Editing Denial-of-service attack

{{Short description|Type of cyber-attack}}
{{Redirect|DoS|other uses|DOS (disambiguation)}}
{{Redirect|DDoS|the non-profit whistleblower site|Distributed Denial of Secrets}}
{{More citations needed|date=February 2024}}
{{Use dmy dates|date=May 2024}}

[[File:Stachledraht DDos Attack.svg|thumb|Diagram of a DDoS attack. Note how multiple computers are attacking a single computer.]]

In [[computing]], a '''denial-of-service attack''' ('''DoS attack''') is a [[cyberattack]] in which the perpetrator seeks to make a machine or network resource unavailable to its intended [[user (computing)|users]] by temporarily or indefinitely disrupting [[network service|services]] of a [[host (network)|host]] connected to a [[Computer network|network]]. Denial of service is typically accomplished by [[Flooding (computer networking)|flooding]] the targeted machine or resource with superfluous requests in an attempt to overload systems and prevent some or all legitimate requests from being fulfilled.<ref>{{cite web |date=6 February 2013 |title=Understanding Denial-of-Service Attacks |url=https://www.cisa.gov/news-events/news/understanding-denial-service-attacks |access-date=26 May 2016 |publisher=US-CERT}}</ref> The range of attacks varies widely, spanning from inundating a server with millions of requests to slow its performance, overwhelming a server with a substantial amount of invalid data, to submitting requests with an illegitimate [[IP address]].<ref>{{Cite journal |last1=Elleithy |first1=Khaled |last2=Blagovic |first2=Drazen |last3=Cheng |first3=Wang |last4=Sideleau |first4=Paul |date=2005-01-01 |title=Denial of Service Attack Techniques: Analysis, Implementation and Comparison |url=https://digitalcommons.sacredheart.edu/computersci_fac/52 |journal=School of Computer Science & Engineering Faculty Publications}}</ref>

In a '''distributed denial-of-service attack''' ('''DDoS attack'''), the incoming traffic flooding the victim originates from many different sources. More sophisticated strategies are required to mitigate this type of attack; simply attempting to block a single source is insufficient as there are multiple sources.<ref>{{Cite web|date=2021-01-13|title=What is a DDoS Attack? - DDoS Meaning|url=https://usa.kaspersky.com/resource-center/threats/ddos-attacks|access-date=2021-09-05|website=Kaspersky |language=en}}</ref><ref>{{Cite web|title=What is a DDoS Attack?|url=https://www.cloudflare.com/en-gb/learning/ddos/what-is-a-ddos-attack/|access-date=2024-12-04|website=Coudflare |language=en}}</ref>  A DDoS attack is analogous to a group of people crowding the entry door of a shop, making it hard for legitimate customers to enter, thus disrupting trade and losing the business money. Criminal perpetrators of DDoS attacks often target sites or services hosted on high-profile [[web server]]s such as [[bank]]s or [[credit card]] [[payment gateway]]s. [[Revenge]] and [[blackmail]],<ref>{{cite web |last1=Prince |first1=Matthew |author-link=Matthew Prince |date=25 April 2016 |title=Empty DDoS Threats: Meet the Armada Collective |url=https://blog.cloudflare.com/empty-ddos-threats-meet-the-armada-collective/ |access-date=18 May 2016 |website=CloudFlare}}</ref><ref>{{cite web|url=http://www.interpacket.com/42882/brand-com-victim-blackmail-attempt-says-president-mike-zammuto/ |title=Brand.com President Mike Zammuto Reveals Blackmail Attempt|date=5 March 2014|archive-url=https://web.archive.org/web/20140311070205/http://www.interpacket.com/42882/brand-com-victim-blackmail-attempt-says-president-mike-zammuto/|archive-date=11 March 2014}}</ref><ref>{{cite web|url=http://dailyglobe.com/61817/brand-coms-mike-zammuto-discusses-meetup-com-extortion/|title=Brand.com's Mike Zammuto Discusses Meetup.com Extortion|date=5 March 2014|archive-url=https://web.archive.org/web/20140513044100/http://dailyglobe.com/61817/brand-coms-mike-zammuto-discusses-meetup-com-extortion/|archive-date=13 May 2014|url-status=dead}}</ref> as well as [[hacktivism]],<ref name=":1">{{cite web |last=Halpin |first=Harry |date=2010-12-17 |title=The Philosophy of Anonymous |url=http://www.radicalphilosophy.com/article/the-philosophy-of-anonymous |access-date=2013-09-10 |publisher=Radicalphilosophy.com}}</ref> can motivate these attacks.

==History==
[[Panix (ISP)|Panix]], the third-oldest [[Internet service provider|ISP]] in the world, was the target of what is thought to be the first DoS attack. On September 6, 1996, Panix was subject to a [[SYN flood]] attack, which brought down its services for several days while hardware vendors, notably [[Cisco]], figured out a proper defense.<ref>{{Cite web|url=https://www.cisco.com/c/en/us/about/press/internet-protocol-journal/back-issues/table-contents-30/dos-attacks.html|title=Distributed Denial of Service Attacks - The Internet Protocol Journal - Volume 7, Number 4|website=Cisco|language=en|access-date=2019-08-26|archive-url=https://web.archive.org/web/20190826143507/https://www.cisco.com/c/en/us/about/press/internet-protocol-journal/back-issues/table-contents-30/dos-attacks.html|archive-date=2019-08-26}}</ref> Another early demonstration of the DoS attack was made by Khan C. Smith in 1997 during a [[DEF CON]] event, disrupting Internet access to the [[Las Vegas Strip]] for over an hour. The release of sample code during the event led to the online attack of [[Sprint Corporation|Sprint]], [[EarthLink]], [[E-Trade]], and other major corporations in the year to follow.{{Citation needed|date=March 2025}} The largest DDoS attack to date happened in September 2017, when [[Google Cloud Platform|Google Cloud]] experienced an attack with a peak volume of {{val|2.54|u=Tb/s}}, revealed by Google on October 17, 2020.<ref>{{Cite web |last=Cimpanu |first=Catalin |title=Google says it mitigated a 2.54 Tbps DDoS attack in 2017, largest known to date |url=https://www.zdnet.com/article/google-says-it-mitigated-a-2-54-tbps-ddos-attack-in-2017-largest-known-to-date/ |access-date=2021-09-16 |website=ZDNet |language=en}}</ref> The record holder was thought to be an attack executed by an unnamed customer of the US-based service provider [[Arbor Networks]], reaching a peak of about {{val|1.7|u=Tb/s}}.<ref>{{cite web|url=https://arstechnica.com/information-technology/2018/03/us-service-provider-survives-the-biggest-recorded-ddos-in-history/|title=US service provider survives the biggest recorded DDoS in history|first=Dan|last=Goodin|date=5 March 2018|website=Ars Technica|access-date=6 March 2018}}</ref>

In February 2020, [[Amazon Web Services]] experienced an attack with a peak volume of {{val|2.3|u=Tb/s}}.<ref>{{Cite news|date=Jun 18, 2020|title=Amazon 'thwarts largest ever DDoS cyber-attack'|work=BBC News|url=https://www.bbc.com/news/technology-53093611|access-date=Nov 11, 2020}}</ref> In July 2021, CDN Provider [[Cloudflare]] boasted of protecting its client from a DDoS attack from a global [[Mirai botnet]] that was up to 17.2 million requests per second.<ref>{{Cite web |date=2021-08-23 |title=Cloudflare Mitigated Record-Setting 17.2 Million RPS DDoS Attack |url=https://www.securityweek.com/cloudflare-mitigated-record-setting-172-million-rps-ddos-attack/ |website=SecurityWeek}}</ref> Russian DDoS prevention provider [[Yandex]] said it blocked a HTTP pipelining DDoS attack on Sept. 5. 2021 that originated from unpatched Mikrotik networking gear.<ref>{{Cite web|title=Yandex Pummeled by Potent Meris DDoS Botnet|url=https://threatpost.com/yandex-meris-botnet/169368/|access-date=2021-12-23|website=threatpost.com|date=10 September 2021 |language=en}}</ref> In the first half of 2022, the [[Russian invasion of Ukraine]] significantly shaped the cyberthreat landscape, with an increase in cyberattacks attributed to both state-sponsored actors and global hacktivist activities. The most notable event was a DDoS attack in February, the largest Ukraine has encountered, disrupting government and financial sector services. This wave of cyber aggression extended to Western allies like the UK, the US, and Germany. Particularly, the UK's financial sector saw an increase in DDoS attacks from [[Nation state|nation-state]] actors and hacktivists, aimed at undermining Ukraine's allies.<ref name=":2">{{Cite web |last=Team |first=Azure Network Security |date=2023-02-21 |title=2022 in review: DDoS attack trends and insights |url=https://www.microsoft.com/en-us/security/blog/2023/02/21/2022-in-review-ddos-attack-trends-and-insights/ |access-date=2024-04-07 |website=Microsoft Security Blog |language=en-US}}</ref>

In February 2023, Cloudflare faced a 71 million/requests per second attack which Cloudflare claims was the largest HTTP DDoS attack at the time.<ref>{{Cite web|title=Cloudflare mitigates record-breaking 71 million request-per-second DDoS attack|url=
https://blog.cloudflare.com/cloudflare-mitigates-record-breaking-71-million-request-per-second-ddos-attack/|access-date=2024-01-13|website=The Cloudflare Blog|date=13 February 2023|language=en}}</ref> HTTP DDoS attacks are measured by HTTP requests per second instead of packets per second or bits per second. On July 10, 2023, the fanfiction platform [[Archive of Our Own]] (AO3) faced DDoS attacks, disrupting services. [[Anonymous Sudan]], claiming the attack for religious and political reasons, was viewed skeptically by AO3 and experts. Flashpoint, a threat intelligence vendor, noted the group's past activities but doubted their stated motives. AO3, supported by the non-profit [[Organization for Transformative Works]] (OTW) and reliant on donations, is unlikely to meet the $30,000 [[Bitcoin]] ransom.<ref>{{Cite web |last=Weatherbed |first=Jess |date=2023-07-11 |title=AO3 fanfiction site forced offline by wave of DDoS attacks |url=https://www.theverge.com/2023/7/11/23790860/ao3-fanfiction-archive-down-outage-ddos-attacks |access-date=2024-04-09 |website=The Verge |language=en}}</ref><ref>{{cite web |date=10 July 2023 |title=Archive of Our Own is down due to a DDoS attack |url=https://www.polygon.com/23790167/ao3-down-ddos-attack-archive-of-our-own |website=Polygon}}</ref> In August 2023, the group of hacktivists [[Noname057(16)|NoName057]] targeted several Italian financial institutions, through the execution of [[slow DoS attack]]s.<ref>{{cite web|url=https://www.redhotcyber.com/post/settimo-giorno-di-attacchi-informatici-allitalia-noname05716-torna-alle-banche-e-alle-telecomunicazioni/ |title=Settimo giorno di attacchi informatici all'Italia. NoName057(16) torna alle Banche e alle Telecomunicazioni |date=6 August 2023 }}</ref> On 14 January 2024, they executed a DDoS attack on Swiss federal websites, prompted by [[President Zelensky]]'s attendance at the [[Davos World Economic Forum]]. Switzerland's National Cyber Security Centre quickly mitigated the attack, ensuring core federal services remained secure, despite temporary accessibility issues on some websites.<ref>{{Cite web |date=2024-01-17 |title=Switzerland hit by cyberattack after Ukraine president's visit |url=https://www.swissinfo.ch/eng/politics/switzerland-hit-by-cyberattack-after-ukraine-president-s-visit/49136116 |access-date=2024-04-08 |website=SWI swissinfo.ch |language=en-GB}}</ref> In October 2023, exploitation of a new vulnerability in the [[HTTP/2]] protocol resulted in the record for largest HTTP DDoS attack being broken twice, once with a 201 million requests per second attack observed by Cloudflare,<ref>{{Cite web|title=HTTP/2 Rapid Reset: deconstructing the record-breaking attack|url=https://blog.cloudflare.com/technical-breakdown-http2-rapid-reset-ddos-attack
|access-date=2024-01-13|website=The Cloudflare Blog|date=10 October 2023|language=en}}</ref> and again with a 398 million requests per second attack observed by [[Google]].<ref>{{Cite web|title=Google mitigated the largest DDoS attack to date, peaking above 398 million rps|url=https://cloud.google.com/blog/products/identity-security/google-cloud-mitigated-largest-ddos-attack-peaking-above-398-million-rps
|access-date=2024-01-13|website=Google Cloud Blog|date=10 October 2023|language=en}}</ref> In August 2024, Global Secure Layer observed and reported on a record-breaking packet DDoS at 3.15 billion packets per second, which targeted an undisclosed number of unofficial [[Minecraft server|Minecraft game servers]].<ref>{{Cite web |title=Unprecedented 3.15 Billion Packet Rate DDoS Attack Mitigated by Global Secure Layer |url=https://globalsecurelayer.com/blog/unprecedented-3-15-billion-packet-rate-ddos-attack |access-date=2024-08-28 |website=globalsecurelayer.com |language=en-AU}}</ref> In October 2024, the [[Internet Archive]] faced two severe DDoS attacks that brought the site completely offline, immediately following a previous attack that leaked records of over 31 million of the site's users.<ref>{{Cite web |title=Internet Archive hacked, data breach impacts 31 million users |url=https://www.bleepingcomputer.com/news/security/internet-archive-hacked-data-breach-impacts-31-million-users/ |access-date=2024-10-10 |website=www.bleepingcomputer.com |language=en-AU}}</ref><ref>{{Cite web |last=Davis |first=Wes |date=2024-10-09 |title=The Internet Archive is under attack, with a breach revealing info for 31 million accounts |url=https://www.theverge.com/2024/10/9/24266419/internet-archive-ddos-attack-pop-up-message |access-date=2024-10-10 |website=The Verge |language=en}}</ref> The hacktivist group [[Anonymous Sudan#Possible link with SN_BLACKMETA|SN_Blackmeta]] claimed the DDoS attack as retribution for American involvement in the [[Gaza war]], despite the Internet Archive being unaffiliated with the United States government; however, their link with the preceding data leak remains unclear.<ref>{{Cite web |last=Boran |first=Marie |date=2024-10-10 |title=Hackers claim 'catastrophic' Internet Archive attack |url=https://www.newsweek.com/catastrophic-internet-archive-hack-hits-31-million-people-1966866 |access-date=2024-10-10 |website=Newsweek |language=en}}</ref>

