In this chapter it's stated that Firewalk, a port scanning tool, will succeed if the firewall is a packet filter or a stateful packet filter, but it will fail if the firewall is an application proxy....

In this chapter it's stated that Firewalk, a port scanning tool, will succeed if the firewall is a packet filter or a stateful packet filter, but it will fail if the firewall is an application proxy.

a. Why is this the case? That is, why does Firewalk succeed when the firewall is a packet filter or stateful packet filter, but fail when the firewall is an application proxy?

b. Can Firewalk be modified to work against an application proxy?