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

Session hijacking is an attack that takes over an already-established authenticated session between two hosts and has nothing to do with probing firewall filtering rules.

Firewalking crafts packets with a TTL set to expire exactly one hop beyond the target firewall gateway. If the firewall's ruleset permits the packet on a given port, it forwards it and a TTL-exceeded ICMP message returns from the next-hop router, confirming the port is open through the filter. If the firewall blocks the port, the packet is dropped silently, producing no response, and this difference in behavior allows an attacker to enumerate permitted ports without ever establishing a connection to the destination host.

A man-in-the-middle attack positions an adversary between two communicating parties to intercept or modify traffic, and does not involve actively probing which ports a firewall allows.

Network sniffing passively captures traffic already traversing a network segment and cannot generate probes to determine which ports are permitted through a firewall.