==Types==
Denial-of-service attacks are characterized by an explicit attempt by attackers to prevent legitimate use of a service. There are two general forms of DoS attacks: those that crash services and those that flood services. The most serious attacks are distributed.<ref name="Taghavi Zargar 2046–2069">{{cite web|url=http://d-scholarship.pitt.edu/19225/1/FinalVersion.pdf |archive-url=https://web.archive.org/web/20140307201217/http://d-scholarship.pitt.edu/19225/1/FinalVersion.pdf |archive-date=2014-03-07 |url-status=live |title=A Survey of Defense Mechanisms Against Distributed Denial of Service (DDoS) Flooding Attacks |first=Saman |last=Taghavi Zargar |publisher=IEEE Communications Surveys & Tutorials |volume=15 |issue=4 |pages=2046–2069 |date=November 2013 |access-date=2014-03-07}}</ref>

==={{visible anchor|Distributed DoS|Distributed_attack}}===
A distributed denial-of-service (DDoS) attack occurs when multiple systems flood the [[Bandwidth (computing)|bandwidth]] or resources of a targeted system, usually one or more web servers.<ref name="Taghavi Zargar 2046–2069"/> A DDoS attack uses more than one unique IP address or machines, often from thousands of hosts infected with [[malware]].<ref>{{cite book | last1=Amiri | first1=I.S. | last2=Soltanian | first2=M.R.K. | title=Theoretical and Experimental Methods for Defending Against DDoS Attacks | publisher=Syngress | year=2015 | isbn=978-0-12-805399-7}}</ref><ref>{{cite news|title=Has Your Website Been Bitten By a Zombie?|url=http://blog.cloudbric.com/2015/08/has-your-website-been-bitten-by-zombie.html|access-date=15 September 2015|agency=Cloudbric|date=3 August 2015}}</ref> A distributed denial of service attack typically involves more than around 3–5 nodes on different networks; fewer nodes may qualify as a DoS attack but is not a DDoS attack.<ref name="Infosec7Layer"/><ref>{{cite book | last =Raghavan | first =S.V. | title =An Investigation into the Detection and Mitigation of Denial of Service (DoS) Attacks | publisher =Springer | date =2011 | isbn =9788132202776}}</ref>

Multiple attack machines can generate more attack traffic than a single machine and are harder to disable, and the behavior of each attack machine can be stealthier, making the attack harder to track and shut down. Since the incoming traffic flooding the victim originates from different sources, it may be impossible to stop the attack simply by using [[ingress filtering]]. It also makes it difficult to distinguish legitimate user traffic from attack traffic when spread across multiple points of origin. As an alternative or augmentation of a DDoS, attacks may involve forging of IP sender addresses ([[IP address spoofing]]) further complicating identifying and defeating the attack. 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.{{Citation needed|date=April 2024}} The scale of DDoS attacks has continued to rise over recent years, by 2016 exceeding a [[terabit per second]].<ref name="Goodin">{{cite web|last=Goodin |first=Dan |date=28 September 2016 |title=Record-breaking DDoS reportedly delivered by >145k hacked cameras |website=Ars Technica |url=https://arstechnica.com/security/2016/09/botnet-of-145k-cameras-reportedly-deliver-internets-biggest-ddos-ever/ |archive-url=https://web.archive.org/web/20161002000235/http://arstechnica.com/security/2016/09/botnet-of-145k-cameras-reportedly-deliver-internets-biggest-ddos-ever/ |archive-date=2 October 2016 |url-status=live}}</ref><ref>{{Cite web |url=https://thehackernews.com/2016/09/ddos-attack-iot.html |title=World's largest 1 Tbps DDoS Attack launched from 152,000 hacked Smart Devices |last=Khandelwal |first=Swati |date=26 September 2016 |publisher=The Hacker News |archive-url=https://web.archive.org/web/20160930031903/https://thehackernews.com/2016/09/ddos-attack-iot.html |archive-date=30 September 2016 |url-status=live }}</ref> Some common examples of DDoS attacks are [[UDP flood attack|UDP flooding]], [[SYN flooding]] and [[#Amplification|DNS amplification]].<ref>{{Cite book|title=DDoS attacks : evolution, detection, prevention, reaction, and tolerance| last1=Bhattacharyya | first1=Dhruba Kumar | last2=Kalita | first2=Jugal Kumar|author2-link= Jugal Kalita |isbn=9781498729659|location=Boca Raton, FL| publisher=CRC Press|oclc=948286117|date = 2016-04-27}}</ref><ref>{{cite web |title=Imperva, Global DDoS Threat Landscape, 2019 Report |url=https://www.imperva.com/resources/reports/Imperva_DDOS_Report_20200131.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.imperva.com/resources/reports/Imperva_DDOS_Report_20200131.pdf |archive-date=2022-10-09 |url-status=live |website=Imperva.com |publisher=[[Imperva]] |access-date=4 May 2020}}</ref>

====Yo-yo attack====
A '''[[yo-yo]]''' attack is a specific type of DoS/DDoS aimed at [[cloud-hosted]] applications which use [[autoscaling]].<ref>{{cite journal |url=https://dl.acm.org/doi/10.1145/2829988.2790017 |title=Yo-Yo Attack: Vulnerability In Auto-scaling Mechanism |journal=ACM SIGCOMM Computer Communication Review |date=17 August 2015 |volume=45 |issue=4 |pages=103–104 |doi=10.1145/2829988.2790017 |last1=Sides |first1=Mor |last2=Bremler-Barr |first2=Anat |author-link2=Anat Bremler-Barr |last3=Rosensweig |first3=Elisha}}</ref><ref>{{cite book |title=Proceedings of the 11th International Conference on Cloud Computing and Services Science |chapter=Kubernetes Autoscaling: Yo ''Yo'' Attack Vulnerability and Mitigation |year=2021 |doi=10.5220/0010397900340044 |arxiv=2105.00542 |last1=Barr |first1=Anat |last2=Ben David |first2=Ronen |pages=34–44 |isbn=978-989-758-510-4 |s2cid=233482002}}</ref><ref>{{cite journal |title=Towards Yo-Yo attack mitigation in cloud auto-scaling mechanism |year=2020 |doi=10.1016/j.dcan.2019.07.002 |last1=Xu |first1=Xiaoqiong |last2=Li |first2=Jin |last3=Yu |first3=Hongfang |last4=Luo |first4=Long |last5=Wei |first5=Xuetao |last6=Sun |first6=Gang |journal=Digital Communications and Networks |volume=6 |issue=3 |pages=369–376 |s2cid=208093679 |doi-access=free}}</ref> The attacker generates a flood of traffic until a cloud-hosted service scales outwards to handle the increase of traffic, then halts the attack, leaving the victim with over-provisioned resources. When the victim scales back down, the attack resumes, causing resources to scale back up again. This can result in a reduced quality of service during the periods of scaling up and down and a financial drain on resources during periods of over-provisioning while operating with a lower cost for an attacker compared to a normal DDoS attack, as it only needs to be generating traffic for a portion of the attack period.

===Application layer attacks===
An '''application layer DDoS attack''' (sometimes referred to as '''layer 7 DDoS attack''') is a form of DDoS attack where attackers target [[application layer|application-layer]] processes.<ref>{{cite book | last =Lee | first =Newton | title =Counterterrorism and Cybersecurity: Total Information Awareness | publisher =Springer | date =2013 | isbn =9781461472056 }}</ref><ref name="Infosec7Layer">{{cite news | title =Layer Seven DDoS Attacks | newspaper =Infosec Institute }}</ref> The attack over-exercises specific functions or features of a website with the intention to disable those functions or features. This application-layer attack is different from an entire network attack, and is often used against financial institutions to distract IT and security personnel from security breaches.<ref>{{cite news | title =Gartner Says 25 Percent of Distributed Denial of Services Attacks in 2013 Will Be Application - Based | newspaper =Gartner | date =21 February 2013 | url =http://www.gartner.com/newsroom/id/2344217 | archive-url =https://web.archive.org/web/20130225073934/http://www.gartner.com/newsroom/id/2344217 | url-status =dead | archive-date =February 25, 2013 | access-date =28 January 2014 }}</ref> In 2013, application-layer DDoS attacks represented 20% of all DDoS attacks.<ref name="AbABankinJournal">{{cite news | last =Ginovsky | first =John | title =What you should know about worsening DDoS attacks | newspaper =ABA Banking Journal| date =27 January 2014 | url =http://www.ababj.com/component/k2/item/4354-what-you-should-know-about-worsening-ddos-attacks |archive-url=https://web.archive.org/web/20140209003822/http://ababj.com/component/k2/item/4354-what-you-should-know-about-worsening-ddos-attacks | archive-date=2014-02-09 }}</ref> According to research by [[Akamai Technologies]], there have been "51 percent more application layer attacks" from Q4 2013 to Q4 2014 and "16 percent more" from Q3 2014 to Q4 2014.<ref>{{cite web|url=https://blogs.akamai.com/2015/01/q4-2014-state-of-the-internet---security-report-some-numbers.html|title=Q4 2014 State of the Internet - Security Report: Numbers - The Akamai Blog|website=blogs.akamai.com}}</ref> In November 2017; Junade Ali, an engineer at Cloudflare noted that whilst network-level attacks continue to be of high capacity, they were occurring less frequently. Ali further noted that although network-level attacks were becoming less frequent, data from Cloudflare demonstrated that application-layer attacks were still showing no sign of slowing down.<ref>{{cite web|last1=Ali|first1=Junade|title=The New DDoS Landscape|url=https://blog.cloudflare.com/the-new-ddos-landscape/|website=Cloudflare Blog|date=23 November 2017}}</ref>

====Application layer====
The [[OSI model]] (ISO/IEC 7498-1) is a conceptual model that characterizes and standardizes the internal functions of a communication system by partitioning it into [[abstraction layer]]s. The model is a product of the [[Open Systems Interconnection]] project at the [[International Organization for Standardization]] (ISO). The model groups similar communication functions into one of seven logical layers. A layer serves the layer above it and is served by the layer below it. For example, a layer that provides error-free communications across a network provides the communications path needed by applications above it, while it calls the next lower layer to send and receive packets that traverse that path. In the OSI model, the definition of its application layer is narrower in scope than is often implemented. The OSI model defines the application layer as being the user interface. The OSI application layer is responsible for displaying data and images to the user in a human-recognizable format and to interface with the [[presentation layer]] below it. In an implementation, the application and presentation layers are frequently combined.

