Nmap: Network Exploration and Security Auditing Cookbook - Second Edition

Firewalls and traffic filtering

A lot of systems are protected by some kind of traffic filtering, so it is important to always try different ping scanning techniques. In the following example, we will scan a host online that gets marked as offline, but in fact, was just behind some traffic filtering system that did not allow TCP ACK or ICMP requests:

# nmap -sn 0xdeadbeefcafe.com    Note: Host seems down. If it is really up, but blocking our ping      probes, try -Pn    Nmap done: 1 IP address (0 hosts up) scanned in 4.68 seconds    # nmap -sn -PS 0xdeadbeefcafe.com    Nmap scan report for 0xdeadbeefcafe.com (52.20.139.72)    Host is up (0.062s latency).    rDNS record for 52.20.139.72: ec2-52-20-139-72.compute-      1.amazonaws.com  Nmap done: 1 IP address ...