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| Denial of Servie (DoS) |
Extracted from Nelly |
1 Mar 2006 |
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A denial-of-service attack (also, DoS attack) is an attack on a computer system or network that causes a loss of service to users, typically the loss of network connectivity and services by consuming the bandwidth of the victim network or overloading the computational resources of the victim system.
Methods of attack
Primitive attack (i.e. ping example.com -l 1024 -t).
A DoS attack can be perpetrated in a number of ways. There are three basic types of attack:
1. consumption of computational resources, such as bandwidth, disk space, or CPU time
2. disruption of configuration information, such as routing information
3. disruption of physical network components
A nuke attack sends a packet, usually ICMP, which is malformed or fragmented in an invalid way, triggering a bug in the operating system and crashing the targeted computer. This is known as the ping of death.
WinNuke is a similar kind of attack, exploiting the vulnerability in the NetBIOS handler in Windows 95. A string of out-of-band data is sent to TCP port 139 of the victim machine, causing it to lock up and display a Blue Screen of Death. This attack was very popular between the IRC-dwelling script kiddies, due to easy availability of a user-friendly click-and-crash WinNuke program.
Various DoS-causing exploits can cause server-running software to get confused and fill the disk space or consume all available memory or CPU time.
Other kinds of DoS rely primarily on brute force, flooding the target with an overwhelming flux of packets, oversaturating its connection bandwidth or depleting target's system resources. Bandwidth-saturating floods rely on the attacker having higher bandwidth available than the victim; a common way of achieving this today is via Distributed Denial of Service, employing a botnet. Other floods may use specific packet types or connection requests to saturate finite resources by, for example, occupying the maximum number of open connections or filling the victim's disk space with logs.
An attacker with access to a victim computer can bring it to a crawl or even to a crash by using a fork bomb.
On IRC, IRC floods are a common electronic warfare weapon.
Ping flood is based on sending the victim an overwhelming number of ping packets, usually using the "ping -f" command. It is very simple to launch, and a T1 owner can easily defeat a dial-up user.
SYN flood sends a flood of TCP/SYN packets, often with a forged sender address. Each of these packets is handled like a connection request, causing the server to spawn a half-open connection, by sending back a TCP/SYN-ACK packet, and waiting for an TCP/ACK packet in response from the sender address. However, because the sender address is forged, the response never comes. These half-open connections saturate the number of available connections the server is able to make, keeping it from responding to legitimate requests until after the attack ends.
A smurf attack is one particular variant of a flooding DoS attack on the public Internet. It relies on mis-configured network devices that allow packets to be sent to all computer hosts on a particular network via the broadcast address of the network, rather than a specific machine. The network then serves as a smurf amplifier. In such an attack, the perpetrators will send large numbers of IP packets with a faked source address, that is set to the address of the intended victim. To combat Denial of Service attacks on the Internet, services like the Smurf Amplifier Registry have given network service providers the ability to identify misconfigured networks and to take appropriate action such as filtering.
A "banana attack" is another particular type of DoS. It involves redirecting outgoing messages from the client back onto the client, preventing outside access, as well as flooding the client with the sent packets.
Attempts to "flood" a network with bogus packets, thereby preventing legitimate network traffic, are the most common form of attack, often conducted by disrupting network connectivity with the use of multiple hosts in a distributed denial-of-service attack or DDoS. Specific means of attack include: a smurf attack, in which excessive ICMP requests are broadcast to an entire network; bogus HTTP requests on the World Wide Web; incorrectly formed packets; and random traffic. The source addresses of this traffic is usually spoofed in order to hide the true origin of the attack. Due to this and the many vectors of attack, there are no comprehensive rules that can be implemented on network hosts in order to protect against denial-of-service attacks, and it is a difficult feat to determine the source of the attack and the identity of the attacker. This is especially true with distributed attacks.
Attacks can be directed at any network device, including attacks on routing devices and Web, electronic mail, or Domain Name System servers.
Effects of DoS
Denial of Service attacks can also lead to problems in the network 'branches' around the actual computer being attacked. For example, the bandwidth of a router between the Internet and a LAN may be consumed by a DoS, meaning not only will the intended computer be compromised, but the entire network will also be disrupted.
If the DoS is conducted in a sufficiently large scale, entire geographical swathes of Internet connectivity can also be compromised by incorrectly configured or flimsy network infrastructure equipment without the attacker's knowledge or intent. For this reason, most, if not all ISPs ban the practice.
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