How to connect to a docker container from outside the host (same network) [Windows]

Question

I've created my first docker container, it's running a server using Go but I can't access it from outside the host computer. I've just started with docker so I'm a little lost here.

So I have a very simple Go code that starts a server, I have built the docker image which installs Go and builds the code in a Linux base image. I'm running the server on port 8080 so I expose that port to the host running the container like this:

docker run -p 8080:8080 dockertest 

That works and I get to access the server through docker's machine IP (the one that appears on the Docker Quickstart Terminal when initiated), the problem is I can't access the website I'm hosting from outside the host, so if I try to open the same IP address on my phone it just gives me an error: This webpage is not available (ERR_CONNECTION_TIMED_OUT).

I've also tried specifying the IP like this:

docker run -p 192.168.0.157:8080:8080 dockertest 

But when I do that I can access the website through neither the docker machine's IP nor the specified IP on the command line above. I'm also not sure which IP I'm supposed to write in that command I used my computer's IP, I've also tried 127.0.0.1 (localhost) but that gave me the same result: couldn't access the website through any IP whatsoever.

I've googled this problem and found many StackOverflow questions but neither helped me solve my issue, most of them were oriented to Linux or Mac so the solution didn't apply to my situation.

Also, I can run the Go code on my computer and access the website from another device in the same network through my computer's IP. I don't understand why I can't access it when I'm running it in the docker machine, it occurred to me that it may have something to do with IP forwarding or something but I'm a complete noob in networking, I'm mostly a web developer and have almost no experience in native.

Answer 1

1. Open Oracle VM VirtualBox Manager
2. Select the VM used by Docker
3. Click Settings -> Network
4. Adapter 1 should (default?) be "Attached to: NAT"
5. Click Advanced -> Port Forwarding
6. Add rule: Protocol TCP, Host Port 8080, Guest Port 8080 (leave Host IP and Guest IP empty)
7. Guest is your docker container and Host is your machine

You should now be able to browse to your container via localhost:8080 and your-internal-ip:8080.

Answer 2

TL;DR Check the network mode of your VirtualBox host - it should be bridged if you want the virtual machine (and the Docker container it's hosting) accessible on your local network.

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It sounds like your confusion lies in which host to connect to in order to access your application via HTTP. You haven't really spelled out what your configuration is - I'm going to make some guesses, based on the fact that you've got "Windows" and "VirtualBox" in your tags.

I'm guessing that you have Docker running on some flavour of Linux running in VirtualBox on a Windows host. I'm going to label the IP addresses as follows:

D = the IP address of the Docker container

L = the IP address of the Linux host running in VirtualBox

W = the IP address of the Windows host

When you run your Go application on your Windows host, you can connect to it with http://W:8080/ from anywhere on your local network. This works because the Go application binds the port 8080 on the Windows machine and anybody who tries to access port 8080 at the IP address W will get connected.

And here's where it becomes more complicated:

VirtualBox, when it sets up a virtual machine (VM), can configure the network in one of several different modes. I don't remember what all the different options are, but the one you want is bridged. In this mode, VirtualBox connects the virtual machine to your local network as if it were a stand-alone machine on the network, just like any other machine that was plugged in to your network. In bridged mode, the virtual machine appears on your network like any other machine. Other modes set things up differently and the machine will not be visible on your network.

So, assuming you set up networking correctly for the Linux host (bridged), the Linux host will have an IP address on your local network (something like 192.168.0.x) and you will be able to access your Docker container at http://L:8080/.

If the Linux host is set to some mode other than bridged, you might be able to access from the Windows host, but this is going to depend on exactly what mode it's in.

EDIT - based on the comments below, it sounds very much like the situation I've described above is correct.

Let's back up a little: here's how Docker works on my computer (Ubuntu Linux).

Imagine I run the same command you have: docker run -p 8080:8080 dockertest. What this does is start a new container based on the dockertest image and forward (connect) port 8080 on the Linux host (my PC) to port 8080 on the container. Docker sets up it's own internal networking (with its own set of IP addresses) to allow the Docker daemon to communicate and to allow containers to communicate with one another. So basically what you're doing with that -p 8080:8080 is connecting Docker's internal networking with the "external" network - ie. the host's network adapter - on a particular port.

With me so far? OK, now let's take a step back and look at your system. Your machine is running Windows - Docker does not (currently) run on Windows, so the tool you're using has set up a Linux host in a VirtualBox virtual machine. When you do the docker run in your environment, exactly the same thing is happening - port 8080 on the Linux host is connected to port 8080 on the container. The big difference here is that your Windows host is not the Linux host on which the container is running, so there's another layer here and it's communication across this layer where you are running into problems.

What you need is one of two things:

1. to connect port 8080 on the VirtualBox VM to port 8080 on the Windows host, just like you connect the Docker container to the host port.

2. to connect the VirtualBox VM directly to your local network with the bridged network mode I described above.

If you go for the first option, you will be able to access the container at http://W:8080 where W is the IP address or hostname of the Windows host. If you opt for the second, you will be able to access the container at http://L:8080 where L is the IP address or hostname of the Linux VM.

So that's all the higher-level explanation - now you need to figure out how to change the configuration of the VirtualBox VM. And here's where I can't really help you - I don't know what tool you're using to do all this on your Windows machine and I'm not at all familiar with using Docker on Windows.

If you can get to the VirtualBox configuration window, you can make the changes described below. There is also a command line client that will modify VMs, but I'm not familiar with that.

For bridged mode (and this really is the simplest choice), shut down your VM, click the "Settings" button at the top, and change the network mode to bridged, then restart the VM and you're good to go. The VM should pick up an IP address on your local network via DHCP and should be visible to other computers on the network at that IP address.