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.
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. More sophisticated attackers use DDoS tools for the purposes of extortion — even against their business rivals.
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. 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. 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. 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. DNS amplification attacks involve a new mechanism that increased the amplification effect, using a much larger list of DNS servers than seen earlier.
 Unintentional attack
This describes a situation where a website ends up denied, not due to a deliberate attack by a single individual or group of individuals, but simply due to a sudden enormous spike in popularity. This can happen when an extremely popular website posts a prominent link to a second, less well-prepared site, for example, as part of a news story. The result is that a significant proportion of the primary site's regular users — potentially hundreds of thousands of people — click that link in the space of a few hours, having the same effect on the target website as a DDoS attack.
News sites and link sites — sites whose primary function is to provide links to interesting content elsewhere on the Internet — are most likely to cause this phenomenon. The canonical example is the Slashdot effect. Sites such as Digg, Fark, Something Awful, and the webcomic Penny Arcade have their own corresponding "effects", known as "the Digg effect", "farking", "goonrushing" and "wanging"; respectively.
Routers have also been known to create unintentional DoS attacks, as both D-Link and Netgear routers have created NTP vandalism by flooding NTP servers without respecting the restrictions of client types or geographical limitations.
Similar unintentional attacks can also occur via other media, e.g. when a URL is mentioned on television. If a server is being indexed by Google or another search engine during peak periods of activity, or does not have a lot of available bandwidth while being indexed, it can also experience the effects of a DoS attack.
The first major attack involving DNS servers as reflectors occurred in January 2001. The target was Register.com. This attack, which forged requests for the MX records of AOL.com (to amplify the attack) lasted about a week before it could be traced back to all attacking hosts and shut off. It used a list of tens of thousands of DNS records that were a year old at the time of the attack.
In July 2002, the Honeynet Project Reverse Challenge was issued. The binary that was analyzed turned out to be yet another DDoS agent, which implemented several DNS related attacks, including an optimized form of a reflection attack.
On two occasions to date, attackers have performed DNS Backbone DDoS Attacks on the DNS root servers. Since these machines are intended to provide service to all Internet users, these two denial of service attacks might be classified as attempts to take down the entire Internet, though it is unclear what the attackers' true motivations were. The first occurred in October 2002 and disrupted service at 9 of the 13 root servers. The second occurred in February 2007 and caused disruptions at two of the root servers.
In February 2007, more than 10,000 online game servers like Return to Castle Wolfenstein, Halo, Counter-Strike and many others were attacked by "RUS" hacker group. The DDoS attack was made from more than a thousand computer units located in the republics of the former Soviet Union, mostly from Russia, Uzbekistan and Belarus. Minor attacks are still continuing to be made today.
In late January 2008, a group calling themselves Anonymous began a DDoS attack on Scientology web sites as a part of an Anti-Scientology campaign called Project Chanology.
 Prevention and response
 Surviving attacks
The investigative process should begin immediately after the DoS attack begins. There will be multiple phone calls, callbacks, emails, pages and faxes between the victim organization, one's provider, and others involved. This can be a very time consuming process. It has taken some very large networks with plenty of resources several hours to halt a DoS attack.
The easiest way to survive an attack is to have planned for the attack. Having a separate emergency block of IP addresses for critical servers with a separate route can be invaluable. A separate route (perhaps a DSL) is not that extravagant, and it can be used for load balancing or sharing under normal circumstances and switched to emergency mode in the event of an attack.
Filtering is often ineffective, as the route to the filter will normally be swamped so only a trickle of traffic will survive. However, by using an extremely resilient stateful packet filter that will inexpensively drop any unwanted packets, surviving a DoS attack becomes much easier. When such a high performance packet filtering server is attached to an ultra-high bandwidth connection (preferably an internet backbone), communication with the outside world will be unimpaired so long as not all of the available bandwidth is saturated, and performance behind the packet filter will remain normal as long as the packet filter drops all DoS packets. It should be noted however, that in this case the victim of the DoS attack still would need to pay for the excessive bandwidth. The price of service unavailability thus needs to be weighed against the price of truly exorbitant bandwidth/traffic.
Firewalls have simple rules such as to allow or deny protocols, ports or IP addresses. Some DoS attacks are too complex for today's firewalls, e.g. if there is an attack on port 80 (web service), firewalls cannot prevent that attack because they cannot distinguish good traffic from DoS attack traffic. Additionally, firewalls are too deep in the network hierarchy. Your router may be affected even before the firewall gets the traffic. Nonetheless, firewalls can effectively prevent users from launching simple flooding type attacks from machines behind the firewall.
Modern stateful firewalls like Check Point FW1 NGX & Cisco PIX have a built-in capability to differentiate good traffic from DoS attack traffic. This capability is known as a "Defender", as it confirms TCP connections are valid before proxying TCP packets to service networks (including border routers). A similar ability is present in OpenBSD's pF, which is available for other BSDs as well. In that context, it is called "synproxy".
Comodo Firewall Pro has a built-in Emergency Mode which is activated when the number of incoming packets per seconds exceed a set value for more than the specified time, for example, more than 20 packets/sec for more than 20 seconds. If this happens, the firewall classifies it as a DoS attack and switches to Emergency Mode. In this mode, all inbound traffic is blocked except previously established and active connections, but outbound traffic is allowed. The packet number threshold and the time needed for verifying an attack can be adjusted by the user separately for TCP, UDP and ICMP. The firewall also has some other attack prevention mechanisms, like protocol analysis, checksum verification (so that the packets aren't altered since transmission) and NDIS protocol monitoring for attempts at making a DoS attack by using own protocols, thus outmaneuvering older firewalls.
