A DDoS attack is broadly similar to a DoS attack, designed to hinder or stop the normal functioning of a web site, server or other network resource. A DDoS attack differs only in the fact that the attack is conducted using multiple machines. The hacker or virus writer typically use one compromised machine as the ‘master’ and co-ordinates the attack across other, so-called ‘zombie’, machines. Both master and zombie machines are typically compromised by exploiting a vulnerability in an application on the machine to install a Trojan or other piece of malicious code.