Which firewall chain is used to filter ICMP packets from the router itself?

The input chain is specifically designed to handle packets that are destined for the router itself. When it comes to ICMP (Internet Control Message Protocol) packets, which are often used for diagnostic purposes (such as the ping command), they are typically sent to the router's own IP address. Therefore, to filter these ICMP packets that are being received by the router, the input chain is the appropriate choice.

When you configure rules in the input chain, you are controlling the traffic that is trying to access the router, which includes ICMP packets. This allows you to permit or deny these packets based on security policies you wish to enforce.

In contrast, the other chains serve different purposes. The output chain is for packets that the router itself is generating and sending out, the forward chain deals with packets that are passing through the router (not destined for the router but rather for another device on the network), and the postrouting chain is used for modifying packets just before they leave the router, rather than filtering incoming traffic directed at the router itself. Thus, the input chain is clearly the most suitable for managing ICMP packets aimed at the router.