A denial-of-service attack (DoS attack) or distributed denial-of-service attack (DDoS attack) is an attempt to make a computer resource unavailable to its intended users. Although the means to, motives for, and targets of a DoS attack may vary, it generally consists of the concerted, malevolent efforts of a person or persons to prevent an Internet site or service from functioning efficiently or at all, temporarily or indefinitely. The Internet is a global system of interconnected Computer networks A website (alternatively web site or Web site, a back-construction from the Proper noun World Wide Web) is a collection of Web pages A Web service (also Web Service) is defined by the W3C as "a software system designed to support interoperable machine-to-machine interaction
Perpetrators of DoS attacks typically target sites or services hosted on high-profile web servers such as banks, credit card payment gateways, and even DNS root servers. A root name server is a DNS server that answers requests for the DNS root zone, and redirects requests for a particular Top-level domain (TLD to that
One common method of attack involves saturating the target (victim) machine with external communications requests, such that it cannot respond to legitimate traffic, or responds so slowly as to be rendered effectively unavailable. In general terms, DoS attacks are implemented by either forcing the targeted computer(s) to reset, or consume its resources so that it can no longer provide its intended service or obstructing the communication media between the intended users and the victim so that they can no longer communicate adequately. A resource, or system resource, is any physical or virtual component of limited availability within a computer system
Denial-of-service attacks are considered violations of the IAB's Internet proper use policy. The Internet Architecture Board (IAB is the committee charged with oversight of the technical and Engineering development of the Internet by the Internet Society In January 1989 the Internet Architecture Board (IAB issued a statement of policy concerning Internet ethics. They also commonly constitute violations of the laws of individual nations. Computer crime, Cybercrime, e-crime, hi-tech crime or electronic crime generally refers to criminal activity where a Computer 
The United States Computer Emergency Readiness Team defines symptoms of Denial-of-service attacks to include:
Not all service outages, even those that result from malicious activity, are necessarily denial-of-service attacks. Other methods of attack may include a denial of service as one component of a larger offensive.
Denial-of-service attacks can also lead to problems in the network 'branches' around the actual computer being attacked. For example, the bandwidth of a router between the Internet and a LAN may be consumed by an attack, compromising not only the intended computer, but also the entire network. In Computer networking and Computer science, digital bandwidth or just bandwidth is the capacity for a given system to transfer data over a connection
If the attack is conducted on a sufficiently large scale, entire geographical regions of Internet connectivity can be compromised without the attacker's knowledge or intent by incorrectly configured or flimsy network infrastructure equipment.
A "denial-of-service" attack is characterized by an explicit attempt by attackers to prevent legitimate users of a service from using that service. Attacks can be directed at any network device, including attacks on routing devices and web, electronic mail, or Domain Name System servers. Routing is the process of selecting paths in a network along which to send network traffic The World Wide Web (commonly shortened to the Web) is a system of interlinked Hypertext documents accessed via the Internet. Electronic mail, often abbreviated to e-mail, email, or originally eMail, is a Store-and-forward method of writing sending receiving The Domain Name System (DNS is a hierarchical naming system for computers services or any resource participating in the Internet. A server is a Computer dedicated to providing one or more services over a computer network typically through a request-response routine
A DoS attack can be perpetrated in a number of ways. The five basic types of attack are:
A DoS attack may include execution of malware intended to:
A smurf attack is one particular variant of a flooding DoS attack on the public Internet. The Smurf attack is a way of generating a lot of Computer network traffic to a victim host A ping flood is a simple Denial-of-service attack where the attacker overwhelms the victim with ICMP Echo Request (ping packets It only succeeds if the A ping of death ( Abbreviated "POD" is a type of attack on a computer that involves sending a malformed or otherwise malicious Ping to a computer The Smurf attack is a way of generating a lot of Computer network traffic to a victim host It relies on misconfigured network devices that allow packets to be sent to all computer hosts on a particular network via the broadcast address of the network, rather than a specific machine. In Computer networking, a broadcast address is an IP address that allows information to be sent to all machines on a given subnet rather than a specific The network then serves as a smurf amplifier. The Smurf attack is a way of generating a lot of Computer network traffic to a victim host In such an attack, the perpetrators will send large numbers of IP packets with the source address faked to appear to be the address of the victim. The Internet Protocol ( IP) is a protocol used for communicating data across a Packet-switched Internetwork using the Internet Protocol The network's bandwidth is quickly used up causing legitimate packets not to get through to their destination.  To combat Denial of Service attacks on the Internet, services like the Smurf Amplifier Registry have given network service providers the ability to identify misconfigured networks and to take appropriate action such as filtering. The Smurf Amplifier Registry is a Blacklist of networks on the Internet which have been misconfigured in such a way that they can be used as Smurf A network service provider ( NSP) is a business or organization that sells bandwidth or network access by providing direct backbone access to the Internet A firewall is an integrated collection of security measures designed to prevent unauthorized electronic access to a networked computer system
