How can you be sure your network is secure? Before you can patch vulnerabilities you need to discover them. You need to think like a cracker might. You need to hack your own system. This is known as “penetration testing” – a more palatable term to corporations – and the rich tool set of Linux makes it a superb platform for doing this.
The starting point for a penetration test is research: probing the target system to discover anything that can be useful. This includes the type of operating system and particularly what services it exposes through its firewall, and what server applications it is running – both in terms of protocol and software implementation.
Ping
Just as ping is surely the first point of call when troubleshooting a network, so too its underlying protocol – ICMP – is where research must start, to determine the host in question is up and on the network. According to Internet RFC 1122 every TCP/IP host must implement the ICMP echo request and respond to it. Thus, try using ping to elicit a response from your target.
http://www.linuxlinks.com/portal/news/article.php?story=2007081313131243