I found a scary thing in my special Linux cd,
although I know that CD is a very powerful cd,
I don't know it's that scary...

it not only has ethereal, tcpdump,
it also has "ettercap"!
a tool which can hack even in switch env, ssh, OS fingerprint, disconnect connections, type charactors for others!

here's a very very good introduction of this kind of tech and tools. and ways to prevent!