====Method of attack====
The simplest DoS attack relies 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's ability to generate the overwhelming flux of packets. A common way of achieving this today is via distributed denial-of-service, employing a [[botnet]]. An application layer DDoS attack is done mainly for specific targeted purposes, including disrupting transactions and access to databases. It requires fewer resources than network layer attacks but often accompanies them.<ref>{{cite news |last=Higgins |first=Kelly Jackson |title=DDoS Attack Used 'Headless' Browser In 150-Hour Siege |newspaper=Dark Reading |publisher=InformationWeek |date=17 October 2013 |url=http://www.darkreading.com/attacks-breaches/ddos-attack-used-headless-browsers-in-15/240162777 |access-date=28 January 2014 |url-status=dead |archive-url=https://web.archive.org/web/20140122165039/http://www.darkreading.com/attacks-breaches/ddos-attack-used-headless-browsers-in-15/240162777 |archive-date=January 22, 2014 }}</ref> An attack may be disguised to look like legitimate traffic, except it targets specific application packets or functions. The attack on the application layer can disrupt services such as the retrieval of information or search functions on a website.<ref name="AbABankinJournal" />

===Advanced persistent DoS===
An '''advanced persistent DoS''' (APDoS) is associated with an [[advanced persistent threat]] and requires specialized [[DDoS mitigation]].<ref name=":0">{{Cite book|title=Cyberwarfare Sourcebook|last=Kiyuna and Conyers|year=2015|publisher=Lulu.com |isbn=978-1329063945}}</ref> These attacks can persist for weeks; the longest continuous period noted so far lasted 38 days. This attack involved approximately 50+ petabits (50,000+ terabits) of malicious traffic.<ref>{{cite news |last1=Ilascu |first1=Ionut |title=38-Day Long DDoS Siege Amounts to Over 50 Petabits in Bad Traffic |url=https://news.softpedia.com/news/38-Day-Long-DDoS-Siege-Amounts-to-Over-50-Petabits-in-Bad-Traffic-455722.shtml |access-date=29 July 2018 |agency=Softpedia News |date=Aug 21, 2014}}</ref> Attackers in this scenario may tactically switch between several targets to create a diversion to evade defensive DDoS countermeasures but all the while eventually concentrating the main thrust of the attack onto a single victim. In this scenario, attackers with continuous access to several very powerful network resources are capable of sustaining a prolonged campaign generating enormous levels of unamplified DDoS traffic. APDoS attacks are characterized by:
* advanced reconnaissance (pre-attack [[open-source intelligence|OSINT]] and extensive decoyed scanning crafted to evade detection over long periods)
* tactical execution (attack with both primary and secondary victims but the focus is on primary)
* explicit motivation (a calculated end game/goal target)
* large computing capacity (access to substantial computer power and network bandwidth)
* simultaneous multi-threaded OSI layer attacks (sophisticated tools operating at layers 3 through 7)
* persistence over extended periods (combining all the above into a concerted, well-managed attack across a range of targets).<ref>{{cite web|url=http://www.scmagazineuk.com/video-games-company-hit-by-38-day-ddos-attack/article/367329/|archive-url=https://web.archive.org/web/20170201181833/https://www.scmagazineuk.com/video-games-company-hit-by-38-day-ddos-attack/article/541275/|archive-date=2017-02-01|title=Video games company hit by 38-day DDoS attack|last=Gold|first=Steve|date=21 August 2014|work=SC Magazine UK|access-date=4 February 2016}}</ref>

===Denial-of-service as a service===
{{Main|Stresser}}
Some vendors provide so-called ''booter'' or ''stresser'' services, which have simple web-based front ends, and accept payment over the web. Marketed and promoted as stress-testing tools, they can be used to perform unauthorized denial-of-service attacks, and allow technically unsophisticated attackers access to sophisticated attack tools.<ref>{{Cite web|url=http://krebsonsecurity.com/2015/08/stress-testing-the-booter-services-financially/|title=Stress-Testing the Booter Services, Financially|last=Krebs|first=Brian|date=August 15, 2015|website=Krebs on Security|access-date=2016-09-09}}</ref> Usually powered by a botnet, the traffic produced by a consumer stresser can range anywhere from 5-50&nbsp;Gbit/s, which can, in most cases, deny the average home user internet access.<ref>{{Cite journal|last1=Mubarakali|first1=Azath|last2=Srinivasan|first2=Karthik|last3=Mukhalid|first3=Reham|last4=Jaganathan|first4=Subash C. B.|last5=Marina|first5=Ninoslav|date=2020-01-26|title=Security challenges in internet of things: Distributed denial of service attack detection using support vector machine-based expert systems|url=https://onlinelibrary.wiley.com/doi/10.1111/coin.12293|journal=Computational Intelligence|language=en|volume=36|issue=4|pages=1580–1592|doi=10.1111/coin.12293|s2cid=214114645|issn=0824-7935}}</ref>

===Markov-modulated denial-of-service attack===
A Markov-modulated denial-of-service attack occurs when the attacker disrupts control packets using a [[hidden Markov model]]. A setting in which Markov-model based attacks are prevalent is online gaming as the disruption of the control packet undermines game play and system functionality.<ref>{{Cite journal |title=Risk-Sensitive Control Under Markov Modulated Denial-of-Service (DoS) Attack Strategies |url=https://ieeexplore.ieee.org/document/7070734 |access-date=2023-10-19 |journal=IEEE Transactions on Automatic Control |date=2015 |doi=10.1109/TAC.2015.2416926 |s2cid=9510043 |language=en-US |last1=Befekadu |first1=Getachew K. |last2=Gupta |first2=Vijay |last3=Antsaklis |first3=Panos J. |volume=60 |issue=12 |pages=3299–3304 }}</ref>

==Symptoms==
The [[United States Computer Emergency Readiness Team]] (US-CERT) has identified symptoms of a denial-of-service attack to include:<ref name="US-CERT1">{{cite web|url=http://www.us-cert.gov/ncas/tips/st04-015|title=Cyber Security Tip ST04-015 - Understanding Denial-of-Service Attacks |publisher=[[United States Computer Emergency Readiness Team]] |date=November 4, 2009 |first=Mindi |last=McDowell |access-date=December 11, 2013 |archive-url=https://web.archive.org/web/20131104052804/http://www.us-cert.gov/ncas/tips/st04-015 |archive-date=2013-11-04 |url-status=live}}</ref>
* unusually slow [[network performance]] (opening files or accessing websites),
* unavailability of a particular website, or
* inability to access any website.

==Attack techniques==

===Attack tools===
In cases such as [[MyDoom]] and [[Slowloris (computer security)|Slowloris]], the tools are embedded in [[malware]] and launch their attacks without the knowledge of the system owner. [[Stacheldraht]] is a classic example of a DDoS tool. It uses a layered structure where the attacker uses a [[Client (computing)|client program]] to connect to handlers which are compromised systems that issue commands to the [[Zombie computer|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.<ref name="Dittrich" />

In other cases a machine may become part of a DDoS attack with the owner's consent, for example, in [[Operation Payback]] organized by the group [[Anonymous (hacker group)|Anonymous]]. The [[Low Orbit Ion Cannon]] has typically been used in this way. Along with [[High Orbit Ion Cannon]] a wide variety of DDoS tools are available today, including paid and free versions, with different features available. There is an underground market for these in hacker-related forums and IRC channels.

===Application-layer attacks===
Application-layer attacks employ DoS-causing [[exploit (computer security)|exploits]] and can cause server-running software to fill the disk space or consume all available memory or [[CPU time]]. Attacks 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 shell-level access to a victim's computer may slow it until it is unusable or crash it by using a [[fork bomb]]. Another kind of application-level DoS attack is XDoS (or XML DoS) which can be controlled by modern web [[application firewall]]s (WAFs). All attacks belonging to the category of ''timeout exploiting''.<ref>Cambiaso, Enrico; Papaleo, Gianluca; Chiola, Giovanni; Aiello, Maurizio (2015). "Designing and modeling the slow next DoS attack". ''Computational Intelligence in Security for Information Systems Conference (CISIS 2015)''. 249-259. Springer.</ref>

[[Slow DoS attack]]s implement an application-layer attack. Examples of threats are Slowloris, establishing pending connections with the victim, or [[Slowdroid|SlowDroid]], an attack running on mobile devices. Another target of DDoS attacks may be to produce added costs for the application operator, when the latter uses resources based on [[cloud computing]]. In this case, normally application-used resources are tied to a needed quality of service (QoS) level (e.g. responses should be less than 200&nbsp;ms) and this rule is usually linked to automated software (e.g. Amazon CloudWatch<ref>{{cite web|url=http://aws.amazon.com/cloudwatch/|title=Amazon CloudWatch|work=Amazon Web Services, Inc.}}</ref>) to raise more virtual resources from the provider to meet the defined QoS levels for the increased requests. The main incentive behind such attacks may be to drive the application owner to raise the elasticity levels to handle the increased application traffic, to cause financial losses, or force them to become less competitive. 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. A [[LAND]] attack is of this type.

===Degradation-of-service attacks===
Pulsing zombies are compromised computers that are directed to launch intermittent and short-lived floodings of victim websites with the intent of merely slowing it rather than crashing it. This type of attack, referred to as ''degradation-of-service'', can be more difficult to detect and can disrupt and hamper connection to websites for prolonged periods of time, potentially causing more overall disruption than a denial-of-service attack.<ref>{{cite book |title=Encyclopaedia Of Information Technology |page=397 |publisher=Atlantic Publishers & Distributors |year=2007 |isbn=978-81-269-0752-6}}</ref><ref>{{cite book|title=Internet and the Law |year=2006 |page=325 |first=Aaron |last=Schwabach |publisher=ABC-CLIO |isbn=978-1-85109-731-9}}</ref> Exposure of degradation-of-service attacks is complicated further by the matter of discerning whether the server is really being attacked or is experiencing higher than normal legitimate traffic loads.<ref>{{cite book|title=Networking and Mobile Computing |first=Xicheng |last=Lu |author2=Wei Zhao |publisher=Birkhäuser |year=2005 |isbn=978-3-540-28102-3 |page=424}}</ref>

===Distributed DoS attack===
If an attacker mounts an attack from a single host, it would be classified as a DoS attack. Any attack against availability would be classed as a denial-of-service attack. On the other hand, if an attacker uses many systems to simultaneously launch attacks against a remote host, this would be classified as a DDoS attack. [[Malware]] can carry DDoS attack mechanisms; one of the better-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]] before releasing the malware and no further interaction was necessary to launch the attack. A system may also be compromised with a [[Trojan horse (computing)|trojan]] containing a [[Zombie computer|zombie agent]]. 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 uses a layered structure where the attacker uses a [[Client (computing)|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. Each handler can control up to a thousand agents.<ref name="Dittrich">{{cite web |url=http://staff.washington.edu/dittrich/misc/stacheldraht.analysis.txt |title=The "stacheldraht" distributed denial of service attack tool |first=David |last=Dittrich |publisher=University of Washington |date=December 31, 1999 |access-date=2013-12-11 |archive-date=2000-08-16 |archive-url=https://web.archive.org/web/20000816021357/http://staff.washington.edu/dittrich/misc/stacheldraht.analysis.txt |url-status=dead }}</ref> In some cases a machine may become part of a DDoS attack with the owner's consent, for example, in [[Operation Payback]], organized by the group [[Anonymous (hacker group)|Anonymous]]. These attacks can use different types of internet packets such as TCP, UDP, ICMP, etc.

