Setting up an OpenSSH bastion host for an entire DNS domain

Overview

Remote administration via ssh is a standard practice for many unix based servers or networking devices. Protecting this critical infrastructure by provisioning internal devices on RFC1918 IP space and/or limiting inbound connections to specific bastion or jump-hosts is a common method to limit external exposure.

Bastion hosts are security hardened devices who’s sole purpose is to act as a gateway into the production environment. These servers should be stripped down to essential services and patched on a regular basis. The server typically sits between the untrusted and trusted network, as shown in the diagram below:

SSH proxy flow

To make the bastion host configuration easier, I prefer to provision all systems on an internal DNS domain name. OpenSSH is configured through the Host and ProxyCommand options to key off the wildcard domain, and force all devices through the bastion host.

Details

Configure OpenSSH to proxy any device in the domain *.internal.example.com through bastion.example.com by adding the following lines to the ~/.ssh/config for individual users or globally by editing /etc/ssh/ssh_config:

[email protected]:~ $ cat .ssh/config
Host *.internal.example.com
        ProxyCommand ssh bastion.example.com -W %h:%p

Any ssh connection made to *.internal.example.com will be automatically proxied through the bastion host:

[email protected]:~ $ ssh router.internal.example.com
***********************************************************************
*        WARNING - Unauthorized access to or use of this system is    *
*                         strictly prohibited.                        *
***********************************************************************

[email protected]'s password:
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