## 48.5. What We Mean by DoS

Another major security problem is one in which users of a Unix system can't access the functionality because access attempts are being blocked in some way. These blocking efforts are called, appropriately enough, denial-of-service attacks, usually abbreviated DoS.

CERT defines three types of DoS attacks:

- An attack that consumes all resources

- Manipulation of configuration information

- Manipulation of network components

Resources in a networked system include memory, bandwidth, Internet connections, and so on. In a DoS attack, the attacker seeks to use these resources in such a way that no one else can connect to the system. Famous examples of this type of attack involve a concept known as the distributed denial-of-service attack, DDoS.

In a DDoS attack, several machines that have not been properly secured against external control are compromised, and an application is placed on each. This application lies dormant until triggered by the attacker. When this happens, these compromised machines -- known as handlers -- direct other compromised machines -- known as agents -- to run an application that generates network packets, all of which are directed to a specific target. These packets overwhelm the available bandwidth of the victim, and they may also overwhelm routers in the path to the victim to the point where entire sections of the Internet may be negatively impacted.

Though Windows-based rather than Unix, the Code Red worm that caused so many problems in 2001 was based on the premise of DDoS.

Though disabling, DoS attacks based on overutilizing ephemeral resources such as bandwidth deny access but don't permanently damage a machine's infrastructure. However, another DoS attack is one in which an attacker gains root access to a machine and modifies configuration information such as usernames and passwords, in such a way that no one can access the network.

How simple is it to access configuration information? Accessing the password file on a system can be as easy as using TFTP (Trivial File Transfer Protocol) to download the password file unless TFTP is disabled or configured to prevent unauthorized access.

In fact, a DDoS attack is dependent on the attacker getting access to several machines in order to launch an attack. Keeping your system clean and protected not only prevents invasion of your own systems, but prevents your Unix boxes from being used to launch attacks on others.

The third type of DoS attack is based on physical attack. Literally, if someone comes after your wires with an axe, no security software is going to protect your system. However, axe-wielding intruders are beyond the scope of this book, so we'll concentrate primarily on software and system adjustments to protect against DoS attacks.

-- SP

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