These collections of compromised systems are known as [[botnet]]s. DDoS tools like [[Stacheldraht]] still use classic DoS attack methods centered on [[IP spoofing]] and amplification like [[smurf attack]]s and [[fraggle attack]]s (types of bandwidth consumption attacks). [[SYN flood]]s (a resource starvation attack) may also be used. Newer tools can use DNS servers for DoS purposes. Unlike MyDoom's DDoS mechanism, botnets can be turned against any IP address. [[Script kiddie]]s use them to deny the availability of well known websites to legitimate users.<ref name="SANS">{{cite web|url=http://www.sans.org/resources/idfaq/trinoo.php|title=SANS Institute – Intrusion Detection FAQ: Distributed Denial of Service Attack Tools: n/a|access-date=2008-05-02|publisher=SANS Institute|year=2000|first=Phillip|last=Boyle|archive-url=https://web.archive.org/web/20080515025103/http://www.sans.org/resources/idfaq/trinoo.php|archive-date=2008-05-15|url-status=dead}}</ref> More sophisticated attackers use DDoS tools for the purposes of [[extortion]]{{spaced ndash}}including against their business rivals.<ref>{{cite web|last=Leyden |first=John |url=https://www.theregister.co.uk/2004/09/23/authorize_ddos_attack/ |title=US credit card firm fights DDoS attack |work=The Register |date=2004-09-23 |access-date=2011-12-02}}</ref> It has been reported that there are new attacks from [[internet of things]] (IoT) devices that have been involved in denial of service attacks.<ref>{{cite web|url=http://thehackernews.com/2015/10/cctv-camera-hacking.html|title=Hacking CCTV Cameras to Launch DDoS Attacks|author=Swati Khandelwal|date=23 October 2015|work=The Hacker News}}</ref> In one noted attack that was made peaked at around 20,000 requests per second which came from around 900 CCTV cameras.<ref>{{cite web|url=https://www.incapsula.com/blog/cctv-ddos-botnet-back-yard.html|title=CCTV DDoS Botnet In Our Own Back Yard|first1=Igal|last1=Zeifman|first2=Ofer|last2=Gayer|first3=Or|last3=Wilder|website=incapsula.com|date=21 October 2015}}</ref> UK's [[GCHQ]] has tools built for DDoS, named PREDATORS FACE and ROLLING THUNDER.<ref name="firstlook.org">{{cite web |date=2014-07-15 |author= Glenn Greenwald |url=https://theintercept.com/2014/07/14/manipulating-online-polls-ways-british-spies-seek-control-internet/ |title=HACKING ONLINE POLLS AND OTHER WAYS BRITISH SPIES SEEK TO CONTROL THE INTERNET |website=The Intercept_ |access-date=2015-12-25}}</ref>

Simple attacks such as SYN floods may appear with a wide range of source IP addresses, giving the appearance of a distributed DoS. These flood attacks do not require completion of the TCP [[three-way handshake]] and attempt to exhaust the destination SYN queue or the server bandwidth. Because the source IP addresses can be trivially spoofed, an attack could come from a limited set of sources, or may even originate from a single host. Stack enhancements such as [[SYN cookies]] may be effective mitigation against SYN queue flooding but do not address bandwidth exhaustion. In 2022, TCP attacks were the leading method in DDoS incidents, accounting for 63% of all DDoS activity. This includes tactics like [[TCP SYN]], TCP ACK, and TCP floods. With TCP being the most widespread networking protocol, its attacks are expected to remain prevalent in the DDoS threat scene.<ref name=":2" />

===DDoS extortion===
In 2015, DDoS botnets such as DD4BC grew in prominence, taking aim at financial institutions.<ref>{{cite news|title=Who's Behind DDoS Attacks and How Can You Protect Your Website?|url=http://blog.cloudbric.com/2015/09/whos-behind-ddos-attacks-and-how-can.html|access-date=15 September 2015|agency=Cloudbric|date=10 September 2015}}</ref> Cyber-extortionists typically begin with a low-level attack and a warning that a larger attack will be carried out if a ransom is not paid in [[bitcoin]].<ref>{{cite news|last1=Solon|first1=Olivia|title=Cyber-Extortionists Targeting the Financial Sector Are Demanding Bitcoin Ransoms|url=https://www.bloomberg.com/news/articles/2015-09-09/bitcoin-ddos-ransom-demands-raise-dd4bc-profile?mod=djemRiskCompliance|access-date=15 September 2015|agency=Bloomberg|date=9 September 2015}}</ref> Security experts recommend targeted websites to not pay the ransom. The attackers tend to get into an extended extortion scheme once they recognize that the target is ready to pay.<ref>{{cite news|last1=Greenberg|first1=Adam|title=Akamai warns of increased activity from DDoS extortion group|url=http://www.scmagazineuk.com/akamai-warns-of-increased-activity-from-ddos-extortion-group/article/438333/|access-date=15 September 2015|agency=SC Magazine|date=14 September 2015}}</ref>

===HTTP slow POST DoS attack===
First discovered in 2009, the HTTP slow POST attack sends a complete, legitimate [[POST (HTTP)|HTTP POST header]], which includes a ''Content-Length'' field to specify the size of the message body to follow. However, the attacker then proceeds to send the actual message body at an extremely slow rate (e.g. 1 byte/110 seconds). Due to the entire message being correct and complete, the target server will attempt to obey the ''Content-Length'' field in the header, and wait for the entire body of the message to be transmitted, which can take a very long time. The attacker establishes hundreds or even thousands of such connections until all resources for incoming connections on the victim server are exhausted, making any further connections impossible until all data has been sent. It is notable that unlike many other DDoS or DDoS attacks, which try to subdue the server by overloading its network or CPU, an HTTP slow POST attack targets the ''logical'' resources of the victim, which means the victim would still have enough network bandwidth and processing power to operate.<ref>{{cite web |url=https://www.owasp.org/images/4/43/Layer_7_DDOS.pdf |archive-url=https://ghostarchive.org/archive/20221009/https://www.owasp.org/images/4/43/Layer_7_DDOS.pdf |archive-date=2022-10-09 |url-status=live|title=OWASP Plan - Strawman - Layer_7_DDOS.pdf |date=18 March 2014|website= Open Web Application Security Project|access-date=18 March 2014}}</ref> Combined with the fact that the [[Apache HTTP Server]] will, by default, accept requests up to 2GB in size, this attack can be particularly powerful. HTTP slow POST attacks are difficult to differentiate from legitimate connections and are therefore able to bypass some protection systems. [[OWASP]], an [[Open-source model|open source]] web application security project, released a tool to test the security of servers against this type of attack.<ref>{{cite web |url=https://www.owasp.org/index.php/OWASP_HTTP_Post_Tool |title=OWASP HTTP Post Tool |archive-url=https://web.archive.org/web/20101221131703/https://www.owasp.org/index.php/OWASP_HTTP_Post_Tool |archive-date=2010-12-21}}</ref>

===Challenge Collapsar (CC) attack===
A Challenge Collapsar (CC) attack is an attack where standard HTTP requests are sent to a targeted web server frequently. The [[Uniform Resource Identifier]]s (URIs) in the requests require complicated time-consuming algorithms or database operations which may exhaust the resources of the targeted web server.<ref>{{cite web|url=https://support.huaweicloud.com/en-us/antiddos_faq/antiddos_01_0020.html|title=What Is a CC Attack?|website=HUAWEI CLOUD-Grow With Intelligence|language=en|access-date=2019-03-05|url-status=live|archive-url=https://web.archive.org/web/20190305080627/https://support.huaweicloud.com/en-us/antiddos_faq/antiddos_01_0020.html|archive-date=2019-03-05}}</ref><ref>{{cite web|url=https://patents.google.com/patent/CN106161451A/en|author=刘鹏|author2=郭洋|title=CC (challenge collapsar) attack defending method, device and system|website=Google Patents|language=en|access-date=2018-03-05|url-status=live|archive-url=https://web.archive.org/web/20190305080850/https://patents.google.com/patent/CN106161451A/en|archive-date=2019-03-05}}</ref><ref>{{cite web|url=https://patents.google.com/patent/CN106330911A/en|author=曾宪力|author2=史伟|author3=关志来|author4=彭国柱|title=CC (Challenge Collapsar) attack protection method and device|website=Google Patents|language=en|access-date=2018-03-05|url-status=live|archive-url=https://web.archive.org/web/20190305081050/https://patents.google.com/patent/CN106330911A/en|archive-date=2019-03-05}}</ref> In 2004, a Chinese hacker nicknamed KiKi invented a hacking tool to send these kinds of requests to attack a NSFOCUS firewall named Collapsar, and thus the hacking tool was known as Challenge Collapsar, or ''CC'' for short. Consequently, this type of attack got the name ''CC attack''.<ref>{{cite web|url=http://digi.163.com/14/0724/19/A1UL2O95001618JV.html|title=史上最臭名昭著的黑客工具 CC的前世今生|language=zh-hans|website=NetEase|publisher=驱动中国网(北京)|date=2014-07-24|access-date=2019-03-05|url-status=dead|archive-url=https://web.archive.org/web/20190305080935/http://digi.163.com/14/0724/19/A1UL2O95001618JV.html|archive-date=2019-03-05}}</ref>

===Internet Control Message Protocol (ICMP) flood===
A [[smurf attack]] 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 attacker will send large numbers of [[Internet Protocol|IP]] packets with the source address faked to appear to be the address of the victim.<ref>{{Cite journal |last=Sun |first=Fei Xian |date=2011 |title=Danger Theory Based Risk Evaluation Model for Smurf Attacks |url=https://www.scientific.net/KEM.467-469.515 |journal=Key Engineering Materials |language=en |volume=467-469 |pages=515–521 |doi=10.4028/www.scientific.net/KEM.467-469.515 |s2cid=110045205 |issn=1662-9795}}</ref> Most devices on a network will, by default, respond to this by sending a reply to the source IP address. If the number of machines on the network that receive and respond to these packets is very large, the victim's computer will be flooded with traffic. This overloads the victim's computer and can even make it unusable during such an attack.<ref name="ANML-DDoS">{{cite web|url=http://anml.iu.edu/ddos/types.html|title=Types of DDoS Attacks |publisher=Advanced Networking Management Lab (ANML) |work=Distributed Denial of Service Attacks(DDoS) Resources, Pervasive Technology Labs at Indiana University |date=December 3, 2009 |archive-url=https://web.archive.org/web/20100914222536/http://anml.iu.edu/ddos/types.html |archive-date=2010-09-14 |access-date=December 11, 2013 }}</ref>

[[Ping flood]] is based on sending the victim an overwhelming number of [[ping (networking utility)|ping]] packets, usually using the ''ping'' command from [[Unix-like]] hosts.{{Efn|The -t flag on [[Microsoft Windows|Windows]] systems is much less capable of overwhelming a target, also the -l (size) flag does not allow sent packet size greater than 65500 in Windows.}} It is very simple to launch, the primary requirement being access to greater [[bandwidth (computing)|bandwidth]] than the victim. [[Ping of death]] is based on sending the victim a malformed ping packet, which will lead to a system crash on a vulnerable system. The [[BlackNurse (Computer Security)|BlackNurse]] attack is an example of an attack taking advantage of the required Destination Port Unreachable ICMP packets.