Most switches have some rate-limiting and ACL capability. Some switches provide automatic and or system-wide rate limiting, traffic shaping, delayed binding (TCP splicing), deep packet inspection and Bogon filtering (bogus IP filtering) to detect and remediate denial of service attacks through automatic rate filtering and WAN Link failover and balancing.
These schemes will work as long as the DoS attacks are something that can be prevented by using them. For example SYN flood can be prevented using delayed binding or TCP splicing. Similarly content based DoS can be prevented using deep packet inspection. Attacks originating from dark addresses or going to dark addresses can be prevented using Bogon filtering. Automatic rate filtering can work as long as you have set rate-thresholds correctly and granularly. Wan-link failover will work as long as both links have DoS/DDoS prevention mechanism.
Similar to switches, routers have some rate-limiting and ACL capability. They, too, are manually set. Most routers can be easily overwhelmed under DoS attack. If you add rules to take flow statistics out of the router during the DoS attacks, they further slow down and complicate the matter. Cisco IOS has features that prevents flooding, i.e. example settings.
 Application front end hardware
Application front end hardware is intelligent hardware placed on the network before traffic reaches the servers. It can be used on networks in conjunction with routers and switches. Application front end hardware analyzes data packets as they enter the system, and then identifies them as priority, regular, or dangerous. There are more than 25 bandwidth management vendors. Hardware acceleration is key to bandwidth management. Look for granularity of bandwidth management, hardware acceleration, and automation while selecting an appliance.
 IPS based prevention
Intrusion-prevention systems (IPS) are effective if the attacks have signatures associated with them. However, the trend among the attacks is to have legitimate content but bad intent. IPSs which work on content recognition cannot block behavior based DoS attacks.
An ASIC based IPS can detect and block denial of service attacks because they have the processing power and the granularity to analyze the attacks and act like a circuit breaker in an automated way.
A rate-based IPS (RBIPS) must analyze traffic granularly and continuously monitor the traffic pattern and determine if there is traffic anomaly. It must let the legitimate traffic flow while blocking the DoS attack traffic.
 Side effects of DOS attacks
In computer network security, backscatter is a side-effect of a spoofed denial of service (DoS) attack. In this kind of attack, the attacker spoofs (or forges) the source address in IP packets sent to the victim. In general, the victim machine can not distinguish between the spoofed packets and legitimate packets, so the victim responds to the spoofed packets as it normally would. These response packets are known as backscatter.
If the attacker is spoofing source addresses randomly, the backscatter response packets from the victim will be sent back to random destinations. This effect can be used by network telescopes as an indirect evidence of such attacks.
The term "backscatter analysis" refers to observing backscatter packets arriving at a statistically significant portion of the IP address space to determine characteristics of DoS attacks and victims.
An educational animation describing such backscatter can be found on the animations page maintained by CAIDA, the Cooperative Association for Internet Data Analysis.
 See also
* Barrett Lyon
* Black fax
* Intrusion-detection system
* Network intrusion detection system
* Zombie computer
 Notes and references
1. ^ Intrusion Detection FAQ: Distributed Denial of Service Attack Tools (trinoo and wintrinoo)
2. ^ Understanding Denial-of-Service Attacks (US CERT)
3. ^ "Advisory CA-1997-28 IP Denial-of-Service Attacks" (CERT)
4. ^ Sop, Paul (2007). P2P Distributed Denial of Service Attack Alert.
5. ^ The "stacheldraht" distributed denial of service attack tool
6. ^ Intrusion Detection FAQ: Distributed Denial of Service Attack Tools: trinoo and wintrinoo
7. ^ US credit card firm fights DDoS attack
8. ^ Paxson, Vern (2001), An Analysis of Using Reflectors for Distributed Denial-of-Service Attacks
9. ^ Vaughn, Randal and Evron, Gadi (2006), DNS Amplification Attacks
10. ^ January 2001 thread on the UNISOG mailing list
11. ^ Honeynet Project Reverse Challenge
12. ^ Richards, Johnathan (The Times). "Hackers Declare War on Scientology: A shadowy Internet group has succeeded in taking down a Scientology Web site after effectively declaring war on the church and calling for it to be destroyed.", FOX News, FOX News Network, LLC., January 25, 2008. Retrieved on 2008-01-25.
13. ^ NB: no link in this <ref - ie. without consuming much processing power
14. ^ OpenBSD's pf is a packet filter some providers use for exactly this purpose. 
15. ^ "Some IoS tips for Internet Service (Providers)" (Mehmet Suzen)
 External links
* RFC 4732 Internet Denial-of-Service Considerations
* W3C The World Wide Web Security FAQ
* How to Prevent Denial of Service Attacks
* cert.org CERT's Guide to DoS attacks.
* surasoft.com - DDoS case study, concepts, and protection.
* ATLAS Summary Report - Real-time global report of DDoS attacks.
* newssocket.com An article regarding a DDoS for hire incident.
* linuxsecurity.com An article on preventing DDoS attacks.
* and Zombies:
o Is Your PC a Zombie? on About.com
o Intrusive analysis of a web-based proxy zombie network