Using a bastion host or jump server allows for easier security management as hosts that are not public-facing do not need to be secured as much as the public-facing host. If you need to SSH to the hosts within the private network, you will need to first SSH to the bastion host or jump server and SSH to the other host from there.

Having to connect through multiple hosts adds complexity on the client side. Some ways to help are setting up a VPN or proxy on the gateway and configuring the client connection accordingly.

If the gateway host is an SSH server, you can use the built-in ProxyJump option to automate logging in to reach the end destination through multiple hosts. This option, if paired with public key authentication method, could be a lifesaver for system administrators.

1. Launch terminal.
2. Manually log in to each hosts from one another to make sure it's reachable.

   user@host:~$ ssh user@gateway user@gateway's password:  user@gateway:~$ ssh user@internal user@internal's password:  user@internal:~$ exit logout Connection to internal closed. user@gateway:~$ exit logout Connection to gateway closed. user@host:~$ 

3. Connect to internal host using -J option.

   user@host:~$ ssh -J user@gateway user@internal user@gateway's password:  user@internal's password:  user@internal:~$ exit logout Connection to internal closed. user@host:~$

   Use comma-separated value for jump hosts if multiple jump connection is required

   $ ssh -J user@gateway,user@gateway2 user@internal

   Add : to hostname / IP address to specify port if non-standard is used for SSH server.

   $ ssh -J user@gateway:2222 user@internal

   -J destination         Connect to the target host by first making a ssh connection to         the jump host described by destination and then establishing a         TCP forwarding to the ultimate destination from there.  Multiple         jump hops may be specified separated by comma characters.  This         is a shortcut to specify a ProxyJump configuration directive.         Note that configuration directives supplied on the command-line         generally apply to the destination host and not any specified         jump hosts.  Use ~/.ssh/config to specify configuration for jump         hosts.

   Configure AllowAgentForwarding and AllowTcpForwarding to yes on the jump server if you're using SSH agent or public key authentication.

4. Open SSH user config file using your preferred text editor.

   $ vi ~/.ssh/config

5. Add host and login information of the gateway server.

   Host gateway         hostname 192.168.111.27         user user

6. Add host and login information of the internal server along with ProxyJump configuration

   host internal         hostname 192.168.111.38         user user         proxyjump gateway

7. Directly SSH to internal server without using ProxyJump configuration.

   $ ssh internal [email protected]'s password:  [email protected]'s password:  user@internal:~$