===Nuke===
A nuke is an old-fashioned denial-of-service attack against [[computer network]]s consisting of fragmented or otherwise invalid [[Internet Control Message Protocol|ICMP]] packets sent to the target, achieved by using a modified [[ping (networking utility)|ping]] utility to repeatedly send this [[Data corruption|corrupt data]], thus slowing down the affected computer until it comes to a complete stop.<ref name="Nuke">{{Cite web|url=https://security.radware.com/ddos-knowledge-center/ddospedia/nuke/|title=What Is a Nuke? {{!}} Radware — DDoSPedia|website=security.radware.com|access-date=2019-09-16}}</ref> 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 [[Transmission Control Protocol|TCP]] port 139 of the victim's machine, causing it to lock up and display a [[Blue Screen of Death]].<ref name="Nuke" />

===Peer-to-peer attacks===
{{See also|Direct Connect (protocol)#Direct Connect used for DDoS attacks}}
Attackers have found a way to exploit a number of bugs in [[peer-to-peer]] servers to initiate DDoS attacks. The most aggressive of these peer-to-peer-DDoS attacks exploits [[DC++]].{{cn|date=December 2024}}{{ambiguous|date=December 2024}} With peer-to-peer there is no botnet and the attacker does not have to communicate with the clients it subverts. Instead, the attacker acts as a ''puppet master'', instructing clients of large [[file sharing|peer-to-peer file sharing]] hubs to disconnect from their peer-to-peer network and to connect to the victim's website instead.<ref>{{cite web|url=http://www.prolexic.com/news/20070514-alert.php|title=Prolexic Distributed Denial of Service Attack Alert |access-date=2007-08-22|author=Paul Sop|date=May 2007|work=Prolexic Technologies Inc.|archive-url = https://web.archive.org/web/20070803175513/http://www.prolexic.com/news/20070514-alert.php  |archive-date = 2007-08-03}}</ref><ref>{{cite web|url=http://www.securityfocus.com/news/11466|title=Peer-to-peer networks co-opted for DOS attacks|access-date=2007-08-22|author=Robert Lemos|date=May 2007|publisher=SecurityFocus|archive-date=2015-09-24|archive-url=https://web.archive.org/web/20150924114938/http://www.securityfocus.com/news/11466|url-status=dead}}</ref><ref>{{cite web|url=http://dcpp.wordpress.com/2007/05/22/denying-distributed-attacks/|title=Denying distributed attacks|access-date=2007-08-22|author=Fredrik Ullner|date=May 2007|publisher=DC++: Just These Guys, Ya Know? }}</ref>

===Permanent denial-of-service attacks===
Permanent denial-of-service (PDoS), also known loosely as phlashing,<ref>{{cite news|title=Phlashing attack thrashes embedded systems|first=John|last=Leyden|date=2008-05-21|access-date=2009-03-07|work=The Register|url=https://www.theregister.co.uk/2008/05/21/phlashing/}}</ref> is an attack that damages a system so badly that it requires replacement or reinstallation of hardware.<ref name="TechWeb">{{cite web|url=http://www.darkreading.com/document.asp?doc_id=154270&WT.svl=news1_1|title=Permanent Denial-of-Service Attack Sabotages Hardware|publisher=Dark Reading |date=May 19, 2008 |first=Kelly |last=Jackson Higgins |archive-url=https://web.archive.org/web/20081208002732/http://www.darkreading.com/security/management/showArticle.jhtml?articleID=211201088 |archive-date=December 8, 2008}}</ref> Unlike the distributed denial-of-service attack, a PDoS attack exploits security flaws which allow remote administration on the management interfaces of the victim's hardware, such as [[Router (computing)|routers]], printers, or other [[networking hardware]]. The attacker uses these [[Vulnerability (computer security)|vulnerabilities]] to replace a device's [[firmware]] with a modified, corrupt, or defective firmware image—a process which when done legitimately is known as ''flashing.'' The intent is to [[Brick (electronics)|brick]] the device, rendering it unusable for its original purpose until it can be repaired or replaced. The PDoS is a pure hardware-targeted attack that can be much faster and requires fewer resources than using a botnet in a DDoS attack. Because of these features, and the potential and high probability of security exploits on network-enabled embedded devices, this technique has come to the attention of numerous hacking communities. [[BrickerBot]], a piece of malware that targeted IoT devices, used PDoS attacks to disable its targets.<ref>{{cite web|title="BrickerBot" Results In PDoS Attack|url=https://security.radware.com/ddos-threats-attacks/brickerbot-pdos-permanent-denial-of-service/|website=Radware|access-date=January 22, 2019|date=May 4, 2017}}</ref> PhlashDance is a tool created by Rich Smith (an employee of [[Hewlett-Packard]]'s Systems Security Lab) used to detect and demonstrate PDoS vulnerabilities at the 2008 EUSecWest Applied Security Conference in London, UK.<ref name="EUSecWest">{{cite web|url=http://eusecwest.com/speakers.html#PhlashDance|archive-url=https://web.archive.org/web/20090201173324/http://eusecwest.com/speakers.html#PhlashDance|archive-date=2009-02-01|title=EUSecWest Applied Security Conference: London, U.K.|publisher=EUSecWest|year=2008}}</ref>

===Reflected attack===
A distributed denial-of-service attack may involve sending forged requests of some type to a very large number of computers that will reply to the requests. Using [[IP address spoofing|Internet Protocol address spoofing]], the source address is set to that of the targeted victim, which means all the replies will go to (and flood) the target. This reflected attack form is sometimes called a '''distributed reflective denial-of-service''' ('''DRDoS''') attack.<ref>{{cite web|url=http://www.internetsociety.org/sites/default/files/01_5.pdf|title=Amplification Hell: Revisiting Network Protocols for DDoS Abuse|first=Christian|last=Rossow|publisher=Internet Society|date=February 2014|access-date=4 February 2016|archive-url=https://web.archive.org/web/20160304015033/http://www.internetsociety.org/sites/default/files/01_5.pdf|archive-date=4 March 2016|url-status=dead}}</ref> [[ICMP echo request]] attacks ([[Smurf attack]]s) can be considered one form of reflected attack, as the flooding hosts send Echo Requests to the broadcast addresses of mis-configured networks, thereby enticing hosts to send Echo Reply packets to the victim. Some early DDoS programs implemented a distributed form of this attack.

===Amplification===
Amplification attacks are used to magnify the bandwidth that is sent to a victim. Many services can be exploited to act as reflectors, some harder to block than others.<ref>{{cite web |last=Paxson|first=Vern |year=2001|url=http://www.icir.org/vern/papers/reflectors.CCR.01/reflectors.html |title=An Analysis of Using Reflectors for Distributed Denial-of-Service Attacks |publisher=ICIR.org}}</ref> US-CERT have observed that different services may result in different amplification factors, as tabulated below:<ref>{{cite web |date=July 8, 2014 |title=Alert (TA14-017A) UDP-based Amplification Attacks |publisher=US-CERT |url=http://www.us-cert.gov/ncas/alerts/TA14-017A |access-date=2014-07-08}}</ref>

{| class="wikitable"
|+ UDP-based amplification attacks
|-
! Protocol
! Amplification factor
! Notes
|-
| [[Mitel]] MiCollab
| 2,200,000,000<ref>{{cite web| url=https://blog.cloudflare.com/cve-2022-26143-amplification-attack/ |title=CVE-2022-26143: A Zero-Day vulnerability for launching UDP amplification DDoS attacks|website=[[Cloudflare]] Blog|date=2022-03-08|access-date=16 March 2022}}</ref>
|
|-
| [[Memcached]]
| 50,000
| Fixed in version 1.5.6<ref>{{cite web| url=https://github.com/memcached/memcached/wiki/ReleaseNotes156 |title=Memcached 1.5.6 Release Notes|website=[[GitHub]]|date=2018-02-27|access-date=3 March 2018}}</ref>
|-
| [[Network Time Protocol|NTP]]
| 556.9
| Fixed in version 4.2.7p26<ref>{{cite web|url=http://support.ntp.org/bin/view/Main/SecurityNotice#April_2010_DRDoS_Amplification_A|title=DRDoS / Amplification Attack using ntpdc monlist command|publisher=support.ntp.org|date=2010-04-24|access-date=2014-04-13}}</ref>
|-
| [[CHARGEN]]
| 358.8
|
|-
| [[DNS]]
| up to 179<ref>{{Cite book |last=van Rijswijk-Deij|first=Roland |title=Proceedings of the 2014 Conference on Internet Measurement Conference |chapter=DNSSEC and its potential for DDoS attacks: A comprehensive measurement study |year=2014|pages=449–460 |publisher=ACM Press|doi=10.1145/2663716.2663731 |isbn=9781450332132 |s2cid=2094604 |url=https://research.utwente.nl/en/publications/dnssec-and-its-potential-for-ddos-attacks--a-comprehensive-measurement-study(cb44e199-21c2-4486-ba0e-8a27c80b8a4f).html }}</ref>
|
|-
| [[QOTD]]
| 140.3
|
|-
| [[Quake engine#Network play|Quake Network Protocol]]
| 63.9
| Fixed in version 71
|-
| [[BitTorrent]]
| 4.0 - 54.3<ref>{{cite web |last=Adamsky|first=Florian |year=2015|url=https://www.usenix.org/conference/woot15/workshop-program/presentation/p2p-file-sharing-hell-exploiting-bittorrent|title=P2P File-Sharing in Hell: Exploiting BitTorrent Vulnerabilities to Launch Distributed Reflective DoS Attacks}}</ref>
| Fixed in libuTP since 2015
|-
| [[CoAP]]
| 10 - 50
|
|-
| ARMS
| 33.5
|
|-
| [[Simple Service Discovery Protocol|SSDP]]
| 30.8
|
|-
| [[Kad network|Kad]]
| 16.3
|
|-
| [[SNMPv2]]
| 6.3
|
|-
| [[Steam (service)|Steam Protocol]]
| 5.5
|
|-
| [[NetBIOS]]
| 3.8
|
|}

[[Domain Name System|DNS]] amplification attacks involves an attacker sending a DNS name lookup request to one or more public DNS servers, spoofing the source IP address of the targeted victim. The attacker tries to request as much information as possible, thus amplifying the DNS response that is sent to the targeted victim. Since the size of the request is significantly smaller than the response, the attacker is easily able to increase the amount of traffic directed at the target.<ref>{{cite web |year=2006|url=http://www.isotf.org/news/DNS-Amplification-Attacks.pdf |title=DNS Amplification Attacks |publisher=ISOTF |archive-url=https://web.archive.org/web/20101214074629/http://www.isotf.org/news/DNS-Amplification-Attacks.pdf |archive-date=2010-12-14|author1=Vaughn, Randal |author2=Evron, Gadi }}</ref><ref>{{cite web |date=July 8, 2013 |title=Alert (TA13-088A) DNS Amplification Attacks |publisher=US-CERT |url=http://www.us-cert.gov/ncas/alerts/TA13-088A |access-date=2013-07-17}}</ref>

[[Simple Network Management Protocol]] (SNMP) and [[Network Time Protocol]] (NTP) can also be exploited as reflectors in an amplification attack. An example of an amplified DDoS attack through the NTP is through a command called monlist, which sends the details of the last 600 hosts that have requested the time from the NTP server back to the requester. A small request to this time server can be sent using a spoofed source IP address of some victim, which results in a response 556.9 times the size of the request being sent to the victim. This becomes amplified when using botnets that all send requests with the same spoofed IP source, which will result in a massive amount of data being sent back to the victim. It is very difficult to defend against these types of attacks because the response data is coming from legitimate servers. These attack requests are also sent through UDP, which does not require a connection to the server. This means that the source IP is not verified when a request is received by the server. To bring awareness of these vulnerabilities, campaigns have been started that are dedicated to finding amplification vectors which have led to people fixing their resolvers or having the resolvers shut down completely.{{citation needed|date=May 2022}}

===Mirai botnet===
The [[Mirai (malware)|Mirai botnet]] works by using a [[computer worm]] to infect hundreds of thousands of IoT devices across the internet. The worm propagates through networks and systems taking control of poorly protected IoT devices such as thermostats, Wi-Fi-enabled clocks, and washing machines.<ref name="Mirai">{{Cite journal |title=DDoS in the IoT: Mirai and Other Botnets |journal=Computer |volume=50 |issue=7 |pages=80–84 |language=en-US |doi=10.1109/MC.2017.201|year=2017 |last1=Kolias |first1=Constantinos |last2=Kambourakis |first2=Georgios |last3=Stavrou |first3=Angelos |last4=Voas |first4=Jeffrey |s2cid=35958086 }}</ref> The owner or user will usually have no immediate indication of when the device becomes infected. The IoT device itself is not the direct target of the attack, it is used as a part of a larger attack.<ref name="Kuzmanovic 75–86">{{Cite book|last1=Kuzmanovic|first1=Aleksandar|last2=Knightly|first2=Edward W.|title=Proceedings of the 2003 conference on Applications, technologies, architectures, and protocols for computer communications |chapter=Low-rate TCP-targeted denial of service attacks: The shrew vs. The mice and elephants |date=2003-08-25|publisher=ACM|pages=75–86|doi=10.1145/863955.863966|isbn=978-1581137354|citeseerx=10.1.1.307.4107|s2cid=173992197 }}</ref> Once the hacker has enslaved the desired number of devices, they instruct the devices to try to contact an ISP. In October 2016, a Mirai botnet [[DDoS attacks on Dyn|attacked Dyn]] which is the ISP for sites such as [[Twitter]], [[Netflix]], etc.<ref name="Mirai"/> As soon as this occurred, these websites were all unreachable for several hours.

