docker-portmap-server

This is a 🐳 Docker image containing an OpenSSH server that can be used for remote port forwarding only.

It is meant to act as a server for the dmotte/portmap-client image, but should work with any OpenSSH client.

If you want a rootless version of this image, check out dmotte/docker-portmap-server-rootless.

📦 This image is also on Docker Hub as dmotte/portmap-server and runs on several architectures (e.g. amd64, arm64, ...). To see the full list of supported platforms, please refer to the .github/workflows/main.yml file. If you need an architecture which is currently unsupported, feel free to open an issue.

Usage

Note: this Docker image uses unprivileged users to perform the remote port forwarding stuff. As a result, it will only be possible to use port numbers > 1024. However this is not a problem at all, since you can still leverage the Docker port exposure feature to bind to any port you want on your host (e.g. -p "80:8080").

The first thing you need are host keys for the OpenSSH server. You can generate them with the following commands:

mkdir -p hostkeys/etc/ssh
ssh-keygen -Af hostkeys
mv -thostkeys hostkeys/etc/ssh/*
rm -r hostkeys/etc

This creates a folder named 📁 hostkeys which has to be mounted to /ssh-host-keys inside the container. If you omit this step, the startup script will generate the host keys internally and try to copy them to /ssh-host-keys.

Then you'll have to generate an SSH key pair for each client. For example:

ssh-keygen -t ed25519 -C myclient -N '' -f myclientkey

This will create two files:

• 📄 myclientkey: the client's private SSH key, which should be given to the client
• 📄 myclientkey.pub: the client's public SSH key, which is used by the OpenSSH server running inside the container to authenticate the client

This image supports multiple users and permissions on which ports can be bound by the users. For each user you have to:

• Specify the username and permissions in the container command (mandatory). Example for two users: alice:8001,8002 bob:any
• Mount the SSH public client key(s) to /ssh-client-keys/myuser/myclientkey.pub. If you don't do this, a keypair will be generated and put into the /ssh-client-keys/myuser directory

Note: you can also specify key options in the public key file, e.g. permitlisten="8080" ssh-ed25519 AAAAC3Nza...

When you have everything ready, you can start the server with:

docker run -it --rm \
    -v "$PWD/hostkeys:/ssh-host-keys" \
    -v "$PWD/myclientkey.pub:/ssh-client-keys/myuser/myclientkey.pub:ro" \
    -p80:8080 -p2222:22 \
    dmotte/portmap-server myuser:8080

To test the server on-the-fly, you can connect to it and setup a remote port forwarding tunnel, by running the following OpenSSH command in another shell:

ssh -i myclientkey myuser@localhost -p2222 -NvR8080:google.it:80

This will serve http://google.it/ on port 8080 of the server container, which is exposed to port 80 of your host machine due to the -p 80:8080 docker run flag specified before. Note that, for this to work, the myclientkey must have 600 permissions. If this isn't the case, you can achieve it with:

chmod 600 myclientkey

You can now test that your remote port forwarding tunnel is working with cURL:

curl http://localhost/

For a more complex example, refer to the docker-compose.yml file.

Environment variables

List of supported environment variables:

Variable	Required	Description
KEEPALIVE_INTERVAL	No (default: 30)	Value for the ClientAliveInterval option of the OpenSSH server

Development

If you want to contribute to this project, you can use the following one-liner to rebuild the image and bring up the Docker-Compose stack every time you make a change to the code:

docker-compose down && docker-compose up --build