How can I forward localhost port on my container to localhost on my host?

Question

I have a daemon on my host running on some port (i.e. 8008) and my code normally interacts with the daemon by contacting localhost:8008 for instance.

I've now containerized my code but not yet the daemon. How can I forward the localhost:8008 on my container to localhost:8008 on the host running the container (and therefore the daemon as well).

The following is netstat -tlnp on my host. I'd like the container to forward localhost:2009 to localhost:2009 on the host

Proto Recv-Q Send-Q Local Address           Foreign Address         State       PID/Program name            
tcp        0      0 127.0.0.1:2009          0.0.0.0:*               LISTEN      22547/ssh       
tcp6       0      0 :::22                   :::*                    LISTEN      -               
tcp6       0      0 ::1:2009                :::*                    LISTEN      22547/ssh            

Answer 1

If you still want to access your app locally try this: First, find your docker ip, you can find it using ifconfig command.

use docker0 ip address and add port ahead of it, example:
172.17.0.1:your-port

Done...

Answer 2

If you're doing this on your local machine, you can simple specify the network type as host when starting your container (--network host), which will make your host machine share network with your docker container.

eg:

Start your container:

docker run -it --rm --network host <container>

On your host machine, Run:

python3 -m http.server 8822

Now from your container run:

curl 127.0.0.1:8822

If all went well you'll see traffic on your host terminal.

Serving HTTP on 0.0.0.0 port 8822 (http://0.0.0.0:8822/) ...
127.0.0.1 - - [24/Jan/2023 22:37:01] "GET / HTTP/1.1" 200 -

Answer 3

TL;DR: You can use the special hostname host.docker.internal instead of localhost anywhere inside the container that you want to access localhost on the host. Note that:

• macOS and Windows versions of Docker Desktop have this feature enabled by default.
• Linux hosts (using Docker v 20.10 and above - since December 14th 2020) require you to add --add-host=host.docker.internal:host-gateway to your Docker command to enable the feature.
• Docker Compose on Linux requires you to add the following lines to the container definition:

extra_hosts:
- "host.docker.internal:host-gateway"

Full answer: Is the host running MacOS or Windows? Buried in the documentation for Docker Desktop is the fact that there is no docker0 bridge on MacOS and there is no docker0 bridge on Windows. Apparently that's the cause of this. In both cases the workaround (given right after, in a subsection titled "Use cases and workarounds") is to use the special hostname host.docker.internal in placed of localhost anywhere inside the container that you want to access localhost on the host.

If the host is Linux, there are some Linux-only techniques for achieving this. However, host.docker.internal is also useable with a Linux host, but it has to be enabled first. See the Linux part of the TL;DR, above, for instructions.

By this method, in OP's case host.docker.internal:8008 should be used instead of localhost:8008. Note that this is a code or configuration change of the application code running inside the container. There is no need to mention the port in the container configuration. Do not try to use -p or --expose in the docker run commandline. Not only is it not necessary, but your container will fail to start if the host application you want the container to connect to is already listening on that port.

Answer 4

After checked the answers and did some investigation, I believe there are 2 ways of doing that and these 2 only work in Linux environment.

The first is in this post How to access host port from docker container

The second should be set your --network=host when you docker run or docker container create. In this case, your docker will use the same network interface you use in Mac.

However, both ways above cannot be used in Mac, so I think it is not possible to forward from the container to host in Mac environment. Correct me if I am wrong.

Answer 5

I'm not sure if you can do that just with docker's settings. If my under standing is correct, expose port is not what you looking for. Instead, establish ssh port forwarding from container to host mightbe the answer.

Answer 6

So the way you need to think about this is that Docker containers have their own network stack (unless you explicitly tell it to share the host's stack with --net=host). This means ports need to be exposed both inside the docker container and also on the outside (documentation), when linked with host ports. The ports exposed on the container need to be bound to the host ports explicitly (with -p xxxx:yyyy in your docker run command) or implicitly (using EXPOSE in your Dockerfile and using -P on the command line), like it says here. If your Dockerfile does not contain EXPOSE 8008, or you do not specify --expose 8008 in your docker run command, your container can't talk to the outside world, even if you then use -p 8008:8008 in your docker run command!

So to get tcp/8008 on the host linked with tcp/8008 on the container, you need EXPOSE 8008 inside your Dockerfile (and then docker build your container) OR --expose 8008 in your docker run command. In addition, you need to either use -P to implicitly or -p 8008:8008 to explicitly link that exposed container port to the host port. An example docker run command to do this might look like:

docker run -it --expose 8008 -p 8008:8008 myContainer

It's handy to remember that in the -p 8008:8008 command line option, the order for this operation is -p HOST_PORT:CONTAINER_PORT. Also, don't forget that you won't be able to SSH into your container from another machine on the internet unless you also have this port unblocked in iptables on the host. I always end up forgetting about that and waste half an hour before I remember I forgot to iptables -A INPUT ... for that specific tcp port on the host machine. But you should be able to SSH from your host into the container without the iptables rule, since it uses loopback for local connections. Good luck!

Answer 7

docker run -d --name <NAME OF YOUR CONTAINER> -p 8008:8008 <YOUR IMAGE>

That will publish port 8008 in your container to 8008 on your host.