Question

Suppose you want your firewall to block all incoming Telnet connections but to allow outbound Telnet connections. One approach would be to block all inbound packets to the designated Telnet port (23). We might want to block inbound packets to other ports as well, but what inbound TCP connections must be permitted in order not to interfere with outbound Telnet

Answer 1

Another way to block all inbound packets is to use the firewall layer of the circuit.

Explanation:

The firewall circuit layer is a firewall containing a table with four fields, mainly such as source port, destination port, source IP address, and destination IP address.

They are containing mainly the addresses or the ports that need to be blocked. So we want to block all inbound packets, and also set the source port to 23 while the rest is set to zero, meaning that all inbound packets are blocked.