===R-U-Dead-Yet? (RUDY)===
RUDY attack targets web applications by starvation of available sessions on the web server. Much like Slowloris, RUDY keeps sessions at halt using never-ending POST transmissions and sending an arbitrarily large content-length header value.<ref>{{Cite web|url=https://sourceforge.net/projects/r-u-dead-yet/|title = R-u-dead-yet| date=8 September 2016 }}{{primary source inline|date=March 2023}}</ref>

===SACK Panic===
Manipulating [[maximum segment size]] and [[selective acknowledgement]] (SACK) may be used by a remote peer to cause a denial of service by an [[integer overflow]] in the Linux kernel, potentially causing a [[kernel panic]].<ref name = "SACKPanic, Ubuntu wiki, 2019">{{ cite web | url = https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/SACKPanic | title = SACK Panic and Other TCP Denial of Service Issues | access-date = 21 June 2019 | date = 17 June 2019 | website = [[Ubuntu]] Wiki | archive-url = https://web.archive.org/web/20190619100453/https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/SACKPanic | archive-date = 19 June 2019 | df = dmy-all }}</ref> Jonathan Looney discovered {{CVE|2019-11477|2019-11478|2019-11479}} on June 17, 2019.<ref name = "CVE-2019-11479" >{{ cite web | url = https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-11479 | title = CVE-2019-11479 | access-date = 21 June 2019 | website = [[Common Vulnerabilities and Exposures|CVE]] | archive-url = https://web.archive.org/web/20190621224631/https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-11479 | archive-date = 21 June 2019 | df = dmy-all }}</ref>

===Shrew attack===
The shrew attack is a denial-of-service attack on the [[Transmission Control Protocol]] where the attacker employs [[man-in-the-middle attack|man-in-the-middle techniques]]. It exploits a weakness in TCP's re-transmission timeout mechanism, using short synchronized bursts of traffic to disrupt TCP connections on the same link.<ref>{{Cite book | last1 = Yu Chen | last2 = Kai Hwang | last3 = Yu-Kwong Kwok | doi = 10.1109/LCN.2005.70 | chapter = Filtering of shrew DDoS attacks in frequency domain | title = The IEEE Conference on Local Computer Networks 30th Anniversary (LCN'05)l | pages = 8 pp | year = 2005 | hdl = 10722/45910 | isbn = 978-0-7695-2421-4 | s2cid = 406686 }}</ref>

===Slow read attack===
A slow read attack sends legitimate application layer requests, but reads responses very slowly, keeping connections open longer hoping to exhaust the server's connection pool. The slow read is achieved by advertising a very small number for the TCP Receive Window size, and at the same time emptying clients' TCP receive buffer slowly, which causes a very low data flow rate.<ref>{{cite web |url=https://www.netscout.com/what-is-ddos/slow-read-attacks |title=What is a Slow Read DDoS Attack? |publisher=[[NetScout Systems]]}}</ref>

===Sophisticated low-bandwidth Distributed Denial-of-Service Attack===
A sophisticated low-bandwidth DDoS attack is a form of DoS that uses less traffic and increases its effectiveness by aiming at a weak point in the victim's system design, i.e., the attacker sends traffic consisting of complicated requests to the system.<ref name="Ben-Porat 1031–1043">{{Cite journal|title = Vulnerability of Network Mechanisms to Sophisticated DDoS Attacks|journal = IEEE Transactions on Computers|date = 2013-05-01|issn = 0018-9340|pages = 1031–1043|volume = 62|issue = 5|doi = 10.1109/TC.2012.49|first1 = U.|last1 = Ben-Porat|first2 = A.|last2 = Bremler-Barr|first3 = H.|last3 = Levy|s2cid = 26395831}}</ref> Essentially, a sophisticated DDoS attack is lower in cost due to its use of less traffic, is smaller in size making it more difficult to identify, and it has the ability to hurt systems which are protected by flow control mechanisms.<ref name="Ben-Porat 1031–1043"/><ref>{{cite web|url=https://sourceforge.net/projects/slow-http-test/|title=Slow HTTP Test|author=orbitalsatelite|work=SourceForge|date=8 September 2016 }}</ref>

===SYN flood===
A [[SYN flood]] occurs when a host 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]], send back a TCP/SYN-ACK packet, and wait for a packet in response from the sender address. However, because the sender's address is forged, the response never comes. These half-open connections exhaust the available connections the server can make, keeping it from responding to legitimate requests until after the attack ends.{{ref RFC|4987}}

===Teardrop attacks===
{{see also|IP fragmentation attack}}

A '''teardrop attack''' involves sending [[Mangled packet|mangled]] [[IP fragment]]s with overlapping, oversized payloads to the target machine. This can crash various operating systems because of a bug in their [[TCP/IP]] [[IPv4#Fragmentation and reassembly|fragmentation re-assembly]] code.<ref name="CERT-1">{{cite web |year=1998 |title=CERT Advisory CA-1997-28 IP Denial-of-Service Attacks |url=https://vuls.cert.org/confluence/display/historical/CERT+Advisory+CA-1997-28+IP+Denial-of-Service+Attacks |access-date=July 18, 2014 |publisher=CERT}}</ref> [[Windows 3.1x]], [[Windows 95]] and [[Windows NT]] operating systems, as well as versions of [[Linux]] prior to versions 2.0.32 and 2.1.63 are vulnerable to this attack.{{efn|Although in September 2009, a vulnerability in [[Windows Vista]] was referred to as a ''teardrop attack'', this targeted [[Server Message Block|SMB2]] which is a higher layer than the TCP packets that teardrop used).<ref>{{cite news|url=http://www.zdnet.com/blog/security/windows-7-vista-exposed-to-teardrop-attack/4222 |archive-url=https://web.archive.org/web/20101106101436/http://www.zdnet.com/blog/security/windows-7-vista-exposed-to-teardrop-attack/4222 |url-status=dead |archive-date=6 November 2010 |title=Windows 7, Vista exposed to 'teardrop attack' |work=ZDNet |date=September 8, 2009 |access-date=2013-12-11}}</ref><ref>{{cite web|url=http://www.microsoft.com/technet/security/advisory/975497.mspx |title=Microsoft Security Advisory (975497): Vulnerabilities in SMB Could Allow Remote Code Execution |publisher=Microsoft.com |date=September 8, 2009 |access-date=2011-12-02}}</ref>}} One of the fields in an [[IP header]] is the ''fragment offset'' field, indicating the starting position, or offset, of the data contained in a fragmented packet relative to the data in the original packet. If the sum of the offset and size of one fragmented packet differs from that of the next fragmented packet, the packets overlap. When this happens, a server vulnerable to teardrop attacks is unable to reassemble the packets resulting in a denial-of-service condition.<ref>{{Citation |last=Bhardwaj |first=Akashdeep |title=Solutions for DDoS Attacks on Cloud Environment |date=2023-06-12 |work=New Age Cyber Threat Mitigation for Cloud Computing Networks |pages=42–55 |url=http://dx.doi.org/10.2174/9789815136111123010006 |access-date=2024-02-09 |publisher=BENTHAM SCIENCE PUBLISHERS |doi=10.2174/9789815136111123010006 |isbn=978-981-5136-11-1}}</ref>

===Telephony denial-of-service===
[[Voice over IP]] has made abusive origination of large numbers of [[telephone]] voice calls inexpensive and easily automated while permitting call origins to be misrepresented through [[caller ID spoofing]]. According to the US [[Federal Bureau of Investigation]], telephony denial-of-service (TDoS) has appeared as part of various fraudulent schemes:
* A scammer contacts the victim's banker or broker, impersonating the victim to request a funds transfer. The banker's attempt to contact the victim for verification of the transfer fails as the victim's telephone lines are being flooded with bogus calls, rendering the victim unreachable.<ref>{{cite web|url=https://www.fbi.gov/newark/press-releases/2010/nk051110.htm |title=FBI — Phony Phone Calls Distract Consumers from Genuine Theft |publisher=FBI.gov |date=2010-05-11 |access-date=2013-09-10}}</ref>
* A scammer contacts consumers with a bogus claim to collect an outstanding [[payday loan]] for thousands of dollars. When the consumer objects, the scammer retaliates by flooding the victim's employer with automated calls. In some cases, the displayed caller ID is spoofed to impersonate police or law enforcement agencies.<ref>{{cite web|url=http://www.ic3.gov/media/2013/130107.aspx |title= Internet Crime Complaint Center's (IC3) Scam Alerts January 7, 2013 |work=IC3.gov |date=2013-01-07 |access-date=2013-09-10}}</ref>
* [[Swatting]]: A scammer contacts consumers with a bogus debt collection demand and threatens to send police; when the victim balks, the scammer floods local police numbers with calls on which caller ID is spoofed to display the victim's number. Police soon arrive at the victim's residence attempting to find the origin of the calls.

TDoS can exist even without [[Internet telephony]]. In the [[2002 New Hampshire Senate election phone jamming scandal]], [[telemarketing|telemarketers]] were used to flood political opponents with spurious calls to jam phone banks on election day. Widespread publication of a number can also flood it with enough calls to render it unusable, as happened by accident in 1981 with multiple +1-[[area code]]-867-5309 subscribers inundated by hundreds of calls daily in response to the song "[[867-5309/Jenny]]". TDoS differs from other [[telephone harassment]] (such as [[prank call]]s and [[obscene phone call]]s) by the number of calls originated. By occupying lines continuously with repeated automated calls, the victim is prevented from making or receiving both routine and emergency telephone calls. Related exploits include SMS flooding attacks and [[black fax]] or continuous fax transmission by using a loop of paper at the sender.

===TTL expiry attack===
It takes more router resources to drop a packet with a [[Time to live#IP packets|TTL]] value of 1 or less than it does to forward a packet with a higher TTL value. When a packet is dropped due to TTL expiry, the router CPU must generate and send an [[Internet Control Message Protocol#Time exceeded|ICMP time exceeded]] response. Generating many of these responses can overload the router's CPU.<ref>{{cite web |title=TTL Expiry Attack Identification and Mitigation |url=https://www.cisco.com/c/en/us/about/security-center/ttl-expiry-attack.html |publisher=[[Cisco Systems]] |access-date=2019-05-24}}</ref>

===UPnP attack===
A UPnP attack uses an existing vulnerability in [[Universal Plug and Play]] (UPnP) protocol to get past network security and flood a target's network and servers. The attack is based on a DNS amplification technique, but the attack mechanism is a UPnP router that forwards requests from one outer source to another. The UPnP router returns the data on an unexpected UDP port from a bogus IP address, making it harder to take simple action to shut down the traffic flood. According to the [[Imperva]] researchers, the most effective way to stop this attack is for companies to lock down UPnP routers.<ref>{{Cite news|url=https://www.darkreading.com/new-ddos-attack-method-leverages-upnp/d/d-id/1331799|title=New DDoS Attack Method Leverages UPnP|work=Dark Reading|access-date=2018-05-29|language=en}}</ref><ref>{{Cite news|url=https://www.imperva.com/blog/archive/new-ddos-attack-method-demands-a-fresh-approach-to-amplification-assault-mitigation/|title=New DDoS Attack Method Demands a Fresh Approach to Amplification Assault Mitigation – Blog {{!}} Imperva|date=2018-05-14|work=Blog {{!}} Imperva|access-date=2018-05-29|language=en-US}}</ref>

===SSDP reflection attack===
In 2014, it was discovered that [[Simple Service Discovery Protocol]] (SSDP) was being used in [[DDoS]] attacks known as an [[Simple Service Discovery Protocol#DDoS attack|SSDP reflection attac''k'']] ''with amplification''. Many devices, including some residential routers, have a vulnerability in the UPnP software that allows an attacker to get replies from [[List of TCP and UDP port numbers|UDP port 1900]] to a destination address of their choice. With a [[botnet]] of thousands of devices, the attackers can generate sufficient packet rates and occupy bandwidth to saturate links, causing the denial of services.<ref>{{Cite web|url=https://www.cisecurity.org/ms-isac/|title=Multi-State Information Sharing and Analysis Center|website=CIS}}</ref><ref>{{cite web|url=https://www.us-cert.gov/ncas/alerts/TA14-017A|title=UDP-Based Amplification Attacks|date=18 December 2019 }}</ref><ref name="Cloudflare Blog 2017">{{cite web |last=Majkowski |first=Marek |url=https://blog.cloudflare.com/ssdp-100gbps/ |title=Stupidly Simple DDoS Protocol (SSDP) generates 100 Gbps DDoS |date=2017-06-28 |work=The Cloudflare Blog |access-date=2024-11-20}}</ref> Because of this weakness, the network company [[Cloudflare]] has described SSDP as the "Stupidly Simple DDoS Protocol".<ref name="Cloudflare Blog 2017"/><!--"Stupidly Simple DDoS Protocol" is in the article title only, good general description of the vulnerability though-->

===ARP spoofing===
[[ARP spoofing]] is a common DoS attack that involves a vulnerability in the ARP protocol that allows an attacker to associate their [[MAC address]] to the IP address of another computer or [[Gateway (telecommunications)|gateway]], causing traffic intended for the original authentic IP to be re-routed to that of the attacker, causing a denial of service.

