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Tool for tunneling a port forward into remote Docker containers over SSH
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Latest commit 5d5bce7 Nov 27, 2019


A remote port forwarding tool for accessing services inside Docker containers. Those private admin interfaces, JMX ports, or Erlang/Elixir distribution ports not open to the outside world? No problem.

Shipspotter automates the workflow of connecting to a remote Docker host, finding the container you need to connect to, and then forwarding the ports to the appropriate container on the Docker bridge network.

Example: Nginx with no exposed ports, inside a remote Docker container. Shipspotter demo


Basic Usage

./shipspotter -h docker1 -n elixir -p 9001:9001 -p 7946:7946 -a

This will open an SSH tunnel to the host docker1, leveraging either an SSH agent, or prompting you for the passphrase to your SSHKey. Over the tunnel, shipspotter will connect to Docker on the default Unix socket /var/run/docker.sock and look for a container with an image name that contains elixir. If more than one is present, it will list all of them and then connect to the last one it finds. If you'd rather connect to a different one you can supply the --container-id option and specify one of the others it listed for you. You may specify any number of ports to forward.

Note: only TCP forwarding is supported.

The final -a argument tells shipspotter to bind on the local address This is useful for remotely connecting Erlang/Elixir containers using EPMD and distribution. The EPMD 4369 port is also forwarded by default. If you don't want it to be you may specify --no-forward-epmd.

usage: shipspotter --hostname=HOSTNAME [<flags>]

      --help                   Show context-sensitive help (also try --help-long and --help-man).
  -h, --hostname=HOSTNAME      The remote hostname to connect to
  -p, --port=8080:80 ...       The local:remote port to connect to. e.g. 8080:80
  -a, --local-address=""
                               The local IP address to listen on
  -l, --username="youruser"    The ssh username on the remote host
  -s, --docker-sock="unix:///var/run/docker.sock"
                               The Docker socket address on the remote host
  -n, --image-name=IMAGE-NAME  The Docker image to match on for this application
  -c, --container-id=CONTAINER-ID
                               The Docker container ID to match for this application
  -i, --ssh-key="/Users/youruser/.ssh/id_rsa"
                               Path to the ssh private key to use
  -P, --ssh-port="22"          Port to connect to ssh on the remote host
  -e, --forward-epmd           Shall we also forward the EPMD port?
  -d, --debug                  Turn on debug logging

Shipspotter attempts sane default values for most of the options. You should only override them if you are sure you need to.

Sample Output

$ ./shipspotter -h docker1 -n elixir -p 9001:9001 -d -a

     _     _                       _   _
    | |   (_)                     | | | |
 ___| |__  _ _ __  ___ _ __   ___ | |_| |_ ___ _ __
/ __| '_ \| | '_ \/ __| '_ \ / _ \| __| __/ _ \ '__|
\__ \ | | | | |_) \__ \ |_) | (_) | |_| ||  __/ |
|___/_| |_|_| .__/|___/ .__/ \___/ \__|\__\___|_|
            | |       | |
            |_|       |_|

DEBU[0000] Turning on debug logging
INFO[0000] Found matching container:
INFO[0000]  - id:    9b62e46debe1
INFO[0000]  - image: elixir
INFO[0000]  - name:  /thirsty_keller
INFO[0000]  - up:    22h34m30.789978s
INFO[0000] Using container: 9b62e46debe1
INFO[0000] Container IP address:
INFO[0000] Forwarding ports:
INFO[0000]  - 9001:9001
INFO[0000] Forwarding EPMD on 4369


  1. You must have read access to the Docker Unix socket on the remote system with the user you are logging in with. This can usually be accomplished by adding the user to the docker group on most distros. If you do not you may also connect over TCP on the remote host, by specifying the --docker-sock option. Note that this requires Docker to be listening on the non-SSL TCP port (usually on only). Specify TCP sockets prefixed with tcp:// according to the manner that Docker would normally expect.

  2. You must be using ssh key authentication, either with an agent or without. It would not be hard to add password auth.


For some use cases it's helpful to add an aliased address locally. This is true when, for example, you need to connect an Erlang VM up to a remote container to do distribution or run remote Observer. There is an script provided to do that, with support for macOS and Linux.

Elixir/Erlang Remote Observer

One good use case for Shipspotter is connecting to a remote Erlang VM and running a remote observer process there. To do this you must:

  • Set the remote node name to <your process>@ beforehand
  • Run the script from this distribution
  • Start shipspotter:
    shipspotter -h docker1 -n <your image> -p 9001:9001 -d -a
  • Start a local iex session like:
    iex --name debug@ --cookie <your cookie>
  • Then in the iex session:
    iex(1)> Node.connect :"<your process>@"
    If you don't get true back, look for error output from shipspotter.
  • Start Observer :observer.start. The select the remote node from the menu.

Elixir Remote Shell

Another good use case for Shipspotter is connecting to a remote Elixir shell so that you can run functions directly.

  • Run sudo ./ in the directory where you downloaded it.
  • Start shipspotter:
    shipspotter -h <hostname> -n <service> -p 9001:9001 -d -a
    • Example values for service are hermes or digits. NOTE if you have a different user ID on your local machine than you do on our infrastructure, you will need to also supply --username or -l and provide the correct user ID.
  • Inspect the output to make sure that shipspotter has connected to the correct container. A host may have more than one container. If not, use the -c option to specify the exact container ID and remove the -n option.
  • Start a local iex session like:
    iex --name debug@ --cookie <your cookie> --remsh "<service>@"
    • In place of <service> substitute the name of the service. E.g. digits
  • Then in the iex session:
    iex(<service>@> run the command you want


Contributions are more than welcome. Bug reports with specific reproduction steps are great. If you have a code contribution you'd like to make, open a pull request with suggested code.

Pull requests should:

  • Clearly state their intent in the title
  • Have a description that explains the need for the changes
  • Not break the public API

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