Which firewall chain should be used to filter SSH access to the router itself?

Using the Input chain is essential for filtering SSH access to the router itself because this chain is specifically designed to handle packets that are destined for the router's own interfaces. When a user attempts to connect to the router via SSH, the packets must pass through the Input chain before they can reach the SSH service running on the router.

By placing rules in the Input chain, you can effectively control which sources are allowed or denied access to the router for SSH connections. This allows for fine-tuning security measures directly related to managing administrative access.

In contrast, the other chains serve different purposes: the Output chain is used for packets originating from the router itself, the Prerouting chain is for packets that are entering the router but are not destined for it, and the Forward chain deals with packets that are routed through the router but not intended for local processing. Therefore, they are not applicable for controlling access to the router's own services like SSH.