Quick comment to give you a place to start searching; This is called a stateful firewall. Any modern firewall is stateful and tracks connections in a table. When it receives external traffic (packets) associated with a known established TCP connection, it allows that traffic through.

Quick comment to give you a place to start searching; This is called a stateful firewall. Any modern firewall is stateful and tracks connections in a table. When it receives external traffic (packets) associated with a known established TCP connection, it allows that traffic through.