Warning: Malicious use of SYN-floods are punishable by law.
This post shows howto to establish a synflood attack on an arbitrary remote host. The attack is performed using hping, which is free packet generator and analyzer for the TCP/IP protocol. Hping is one of the de facto tools for security auditing and testing of firewalls and networks. A syn-flood attack is basically a DOS-attack on a bug in TCP - some will argue that TCP is defective by design ;)
The actual attack is initialized by this command:
hping -i u1 -S -p 80 dst-host-or-ip
In most cases DoS attacks like this one renders a webserver totally unable to serve any requests from users.
To get hping installed on a Debian or Ubuntu-based system, type this to install:
aptitude install hping2