'm also a Developer Relations Lead + front-end dev at @inngest

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• Mar 31 '20

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Thank you! I'll add it right away!

mayank joshi

I love system design and most of the time I find myself learning or designing one of them........I am a moderator at Dev.to 😀

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• Mar 31 '20

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Actually the URL to my profile is incorrect.😅😅

Sylwia Vargas

I'm a tech writer and educator advocating for code newbies ✨ I'm also a Developer Relations Lead + front-end dev at @inngest

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• Mar 31 '20

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🤦‍♀️corrected!

Lajos Koszti

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• Mar 31 '20

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It's not closing a port, but stopping a process. You close ports using firewall usually.

mayank joshi

I love system design and most of the time I find myself learning or designing one of them........I am a moderator at Dev.to 😀

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mayank@nlogn.in
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• Mar 31 '20

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When a process is killed, the ports are automatically Freed.

When I was doing TCP up connection I Freed same port for reuse using this method itself.

Lajos Koszti

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Sep 4, 2018

• Mar 31 '20 • Edited on Mar 31 • Edited

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Luckily, if you stop the application, the port will be released. The title says How to properly close a port?, not how to kill a process. What if you want the keep the process running but don't want to listen on that port anymore?

mayank joshi

I love system design and most of the time I find myself learning or designing one of them........I am a moderator at Dev.to 😀

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• Mar 31 '20

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I don't think it is possible to free a port held by a process without killing the same process.

Sylwia Vargas

I'm a tech writer and educator advocating for code newbies ✨ I'm also a Developer Relations Lead + front-end dev at @inngest

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• Mar 31 '20

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Sure. I decided for this title because that's what my students google — the post is in my budding series primarily for my students at a coding bootcamp ❤️

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