Docker use host port without mapping port

Question

I want to send emails from 2 separate docker containers. Both of these containers need port 25 of the host system but I can only map the port to one of those containers. When I try with the second one it throws this error: Bind for 0.0.0.0:25 failed: port is already allocated . How do I use the port of the host system without actually mapping it to one of the containers? The containers need to run on a certain network and not on the host network. I'm using Nodemailer in the docker containers btw.

Answer 1

I figured it out. As mentioned here: From inside of a Docker container, how do I connect to the localhost of the machine? , you need to add the flag --add-host=host.docker.internal:host-gateway to the run command. That way localhost points to the docker host and not the host of the containers own network. No need to change anything in your code.