What is the way to decide how a packet will move from an untrusted outside host to a protected inside that is behind a firewall, which permits the hacker to determine which ports are open and if the p

Firewalking crafts packets with an IP TTL value set to expire exactly one hop beyond the firewall gateway, causing the firewall to forward permitted packets while silently dropping or returning ICMP errors for blocked ones. This allows an attacker to systematically map which ports and protocols can traverse the packet filter without having direct access to hosts behind it.

Session hijacking targets an already-established authenticated session between two hosts and does not probe firewall filtering rules.

Network sniffing passively captures traffic passing a network interface and does not involve sending crafted packets to test firewall access controls.

A man-in-the-middle attack positions the attacker between two communicating parties to intercept or modify traffic, not to enumerate firewall port permissions.