ICMP floods

A smurf attack is one particular variant of a flooding DoS attack on the public Internet. It relies on misconfigured 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 the source address faked to appear to be the address of the 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.
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, the primary requirement being access to greater bandwidth than the victim.
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.

Application level floods

On IRC, IRC floods are a common electronic warfare weapon.
Various DoS-causing exploits such as buffer overflow 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 the 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.
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.
An attacker with access to a victim's computer may slow it until it is unusable or crash it by using a fork bomb.
A 'pulsing zombie' is a term referring to a special denial-of-service attack. A network is subjected to hostile pinging by different attacker computers over an extended amount of time. This results in a degraded quality of service and increased workload for the network's resources. This type of attack is more difficult to detect than traditional denial-of-service attacks due to their surreptitious nature.

Nuke

A Nuke is an old denial-of-service attack against computer networks consisting of fragmented or otherwise invalid ICMP packets to the target, achieved by using a modified ping utility to repeatedly send this corrupt data, thus slowing down the affected computer until it comes to a complete stop.
In online gaming, nuking is used by spamming another user, or all other users, with random repeated messages in quick succession. Such techniques are also seen in instant messaging programs as repeatedly sending text can be assigned to a macro or AppleScript. Modern operating systems are usually resistant to these nuke attacks, and online games now have third party "Flood control."
A specific example of a nuke attack that gained some prominence is the WinNuke, which exploited the vulnerability in the NetBIOS handler in Windows 95. A string of out-of-band data was sent to TCP port 139 of the victim machine, causing it to lock up and display a Blue Screen of Death.

Distributed attack

A distributed denial of service attack (DDoS) occurs when multiple compromised systems flood the bandwidth or resources of a targeted system, usually one or more web servers. These systems are compromised by attackers using a variety of methods.
Malware can carry DDoS attack mechanisms; one of the more well known examples of this was MyDoom. Its DoS mechanism was triggered on a specific date and time. This type of DDoS involved hardcoding the target IP address prior to release of the malware and no further interaction was necessary to launch the attack.
A system may also be compromised with a trojan, allowing the attacker to download a zombie agent (or the trojan may contain one). Attackers can also break into systems using automated tools that exploit flaws in programs that listen for connections from remote hosts. This scenario primarily concerns systems acting as servers on the web.
Stacheldraht is a classic example of a DDoS tool. It utilizes a layered structure where the attacker uses a client program to connect to handlers, which are compromised systems that issue commands to the zombie agents, which in turn facilitate the DDoS attack. Agents are compromised via the handlers by the attacker, using automated routines to exploit vulnerabilities in programs that accept remote connections running on the targeted remote hosts. Each handler can control up to a thousand agents.[4]
These collections of compromised systems are known as botnets. DDoS tools like stacheldraht still use classic DoS attack methods centered around IP spoofing and amplification like smurf attacks and fraggle attacks (these are also known as bandwidth consumption attacks). SYN floods (also known as resource starvation attacks) may also be used. Newer tools can use DNS servers for DoS purposes. (see next section)
Unlike MyDoom's DDoS mechanism, botnets can be turned against any IP address. Script kiddies use them to deny the availability of well known websites to legitimate users.[5] More sophisticated attackers use DDoS tools for the purposes of extortion — even against their business rivals.[6]
It is important to note the difference between a DDoS and DoS attack. If an attacker mounts a smurf attack from a single host it would be classified as a DoS attack. In fact, any attack against availability would be classed as a Denial of Service attack (e.g. using High-energy radio-frequency weapons to render computer equipment inoperable, would be a DoS attack, albeit an exotic one.)[7]. On the other hand, if an attacker uses a thousand zombie systems to simultaneously launch smurf attacks against a remote host, this would be classified as a DDoS attack.
The major advantages to an attacker of using a distributed denial-of-service attack are that multiple machines can generate more attack traffic than one machine, multiple attack machines are harder to turn off than one attack machine, and that the behavior of each attack machine can be stealthier, making it harder to track down and shut down. These attacker advantages cause challenges for defense mechanisms. For example, merely purchasing more incoming bandwidth than the current volume of the attack might not help, because the attacker might be able to simply add more attack machines.
Although most DDoS attacks are malicious in nature, the same technique can be used to aid the Internet community. Internet fraud schemes, such as Nigerian 419 scams or phishing, commonly involve fraudulent websites that either impersonate a real website for purposes of stealing the victim's identity, or lend credibility to a scammer's fictional business venture to lure the victim into a false sense of confidence. Scam baiters, who combat these scams by posing as victims for the purpose of wasting the scammer's time and money and obtaining information that can be used by authorities, will forward sites they encounter during the course of their conversations to groups that specialize in site-killing.[citation needed] The group will first try to have a site taken down by informing the host of said site that the site is being used fraudulently. In the case where that approach fails, the group will organize a "takedown" of the site by encouraging its members to visit the site en masse and continually refresh its content (an intentional form of the Slashdot effect sometimes referred to as flash mobbing, although that term is technically reserved for real-world gatherings). Alternately, some groups have special web pages that link to images hosted by these fake sites and show the images to visitors (usually members or supporters of the site-killing group) while constantly reloading them, which is known as intentional bandwidth hogging.[citation needed] The purpose, similar to malicious DoS attacks, is to (a.) rapidly consume all of the website's allocated monthly bandwidth, after which requests for the site's content are refused, (b.) draw the attention of the site's host, who when faced with the constant onslaught on the entire hosting network's resources, will usually remove the site, and/or (c.) take up all available connections and maximum throughput of the host so that would-be victims cannot access the site.

Reflected attack

A distributed reflected denial of service attack (DRDoS) involves sending forged requests of some type to a very large number of computers that will reply to the requests. Using Internet protocol spoofing, the source address is set to that of the targeted victim, which means all the replies will go to (and flood) the target.
ICMP Echo Request attacks (described above) can be considered one form of reflected attack, as the flooding host(s) send Echo Requests to the broadcast addresses of mis-configured networks, thereby enticing many hosts to send Echo Reply packets to the victim. Some early DDoS programs implemented a distributed form of this attack.
Many services can be exploited to act as reflectors, some harder to block than others.[8] DNS amplification attacks involve a new mechanism that increased the amplification effect, using a much larger list of DNS servers than seen earlier.