==Defense techniques==
{{Main|DDoS mitigation}}
Defensive responses to denial-of-service attacks typically involve the use of a combination of attack detection, traffic classification and response tools, aiming to block traffic the tools identify as illegitimate and allow traffic that they identify as legitimate.<ref>{{Cite journal | last1 = Loukas | first1 = G. | last2 = Oke | first2 = G. | doi = 10.1093/comjnl/bxp078 | title = Protection Against Denial of Service Attacks: A Survey | journal = [[The Computer Journal|Comput. J.]] | volume = 53 | issue = 7 | pages = 1020–1037 | date = September 2010| url = http://staffweb.cms.gre.ac.uk/~lg47/publications/LoukasOke-DoSSurveyComputerJournal.pdf | access-date = 2015-12-02 | archive-url = https://web.archive.org/web/20120324115835/http://staffweb.cms.gre.ac.uk/~lg47/publications/LoukasOke-DoSSurveyComputerJournal.pdf | archive-date = 2012-03-24 | url-status = dead }}</ref> A list of response tools include the following.

===Upstream filtering===
All traffic destined to the victim is diverted to pass through a ''cleaning center'' or a ''scrubbing center'' via various methods such as: changing the victim IP address in the DNS system, tunneling methods (GRE/VRF, MPLS, SDN),<ref>{{cite web |url=https://archive.nanog.org/meetings/nanog23/presentations/afek.ppt |title=MPLS-Based Synchronous Traffic Shunt (NANOG28) |work=Riverhead Networks, Cisco, Colt Telecom |publisher=NANOG28 |date=2003-01-03 |archive-url=https://web.archive.org/web/20210515102444/https://archive.nanog.org/meetings/nanog23/presentations/afek.ppt |archive-date=2021-05-15 |access-date=2003-01-10 |url-status=dead }}</ref> proxies, digital cross connects, or even direct circuits. The cleaning center separates ''bad'' traffic (DDoS and also other common internet attacks) and only passes good legitimate traffic to the victim server.<ref>{{cite web |url=https://archive.nanog.org/meetings/nanog23/presentations/afek.ppt |title=Diversion and Sieving Techniques to Defeat DDoS attacks |work=Cisco, Riverhead Networks |publisher=NANOG23 |date=2001-10-23 |archive-url=https://web.archive.org/web/20210515102444/https://archive.nanog.org/meetings/nanog23/presentations/afek.ppt |archive-date=2021-05-15 |access-date=2001-10-30 |url-status=dead }}</ref> The victim needs central connectivity to the Internet to use this kind of service unless they happen to be located within the same facility as the cleaning center. DDoS attacks can overwhelm any type of hardware firewall, and passing malicious traffic through large and mature networks becomes more and more effective and economically sustainable against DDoS.<ref>{{cite web|url=https://research.sprintlabs.com/publications/uploads/RR04-ATL-013177.pdf |title=DDoS Mitigation via Regional Cleaning Centers (Jan 2004) |work=SprintLabs.com |publisher=Sprint ATL Research |archive-url=https://web.archive.org/web/20080921012859/http://research.sprintlabs.com/publications/uploads/RR04-ATL-013177.pdf |archive-date=2008-09-21 |access-date=2011-12-02 |url-status=dead}}</ref>

===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 [[Network switch|switches]] and as part of [[bandwidth management]]. Application front-end hardware analyzes data packets as they enter the network, and identifies and drops dangerous or suspicious flows.

===Application level key completion indicators===
Approaches to detection of DDoS attacks against cloud-based applications may be based on an application layer analysis, indicating whether incoming bulk traffic is legitimate.<ref>{{cite book |last1=Alqahtani |first1=S. |last2=Gamble |first2=R. F. |title=2015 48th Hawaii International Conference on System Sciences |chapter=DDoS Attacks in Service Clouds |date=1 January 2015 |pages=5331–5340 |doi=10.1109/HICSS.2015.627 |isbn=978-1-4799-7367-5 |s2cid=32238160}}</ref> These approaches mainly rely on an identified path of value inside the application and monitor the progress of requests on this path, through markers called ''key completion indicators''.<ref>{{cite news|last=Kousiouris|first=George|title=KEY COMPLETION INDICATORS:minimizing the effect of DoS attacks on elastic Cloud-based applications based on application-level markov chain checkpoints |newspaper=CLOSER Conference|date=2014|pages=622–628 |doi=10.5220/0004963006220628|isbn=978-989-758-019-2 }}</ref> In essence, these techniques are statistical methods of assessing the behavior of incoming requests to detect if something unusual or abnormal is going on. An analogy is to a brick-and-mortar department store where customers spend, on average, a known percentage of their time on different activities such as picking up items and examining them, putting them back, filling a basket, waiting to pay, paying, and leaving. If a mob of customers arrived in the store and spent all their time picking up items and putting them back, but never made any purchases, this could be flagged as unusual behavior.

===Blackholing and sinkholing===
With [[blackhole routing]], all the traffic to the attacked DNS or IP address is sent to a ''black hole'' (null interface or a non-existent server). To be more efficient and avoid affecting network connectivity, it can be managed by the ISP.<ref>{{cite journal |last1=Patrikakis |first1=C. |last2=Masikos |first2=M. |last3=Zouraraki |first3=O. |title=Distributed Denial of Service Attacks |journal=The Internet Protocol Journal |volume=7 |issue=4 |pages=13–35 |date=December 2004 |url=http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_7-4/dos_attacks.html |access-date=2010-01-13 |archive-date=2015-12-27 |archive-url=https://web.archive.org/web/20151227060036/http://www.cisco.com/web/about/ac123/ac147/archived_issues/ipj_7-4/dos_attacks.html |url-status=dead }}</ref> A [[DNS sinkhole]] routes traffic to a valid IP address which analyzes traffic and rejects bad packets. Sinkholing may not be efficient for severe attacks.

===IPS based prevention===
[[Intrusion prevention system]]s (IPS) are effective if the attacks have signatures associated with them. However, the trend among attacks is to have legitimate content but bad intent. Intrusion-prevention systems that work on content recognition cannot block behavior-based DoS attacks.<ref name=":0" /> An [[Application-specific integrated circuit|ASIC]] based IPS may detect and block denial-of-service attacks because they have the [[Bandwidth (computing)|processing power]] and the granularity to analyze the attacks and act like a [[circuit breaker]] in an automated way.<ref name=":0" />

===DDS based defense===
More focused on the problem than IPS, a DoS defense system (DDS) can block connection-based DoS attacks and those with legitimate content but bad intent. A DDS can also address both protocol attacks (such as teardrop and ping of death) and rate-based attacks (such as ICMP floods and SYN floods). DDS has a purpose-built system that can easily identify and obstruct denial of service attacks at a greater speed than a software-based system.<ref>{{Cite web|last=Popeskic|first=Valter|title=How to prevent or stop DoS attacks?|date=16 October 2012|url=https://howdoesinternetwork.com/2012/prevent-stop-dos-attacks}}</ref>

===Firewalls===
In the case of a simple attack, a [[Firewall (computing)|firewall]] can be adjusted to deny all incoming traffic from the attackers, based on protocols, ports, or the originating IP addresses. More complex attacks will however be hard to block with simple rules: for example, if there is an ongoing attack on port 80 (web service), it is not possible to drop all incoming traffic on this port because doing so will prevent the server from receiving and serving legitimate traffic.<ref>{{cite web|url=http://www.computerworld.com/s/article/94014/How_to_defend_against_DDoS_attacks|first=Paul|last=Froutan|title=How to defend against DDoS attacks|work=[[Computerworld]]|date=June 24, 2004|access-date=May 15, 2010|archive-date=2 July 2014|archive-url=https://web.archive.org/web/20140702140309/http://www.computerworld.com/s/article/94014/How_to_defend_against_DDoS_attacks|url-status=dead}}</ref> Additionally, firewalls may be too deep in the network hierarchy, with routers being adversely affected before the traffic gets to the firewall. Also, many security tools still do not support IPv6 or may not be configured properly, so the firewalls may be bypassed during the attacks.<ref>{{Cite news|url=https://www.computerweekly.com/news/252445613/Cyber-security-vulnerability-concerns-skyrocket|title=Cyber security vulnerability concerns skyrocket|work=ComputerWeekly.com|access-date=2018-08-13|language=en-GB}}</ref>

===Routers===
Similar to switches, routers have some [[rate limiting|rate-limiting]] and [[access control list|ACL]] capabilities. They, too, are manually set. Most routers can be easily overwhelmed under a DoS attack. Nokia SR-OS using FP4 or FP5 processors offers DDoS protection.<ref>{{cite web |url=https://www.nokia.com/networks/technologies/fp-network-processor-technology/ |title=FP Network Processor Technology |access-date=2024-06-15}}</ref> Nokia SR-OS also uses big data analytics-based Nokia Deepfield Defender for DDoS protection.<ref>[https://www.nokia.com/networks/ip-networks/deepfield/defender/ Nokia Deepfield Defender]</ref> [[Cisco IOS]] has optional features that can reduce the impact of flooding.<ref>{{cite web |url=http://mehmet.suzen.googlepages.com/qos_ios_dos_suzen2005.pdf |title=Some IoS tips for Internet Service (Providers) |first=Mehmet |last=Suzen |archive-url=https://web.archive.org/web/20080910202908/http://mehmet.suzen.googlepages.com/qos_ios_dos_suzen2005.pdf |archive-date=2008-09-10 }}</ref>

===Switches===
Most switches have some rate-limiting and [[access control list|ACL]] capability. Some switches provide automatic or system-wide [[rate limiting]], [[traffic shaping]], [[delayed binding]] ([[TCP splicing]]), [[deep packet inspection]] and [[bogon filtering]] (bogus IP filtering) to detect and remediate DoS attacks through automatic rate filtering and WAN Link failover and balancing. These schemes will work as long as the DoS attacks can be prevented by using them. For example, SYN flood can be prevented using delayed binding or TCP splicing. Similarly, content-based DoS may be prevented using deep packet inspection. Attacks using [[Martian packet]]s can be prevented using bogon filtering. Automatic rate filtering can work as long as set rate thresholds have been set correctly. WAN-link failover will work as long as both links have a DoS prevention mechanism.<ref name=":0" />

==Blocking vulnerable ports==
Threats may be associated with specific TCP or UDP port numbers. Blocking these ports at the firewall can mitigate an attack. For example, in an SSDP reflection attack, the key mitigation is to block incoming UDP traffic on port 1900.<ref>{{Cite web|url=https://www.cloudflare.com/learning/ddos/ssdp-ddos-attack/|title=SSDP DDoS attack &#124; Cloudflare}}</ref>

==Unintentional denial-of-service==
An unintentional denial-of-service can occur when a system 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{{spaced ndash}}potentially hundreds of thousands of people{{spaced ndash}}click that link in the space of a few hours, having the same effect on the target website as a DDoS attack. A VIPDoS is the same, but specifically when the link was posted by a celebrity. When [[Death of Michael Jackson|Michael Jackson died]] in 2009, websites such as Google and Twitter slowed down or even crashed.<ref>{{cite news| url=http://news.bbc.co.uk/1/hi/8120324.stm | work=BBC News | first=Maggie | last=Shiels | title=Web slows after Jackson's death | date=2009-06-26}}</ref> Many sites' servers thought the requests were from a virus or spyware trying to cause a denial-of-service attack, warning users that their queries looked like "automated requests from a [[computer virus]] or spyware application".<ref>{{cite web|date=October 20, 2009|title=Unexpected Search Results|url=http://productforums.google.com/forum/?#!category-topic/websearch/unexpected-search-results/uFcXXixhiBw|access-date=2012-02-11|work=Google Product Forums › Google Search Forum}}{{dead link|date=April 2025}}</ref>

