docker-autossh

Highly customizable AutoSSH docker container

Overview

jnovack/autossh is a small lightweight (~15MB) 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 are thousands of autossh docker containers, why use this one? I hope you find it easier to use. It is smaller, more customizable, an automated build, easy to use, and I hope you learn something. I tried to follow standards and established conventions where I could to make it easier to understand and copy and paste lines from this project to others to grow your knowledge!

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@github
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.

Mounts

/id_rsa

Mount the key you generated within the Setup step, or set SSH_KEY_FILE.

-v /path/to/id_rsa:/id_rsa

/known_hosts

Mount the known_hosts file if you want to enable STRICT_KEY_CHECKING, or set SSH_KNOWN_HOSTS.

-v /path/to/known_hosts:/known_hosts

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.

This option reverses if you set SSH_MODE (see below). To bind a local forward tunnel to all interfaces, use an asterisk then the port desigation (e.g. *:2222).

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).

SSH_STRICT_HOST_IP_CHECK

Set to false if you want the IP addresses of hosts to not be checked if the known_hosts file is provided. This can avoid issues for hosts with dynamic IP addresses, but removes some additional protection against DNS spoofing attacks. Host IP Checking is enabled by default.

SSH_KEY_FILE

In the event you wish to store the key in Docker Secrets, you may wish to set this to /run/secrets/*secret-name*

SSH_KNOWN_HOSTS

In the event you wish to store the known_hosts in Docker Secrets, you may wish to set this to /run/secrets/*secret-name*

SSH_MODE

Defines how the tunnel will be set up:

• -R is default, remote forward mode.
• -L means local forward mode.

SERVER_ALIVE_INTERVAL

Sets a timeout interval in seconds after which if no data has been received from the server, ssh(1) will send a message through the encrypted channel to request a response from the server.

• 0 turns the option off.
• 10 is default for this image.

Additional details are available from ssh_config(5)

SERVER_ALIVE_COUNT_MAX

Sets the threshold of alive messages after which the connection is terminated and reestablished.

• 3 is the default for this image.
• SERVER_ALIVE_INTERVAL=0 turns this variable ineffective.

Additional details are available from ssh_config(5)

Additional Environment variables

• autossh(1)
• ssh_config(5)

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: '3.7'

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
      - 1.1.1.1

  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

ARM Support

For running on an ARM processor, you may use the tag latest-arm32v7

docker pull jnovack/autossh:latest-arm32v7