docker-autossh

Highly customizable AutoSSH docker container

Overview

jnovack/autossh is a small lightweight (8.07MB) image that attempts to
provide a secure way to establish an SSH Tunnel without including your keys in
the image itself or linking to the host.

There's thousands of autossh docker containers, why use this one? I hope you
find it easier to use. It's smaller, more customizable, an automated build, so
it's easy to use, and I hope you learn something!

Description

autossh is a program to start a copy of ssh and monitor it, restarting it as
necessary should it die or stop passing traffic.

Before we begin, I want to define some terms.

• local - THIS docker container.

• target - The endpoint and ultimate destination of the tunnel.

• remote - The 'middle-man', or proxy server you are tunnelling through to
  get to your target.

• source - The initial endpoint you are starting from that does not have
  access to the target endpoint, but does have access to the remote
  endpoint.

The local machine is USUALLY the same as the target but since we are using
Docker, we have to abstract out the local container from the target
endpoint where we want autossh to land. Normally, this is where
autossh is usually run from.

Typically, the target can be on a Home LAN segment without a publicly
addressible IP address; whereas the remote machine has an address that is
reachable by both target and source. And source can only reach remote.

target ---> |firewall| >--- remote ---< |firewall| <--- source
10.1.1.101             [public.ip.addr]          192.168.1.101

The target (running autossh) connects up to the remote server and
keeps a tunnel alive so that source can proxy through remote and reach
resources on target. Think of it as "long distance port-forwarding".

Setup

To start, you will need to generate an SSH key on the Docker host. This will
ensure the key for the container is separate from your normal user key in the
event there is ever a need to revoke one or the other.

$ ssh-keygen -t rsa -b 4096 -C "autossh"
Generating public/private rsa key pair.
Enter file in which to save the key (/home/jnovack/.ssh/id_rsa):
Enter passphrase (empty for no passphrase):
Enter same passphrase again:
Your identification has been saved in /home/jnovack/.ssh/id_rsa.
Your public key has been saved in /home/jnovack/.ssh/id_rsa.pub.
The key fingerprint is:
00:11:22:33:44:55:66:77:88:99:aa:bb:cc:dd:ee:ff jnovack@yourmom
The key's randomart image is:
+-----[ RSA 4096]-----+
|     _.-'''''-._     |
|   .'  _     _  '.   |
|  /   (_)   (_)   \  |
| |  ,           ,  | |
| |  \`.       .`/  | |
|  \  '.`'""'"`.'  /  |
|   '.  `'---'`  .'   |
|     '-._____.-'     |
+---------------------+

Command-line Options

What would a docker container be without customization? I have an extensive
list of environment variables that can be set.

Environment Variables

SSH_HOSTUSER

Specify the usename on the remote endpoint. (Default: root)

SSH_HOSTNAME

Specify the address (ip preferred) of the remote endpoint. (Default:
localhost)

SSH_TUNNEL_REMOTE

Specify the port number on the remote endpoint which will serve as the
tunnel entrance. (Default: random > 32768) If you do not want a new port
every time you restart jnovack/autossh you may wish to explicitly set
this.

SSH_TUNNEL_HOST

Specify the address (ip preferred) of the target.

SSH_TUNNEL_LOCAL

Specify the port number on the target endpoint which will serve as the
tunnel exit, or destination service. Typically this is ssh (port: 22),
however, you can tunnel other services such as redis (port: 6379),
elasticsearch (port: 9200) or good old http (port: 80) and https (port: 443).

Docker Volumes

/id_rsa

Mount the key you generated within the Setup step.

-v /path/to/id_rsa:/id_rsa

Examples

docker-compose.yml

In the top example ssh-to-docker-host, a tunnel will be made from the docker
container (aptly named autossh-ssh-to-docker-host) to the host running the
docker container.

To use, ssh to fake internet address 203.0.113.10:2222 and you will be
forwarded to 172.17.0.1:22 (the host running the docker container).

In the lower example, ssh-to-lan-endpoint, a tunnel will be made to a host
on the private LAN of the docker host. sshing to fake internet address
203.0.113.10:22222 will traverse through the docker container through the
docker host, and onto the private lan where the connection will terminate
192.168.123.45:22.

version: '2.1'

services:
  ssh-to-docker-host:
    image: jnovack/autossh
    container_name: autossh-ssh-to-docker-host
    environment:
      - SSH_HOSTUSER=sshuser
      - SSH_HOSTNAME=203.0.113.10
      - SSH_TUNNEL_REMOTE=2222
      - SSH_TUNNEL_HOST=172.17.0.1
      - SSH_TUNNEL_LOCAL=22
    restart: always
    volumes:
     - /etc/autossh/id_rsa:/id_rsa
    dns:
     - 8.8.8.8
     - 4.2.2.4

  ssh-to-lan-endpoint:
    image: jnovack/autossh
    container_name: autossh-ssh-to-lan-endpoint
    environment:
      - SSH_HOSTUSER=sshuser
      - SSH_HOSTNAME=203.0.113.10
      - SSH_TUNNEL_REMOTE=22222
      - SSH_TUNNEL_HOST=198.168.123.45
      - SSH_TUNNEL_LOCAL=22
    restart: always
    volumes:
      - /etc/autossh/id_rsa:/id_rsa
    dns:
      - 8.8.8.8
      - 4.2.2.4