A bastion host is a special purpose computer on a network specifically designed and configured to withstand attacks. The computer generallyhosts a single application, for example a proxy server, and all otherservices are removed or limited to reduce the threat to the computer. It is hardened in this manner primarily due to its location and purpose,which is either on the outside of a firewall or in a demilitarized zone (DMZ) and usually involves access from untrusted networks orcomputers.

Bastion is an isolated Docker image that can work as a link between Public and Private network. It can be also useful for reverse SSH tunneling for a host behind a NAT. This image based on Alpine Linux last version.

• PUBKEY_AUTHENTICATION [true | false] - Specifies whether public key authentication is allowed. The default is true. Note that this option applies to protocol version 2 only.

• AUTHORIZED_KEYS [/relative/or/not/path/to/file] - Specifies the file that contains the public keys that can be used for user authentication. AUTHORIZED_KEYS may contain tokens of the form %T which are substituted during connection setup. The following tokens are defined: %% is replaced by a literal %, %h is replaced by the home directory of the user being authenticated, and %u is replaced by the username of that user. After expansion, AUTHORIZED_KEYS is taken to be an absolute path or one relative to the user's home directory. The default file is authorized_keys and the default home directory is /var/lib/bastion and should be present by Docker volume mount by -v $PWD/authorized_keys:/var/lib/bastion/authorized_keys:ro.

• TRUSTED_USER_CA_KEYS [/full/path/to/file] - Specifies a file containing public keys of certificate authorities that are trusted to sign user certificates for authentication, or none to not use one. Keys are listed one per line; empty lines and comments starting with # are allowed. If a certificate is presented for authentication and has its signing CA key listed in this file, then it may be used for authentication for any user listed in the certificate's principals list. Note that certificates that lack a list of principals will not be permitted for authentication using TRUSTED_USER_CA_KEYS. Directive AuthorizedPrincipalsFile hardcoded to /etc/ssh/auth_principals/%u and in time of build and generated one principals file for presented user - /etc/ssh/auth_principals/bastion with the one row bastion, and this principal should be listed in the certificate's principals list.

• GATEWAY_PORTS [true | false] - Specifies whether remote hosts are allowed to connect to ports forwarded for the client. By default, sshd binds remote port forwardings to the loopback address. This prevents other remote hosts from connecting to forwarded ports. GATEWAY_PORTS can be used to specify that sshd should allow remote port forwardings to bind to non-loopback addresses, thus allowing other hosts to connect. The argument may be false to force remote port forwardings to be available to the local host only, true to force remote port forwardings to bind to the wildcard address. The default is false.

• PERMIT_TUNNEL [true | false] - Specifies whether tun device forwarding is allowed. The argument must be true or false. Specifying true permits both point-to-point (layer 3) and ethernet (layer 2). The default is false.

• X11_FORWARDING [true | false] - Specifies whether X11 forwarding is permitted. The argument must be true or false. The default is false.

• TCP_FORWARDING [true | false] - Specifies whether TCP forwarding is permitted. The default is true. Note that disabling TCP forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders.

• AGENT_FORWARDING [true | false] - Specifies whether ssh-agent forwarding is permitted. The default is true. Note that disabling agent forwarding does not improve security unless users are also denied shell access, as they can always install their own forwarders.

• LISTEN_ADDRESS [0.0.0.0] - Specifies the local addresses should listen on. By default it 0.0.0.0. Useful when Docker container runs in Host mode

• LISTEN_PORT [22] - Specifies the port number that listens on. The default is 22. Useful when Docker container runs in Host mode

The container assumes your authorized_keys file with 644 permissions and mounted under /var/lib/bastion/authorized_keys.

Docker example:

$ docker volume create bastion
$ docker run -d \
    --name bastion \
    --hostname bastion \
    --restart unless-stopped \
    -v $PWD/authorized_keys:/var/lib/bastion/authorized_keys:ro \
    -v bastion:/usr/etc/ssh:rw \
    --add-host docker-host:172.17.0.1 \
    -p 22222:22/tcp \
    -e "PUBKEY_AUTHENTICATION=true" \
    -e "GATEWAY_PORTS=false" \
    -e "PERMIT_TUNNEL=false" \
    -e "X11_FORWARDING=false" \
    -e "TCP_FORWARDING=true" \
    -e "AGENT_FORWARDING=true" \
    binlab/bastion

Docker-compose example:

version: "3.6"
services:
  bastion:
    image: binlab/bastion
    container_name: bastion
    hostname: bastion
    restart: unless-stopped
    expose:
      - 22/tcp
    ports:
      - 22222:22/tcp
    environment:
      PUBKEY_AUTHENTICATION: "true"
      GATEWAY_PORTS: "false"
      PERMIT_TUNNEL: "false"
      X11_FORWARDING: "false"
      TCP_FORWARDING: "true"
      AGENT_FORWARDING: "true"
    volumes:
      - $PWD/authorized_keys:/var/lib/bastion/authorized_keys:ro
      - bastion:/usr/etc/ssh:rw
    extra_hosts:
      - docker-host:172.17.0.1
    networks:
      - bastion

networks:
  bastion:
    driver: bridge
    
volumes:
  bastion:

* When you are run Bastion container first time it generates dsa, ecdsa, ed25519 and rsa key pair and saves them in permanent volume bastion, When you need to regenerate key pair, you should remove volume bastion.

• Add your user to group docker to have possibility run docker-compose and docker from your user without sudo. After you should re-login or open a new terminal window.

$ sudo usermod -aG docker <your_user>

• Create custom work dir e.g. docker, enter to it and clone repository

$ mkdir $HOME/docker 
$ cd $HOME/docker
$ git clone https://github.com/binlab/docker-bastion.git
$ cd docker-bastion

• Generate rsa pair (if you have one, skip this)

$ ssh-keygen -t rsa -b 4096 -C "[email protected]" -f $HOME/.ssh/id_rsa

• Add rsa public key to .bastion_keys file

$ cat $HOME/.ssh/id_rsa.pub > $PWD/.bastion_keys

• Run docker-compose.yml configuration - bastion & docker-ssh

$ docker-compose up

• And then you are can connect to it (in another terminal window)

$ ssh -i $HOME/.ssh/id_rsa -p 22222 [email protected]

• You should see like this:

user@localhost:~$ ssh -p 22222 [email protected]
The authenticity of host '[127.0.0.1]:22222 ([127.0.0.1]:22222)' can't be established.
ECDSA key fingerprint is 
SHA256:********************************************
ECDSA key fingerprint is MD5:**:**:**:**:**:**:**:**:**:**:**:**:**:**:**:**.
Are you sure you want to continue connecting (yes/no)? yes
Warning: Permanently added '[127.0.0.1]:22222' (ECDSA) to the list of known hosts.
Welcome to Alpine!

The Alpine Wiki contains a large amount of how-to guides and general
information about administrating Alpine systems.
See <http://wiki.alpinelinux.org>.

You can setup the system with the command: setup-alpine

You may change this message by editing /etc/motd.

bastion:~$ 

To achieve this you should add your private key to SSH agent and turn on ForwardAgent in ~/.ssh/config or from a command line via flag -A

-A option enables forwarding of the authentication agent connection.

It means that, it forwards your SSH auth schema to the remote host. > So you can use SSH over there as if you were on your local machine.

• Add private key to SSH agent

$ ssh-add $HOME/.ssh/id_rsa

• Test Bastion bridge in action

$ ssh -A -J [email protected]:22222 <your_user>@docker-host

$ ssh -A -J [email protected]:22222 bastion@docker-ssh