Evading firewalls

Many firewalls and routers block or drop the ICMP (echo request and echo reply) ping. This is meant to obscure the presence of the hosts behind the firewall and protect against a possible DoS using the ping packet. When you use Nmap to scan a system or network, by default, it sends out a ping to see if the host is up. If it gets a response, it then sends the specified packets to scan the system. If the ping is blocked or dropped, Nmap gives up and says, host is down. To get around firewalls and routers that block or drop the ping, we need to suppress Nmap's default behavior of sending out that initial ping and get past the firewall that is blocking us. We can do this by using the -P0 switch. Type nmap -sS -P0 192.168.10.70: