When port forwarding didn't seem to work with Docker

I just had an issue with Docker which took up a couple of hours, and which, like most such problems, was just plain stupidity on my part. And being new to Docker didn’t help either.

I had a spray server app that I needed to run inside a docker container. Don’t ask why. The spray app ran on localhost at port 9090. Now, I just wanted to make the service available from outside the container. And it’s pretty simple. I built the image, and then just ran

sudo docker run -p 9090:9090 my-spray-app

This should work. And it does work, but I’ll come to that later. So, after running this, when I tried to access the service via localhost:9090, all I got was “Remotely Closed” errors.

I thought maybe I had made some mistake, which I surely had, but coming to that later. So, I looked around, Google and stuff, and I was sure that the service was running inside the container after connecting to the docker in an interactive terminal and checking stuff out. Nope, no issues there. The service was running fine.

Then, there was this issue on github, which was exactly what was happening with me too. Using --net=host worked, but nothing else did. And just to be clear, this is probably not an issue. The Docker developers are being very nice in trying to address the issue, but it seems like those people are making the same mistake I was.

Then, there was this issue. I could see the same kind of output in netstat, but that clearly is not an issue. I am at least a little familiar with how networking works in linux.

The issue is that the app that is running in the container is bound on localhost. Which is not accessible from outside at all. As this answer by a docker staff member says,

It isn’t really possible to map the container’s localhost to the outside. The process in the container must bind to the container’s eth0

So, that was all. And like the idiot that I am, this was plainly visible and I still didn’t think of it. When I saw the output of docker ps, the port column clearly said that the binding was to the “0.0.0.0:9090”, and not “localhost:9090”. I saw it, but I didn’t observe. That’s what happens when you don’t listen to Sherlock Holmes.

The only thing I needed to do was set the host of the spray app to “0.0.0.0”. I am sure there are other ways of doing this, but this was all I needed for the time being.

In my defense… Ah, forget it, it was just stupid. Putting this up in case it helps some fellow morons.