News sites and link sites{{spaced ndash}}sites whose primary function is to provide links to interesting content elsewhere on the Internet{{spaced ndash}}are most likely to cause this phenomenon. The canonical example is the [[Slashdot effect]] when receiving traffic from [[Slashdot]]. It is also known as "the [[Reddit]] hug of death"<ref>{{cite web |url=https://medium.com/codingame/story-of-a-reddit-hug-of-death-and-lessons-learned-3565bb8a6793 |title=Story of a Reddit Hug of Death and Lessons Learned |date=16 November 2017 |access-date=2024-09-24}}</ref> and "the [[Digg]] effect".<ref>{{cite web |url=http://socialkeith.com/the-digg-effect-v4/ |title=The Digg Effect v4 |publisher=Social Keith |access-date=October 20, 2010 |first1=Keith |last1=Plocek |archive-url=https://web.archive.org/web/20101022060115/http://socialkeith.com/the-digg-effect-v4/ |archive-date=October 22, 2010 |url-status=dead }}</ref>

Similar unintentional denial-of-service can also occur via other media, e.g. when a URL is mentioned on television. In March 2014, after [[Malaysia Airlines Flight 370]] went missing, [[DigitalGlobe]] launched a [[crowdsourcing]] service on which users could help search for the missing jet in satellite images. The response overwhelmed the company's servers.<ref>{{cite web|url=http://wnmufm.org/post/people-overload-website-hoping-help-search-missing-jet|title=People Overload Website, Hoping To Help Search For Missing Jet|author=Bill Chappell|publisher=NPR|date=12 March 2014|access-date=4 February 2016}}</ref> An unintentional denial-of-service may also result from a prescheduled event created by the website itself, as was the case of the [[Census in Australia]] in 2016.<ref>{{cite web|url=https://delimiter.com.au/2016/08/19/experts-cast-doubt-census-ddos-claims/|title=Experts cast doubt on Census DDoS claims|date=19 August 2016|access-date=31 January 2018|last=Palmer|first=Daniel|publisher=Delimiter}}</ref>

Legal action has been taken in at least one such case. In 2006, [[Universal Tube & Rollform Equipment|Universal Tube & Rollform Equipment Corporation]] sued [[YouTube]]: massive numbers of would-be YouTube.com users accidentally typed the tube company's URL, utube.com. As a result, the tube company ended up having to spend large amounts of money on upgrading its bandwidth.<ref>{{cite news |title=YouTube sued by sound-alike site |work=BBC News |date=2006-11-02 |url=http://news.bbc.co.uk/2/hi/business/6108502.stm }}</ref> The company appears to have taken advantage of the situation, with utube.com now containing ads and receiving advertisement revenue. 

Routers have also been known to create unintentional DoS attacks, as both [[D-Link]] and [[Netgear]] routers have [[NTP server misuse and abuse|overloaded NTP servers]] by flooding them without respecting the restrictions of client types or geographical limitations.

==Side effects of attacks==

===Backscatter===
{{See also|Backscatter (email)|Internet background noise}}

In computer network security, backscatter is a side-effect of a spoofed denial-of-service attack. In this kind of attack, the attacker spoofs the source address in [[IP packet (disambiguation)|IP packets]] sent to the victim. In general, the victim machine cannot 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.<ref>{{cite web |url=http://www.caida.org/publications/animations/ |work=Animations |title=Backscatter Analysis (2001) |type=video |publisher=[[Cooperative Association for Internet Data Analysis]] |access-date=December 11, 2013}}</ref>

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 telescope]]s as 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 the characteristics of DoS attacks and victims.

==Legality==
[[File:FBI DDoS domain seized.png|thumb|Numerous websites offering tools to conduct a DDoS attack were seized by the FBI under the [[Computer Fraud and Abuse Act]].<ref>{{cite web|title=FBI Seizes 15 DDoS-For-Hire Websites|url=https://kotaku.com/fbi-seizes-15-ddos-for-hire-websites-1831239141|website=Kotaku|date=6 January 2019}}</ref>]]
{{See also|Cybercrime|DPP v Lennon}}

Many jurisdictions have laws under which denial-of-service attacks are illegal. [[UNCTAD]] highlights that 156 countries, or 80% globally, have enacted [[cybercrime]] laws to combat its widespread impact. Adoption rates vary by region, with Europe at a 91% rate, and Africa at 72%.<ref>{{Cite web |title=Cybercrime Legislation Worldwide {{!}} UNCTAD |url=https://unctad.org/page/cybercrime-legislation-worldwide |access-date=2024-04-08 |website=unctad.org |language=en}}</ref>

In the US, denial-of-service attacks may be considered a federal crime under the [[Computer Fraud and Abuse Act]] with penalties that include years of imprisonment.<ref>{{cite web|url=http://www.gpo.gov/fdsys/pkg/USCODE-2010-title18/html/USCODE-2010-title18-partI-chap47-sec1030.htm |title=United States Code: Title 18,1030. Fraud and related activity in connection with computers &#124; Government Printing Office |publisher=gpo.gov |date=2002-10-25|access-date=2014-01-15}}</ref> The [[Computer Crime and Intellectual Property Section]] of the [[US Department of Justice]] handles cases of DoS and DDoS. In one example, in July 2019, Austin Thompson, aka [[DerpTrolling]], was sentenced to 27 months in prison and $95,000 restitution by a federal court for conducting multiple DDoS attacks on major video gaming companies, disrupting their systems from hours to days.<ref name=Thompson_sentenced_1 >{{cite web | url=https://www.justice.gov/usao-sdca/pr/utah-man-sentenced-computer-hacking-crime | title=Utah Man Sentenced for Computer Hacking Crime | date=2019-07-02 | archive-url=https://web.archive.org/web/20190710153706/https://www.justice.gov/usao-sdca/pr/utah-man-sentenced-computer-hacking-crime | archive-date=2019-07-10 | url-status=live }}</ref><ref name=Thompson_sentenced_2 >{{ cite news | url=https://www.theregister.co.uk/2019/07/04/gamebusting_ddos_wielder_derptrolling_sentenced_to_two_years_in_the_clink/ | title=Get rekt: Two years in clink for game-busting DDoS brat DerpTrolling | last=Smolaks  | first=Max | publisher=[[The Register]] | date=2019-07-04 | access-date=2019-09-27 | quote=Austin Thompson, aka DerpTrolling, who came to prominence in 2013 by launching Distributed Denial of Service (DDoS) attacks against major video game companies, has been sentenced to 27 months in prison by a federal court. Thompson, a resident of Utah, will also have to pay $95,000 to Daybreak Games, which was owned by Sony when it suffered at the hands of DerpTrolling. Between December 2013 and January 2014, Thompson also brought down Valve’s Steam – the largest digital distribution platform for PC gaming – as well as Electronic Arts' Origin service and Blizzard's BattleNet. The disruption lasted anywhere from hours to days. }}</ref>

In European countries, committing criminal denial-of-service attacks may, as a minimum, lead to arrest.<ref>{{cite web|title=International Action Against DD4BC Cybercriminal Group|url=https://www.europol.europa.eu/content/international-action-against-dd4bc-cybercriminal-group|website=EUROPOL|date=12 January 2016}}</ref> The United Kingdom is unusual in that it specifically outlawed denial-of-service attacks and set a maximum penalty of 10 years in prison with the [[Police and Justice Act 2006]], which amended Section 3 of the [[Computer Misuse Act 1990]].<ref>{{cite web|title=Computer Misuse Act 1990|url=http://www.legislation.gov.uk/ukpga/1990/18/section/3|website=legislation.gov.uk — The National Archives, of UK|date=10 January 2008}}</ref>

In January 2019, [[Europol]] announced that "actions are currently underway worldwide to track down the users" of Webstresser.org, a former DDoS marketplace that was shut down in April 2018 as part of [[Operation PowerOFF]].<ref>{{cite web |title=Newsroom |url=https://www.europol.europa.eu/newsroom/news/world%E2%80%99s-biggest-marketplace-selling-internet-paralysing-ddos-attacks-taken-down |website=Europol |access-date=29 January 2019 |language=en}}</ref> Europol said UK police were conducting a number of "live operations" targeting over 250 users of Webstresser and other DDoS services.<ref>{{cite web |title=Authorities across the world going after users of biggest DDoS-for-hire website |url=https://www.europol.europa.eu/newsroom/news/authorities-across-world-going-after-users-of-biggest-ddos-for-hire-website |website=Europol |access-date=29 January 2019 |language=en}}</ref>

On January 7, 2013, [[Anonymous (hacker group)|Anonymous]] posted a [[We the People (petitioning system)|petition]] on the [[whitehouse.gov]] site asking that DDoS be recognized as a legal form of protest similar to the [[Occupy movement]], the claim being that the similarity in the purpose of both is same.<ref>{{cite web|url=https://huffingtonpost.com/2013/01/12/anonymous-ddos-petition-white-house_n_2463009.html |title=Anonymous DDoS Petition: Group Calls On White House To Recognize Distributed Denial Of Service As Protest. |publisher=HuffingtonPost.com |date=2013-01-12}}</ref>

==See also==<!-- please respect alphabetical order -->
{{Columns-list|
* {{annotated link|BASHLITE}}
* {{annotated link|Billion laughs attack}}
* {{annotated link|Blaster (computer worm)}}
* {{annotated link|Clear channel assessment attack}}
* {{annotated link|Dendroid (malware)}}
* {{annotated link|Distributed denial-of-service attacks on root nameservers}}
* {{annotated link|DNS Flood}}
* {{annotated link|Hit-and-run DDoS}}
* {{annotated link|Industrial espionage}}
* {{annotated link|Infinite loop}}
* {{annotated link|Intrusion detection system}}
* {{annotated link|Killer poke}}
* {{annotated link|Lace card}}
* {{annotated link|Mixed threat attack}}
* {{annotated link|Network intrusion detection system}}
* {{annotated link|2016 Dyn cyberattack}}
* {{annotated link|Paper terrorism}}
* {{annotated link|Project Shield}}
* {{annotated link|ReDoS}}
* {{annotated link|Resource exhaustion attack}}
* {{annotated link|Virtual sit-in}}
* {{annotated link|Web shell}}
* {{annotated link|Radio jamming}}
* {{annotated link|Xor DDoS}}
* {{annotated link|Zemra}}
* {{annotated link|Zip bomb}}
}}

==Notes==
{{Notelist}}

==References==
{{Reflist}}

==Further reading==
*{{cite web|url=http://cyber.law.harvard.edu/sites/cyber.law.harvard.edu/files/2010_DDoS_Attacks_Human_Rights_and_Media.pdf|title=Distributed Denial of Service Attacks Against Independent Media and Human Rights Sites|date=December 2011|publisher=The Berkman Center for Internet & Society at Harvard University|access-date=2011-03-02|archive-url=https://web.archive.org/web/20110226113832/https://cyber.law.harvard.edu/sites/cyber.law.harvard.edu/files/2010_DDoS_Attacks_Human_Rights_and_Media.pdf|archive-date=2011-02-26|author1=Ethan Zuckerman|author2=Hal Roberts|author3=Ryan McGrady|author4=Jillian York|author5=John Palfrey|url-status=dead}}
*[http://www.pcworld.com/article/2056805/applicationlayer-ddos-attacks-are-becoming-increasingly-sophisticated.html PC World - Application Layer DDoS Attacks are Becoming Increasingly Sophisticated]
*{{Cite web |last=Neil |first=Patel |date=2025-01-28 |title=How Hybrid Cloud Environments Boost Resilience Against DDoS Attacks? |url=https://kemotech.co.uk/how-hybrid-cloud-environments-boost-resilience-against-ddos-attacks/ |access-date=2025-02-27 |website=KemoTech |language=en-US}}

==External links==
* {{IETF RFC|4732}} Internet Denial-of-Service Considerations
* [https://www.w3.org/Security/Faq/wwwsf6.html W3C The World Wide Web Security FAQ - Securing against Denial of Service attacks]
* {{web archive |url=https://web.archive.org/web/20081109003822/https://www.cert.org/tech_tips/denial_of_service.html |title=CERT's Guide to DoS attacks}} (historic document)

{{Information security}}
{{Authority control}}

{{DEFAULTSORT:Denial-Of-Service Attack}}
[[Category:Denial-of-service attacks]]
[[Category:Cyberwarfare]]
[[Category:Types of cyberattacks]]
[[Category:Internet outages]]