1. An exploit whose purpose is to deny somebody the use of the service: namely to crash or hang a program or the entire system. Example: Some classes of DoS are: traffic flood Overwhelms the Internet connection. Because it is the Internet connection itself that is attacked, there isn't much the victim can do to stop the attack. A firewall might block the flood from going any further, but the Internet connection in front of the firewall is still overloaded. application floods or bombs Overwhelms a program with too many events. A firewall that allows the traffic cannot block these attacks. For example, a firewall configured to allow IRC cannot selectively block just the flood but allow all other traffic. Common services attacked this way are IRC, HTTP, and e-mail (SMTP). remote system crash/hang Historically, there have been a lot of ways of remotely crashing machines. These attack the TCP/IP stack within the system causing it to crash or hang. This affects all software running on the system. remote service crash Crashes just the application/service. This doesn't affect other software running on the system. Example: Some famous DoS attacks are: Ping of Death This exploit crashed most machines vintage 1995 by sending illegally fragmented packets at a victim. Even as late as 2000, some systems were vulnerable to variants of this DoS, such as the Jolt2 attack against Windows systems. SYN flood ping flood WinNuke Sending OOB/URG data across a TCP connection to Windows. teardrop Sends overlapping IP fragments at the victim. Overlapping IP fragments should normally never happen. This means that the code to handle them has never been tested -- and therefore many bugs exist. land (latierra) An attacker can forge a packet that is sent from the victim's machine to the victim's machine, which can cause it to reply back to itself in an infinite loop. targa A tool that includes many popular DoS attacks (by Mixter). Culture: A common word for DoS is "nuke", which was first popularized by the WinNuke program (a simple ping-of-death expoit script. These days, "nukes" are those DoS exploits that script kiddies in chat rooms use against each other. See also: SYN flood From Hacking-Lexicon