Docker and Port Forwarding

Sometimes just running a simple command isn’t enough. Sometimes we need to run a service inside a container, like a webserver, and then access the webserver on its port.

Even better would be, to say “Hey, map port 80 inside the container to port 8080 outside of the container”. And that’s exactly what you can do with Port forwarding!

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In this example we are starting an apache webserver hosting a single file


docker run --rm -d --name apacheserver httpd

Will start an apache in detached mode (-d)
It should open a webserver on port 80
It will give the container a name “apacheserver”
It will remove the container when it stops

Open http://localhost:80

That doesn’t do anything
But is the Web-Server really running?
How can we test this, even though we can’t access it?! 🤔


docker exec -it apacheserver /bin/bash

You get an interactive shell inside the container


apt-get update && apt-get install curl

and then


curl localhost:80

Should bring up a “it works” message, which means the webserver is running (the exact output is: <html><body><h1>It works!</h1></body></html>)
But why can’t we see it on the host?

Let’s exit the container and try something else…


exit

Exit the interactive shell


docker logs apacheserver

This will give you the log output of the container
You even see the request from inside the container


docker logs apacheserver -f

Will follow the log output until you hit ctrl+c
But still, http://localhost:80 doesn’t do anything on the host

Let’s have a look into the container details:


docker inspect apacheserver

Will print out the container information
You see there are no ports bound to the host


docker stop apacheserver

Stop and remove the container
You could also do docker rm -f apacheserver


docker ps -a

Should be empty

Docker Port Forwarding


docker run -d --rm --name apacheserver -p 8080:80 httpd

-p 8080:80 forwards HOST-PORT:GUEST-PORT
On the host machine port 8080 is now mapped to port 80 in the guest machine

Open http://localhost:8080

Should bring up now “it works”


docker inspect apacheserver

Now shows the forwarded port


docker stop apacheserver

Should stop and remove the container again

http://localhost:8080

Is again unreachable, because the server stopped


docker run -d --rm --name phpserver -p 8080:80 -v ${PWD}:/var/www/html php:7.2-apache

Starts a new container named “phpserver” and maps port 80 from the container onto port 8080 on the host
Maps the current directory into /var/www/html, which is the document-root for apache
If you still have the index.php there it will now be served on http://localhost:8080


docker stop phpserver

Let’s remove the container and clean up


docker ps -a

Should be empty again.