Ping flood is based on sending the victim an overwhelming number of ping packets, usually using the "ping -f" command. A ping flood is a simple Denial-of-service attack where the attacker overwhelms the victim with ICMP Echo Request (ping packets It only succeeds if the Ping is a Computer network tool used to test whether a particular host is reachable across an IP network it is also used to self test the network interface It is very simple to launch, the primary requirement being access to greater bandwidth than the victim. In Computer networking and Computer science, digital bandwidth or just bandwidth is the capacity for a given system to transfer data over a connection
SYN flood sends a flood of TCP/SYN packets, often with a forged sender address. A SYN flood is a form of Denial-of-service attack in which an attacker sends a succession of SYN requests to a target's system Each of these packets is handled like a connection request, causing the server to spawn a half-open connection, by sending back a TCP/SYN-ACK packet, and waiting for packet in response from the sender address. However, because the sender address is forged, the response never comes. These half-open connections saturate the number of available connections the server is able to make, keeping it from responding to legitimate requests until after the attack ends.
A Teardrop attack involves sending mangled IP fragments with overlapping, over-sized, payloads to the target machine. In Computer networking, a mangled or invalid packet is a packet &mdashespecially IP packet&mdashwhich lacks sound order self-coherence or content The Internet Protocol ( IP) is a protocol used for communicating data across a Packet-switched Internetwork using the Internet Protocol A bug in the TCP/IP fragmentation re-assembly code of various operating systems causes the fragments to be improperly handled, crashing them as a result of this. The Internet Protocol Suite (commonly TCP/IP) is the set of Communications protocols used for the Internet and other similar networks Internet Protocol version 4 ( IPv4) is the fourth revision in the development of the Internet Protocol (IP and it is the first version of the protocol to be widely  Windows 3.1x, Windows 95 and Windows NT operating systems, as well as versions of Linux prior to versions 2. Windows 31x was a major release of Microsoft Windows. Several editions were released between 1992 and 1994 succeeding Windows 3 Windows 95 is a consumer-oriented Graphical user interface -based Operating system. Windows NT is a family of Operating systems produced by Microsoft, the first version of which was released in July 1993 Linux (commonly pronounced ˈlɪnəks 0. 32 and 2. 1. 63 are vulnerable to this attack.
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++. DC++ is a free and open-source, Peer-to-peer file-sharing client that can be used to connect to the Direct Connect network Peer-to-peer attacks are different from regular botnet-based attacks. 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 peer-to-peer file sharing hubs to disconnect from their peer-to-peer network and to connect to the victim’s website instead. As a result, several thousand computers may aggressively try to connect to a target website. While a typical web server can handle a few hundred connections/sec before performance begins to degrade, most web servers fail almost instantly under five or six thousand connections/sec. With a moderately big peer-to-peer attack a site could potentially be hit with up to 750,000 connections in a short order. The targeted web server will be plugged up and confused by the incoming connections. While peer-to-peer attacks are easy to identify with signatures, the large number of IP addresses that need to be blocked (often over 250,000 during the course of a big attack) means that this type of attack can overwhelm mitigation defenses. Even if a mitigation device can keep blocking IP addresses, there are other problems to consider. For instance, there is a brief moment where the connection is opened on the server side before the signature itself comes through. Only once the connection is opened to the server can the identifying signature be sent and detected, and the connection torn down. Even tearing down connections rapidly takes server resources and can harm the server.
A permanent denial-of-service (PDOS), also know loosely as phlashing, is an attack that damages a system so badly that it requires replacement or reinstallation of hardware.  Unlike the distributed denial-of-service attack a PDOS attack exploits security flaws in the remote management interfaces of the victim's hardware, be it routers, printers, or other networking hardware. These flaws leave the door opens for an attacker to remotely 'update' the hardware firmware to a modified, corrupt or defective firmware image, therefore bricking the device and making it permanently unusable of it's original purpose. In Computing, firmware is a computer program that is Embedded in a hardware device for example a Microcontroller. When used in reference to electronics "brick" describes a device that cannot function in any capacity (such as a machine with damaged Firmware) The PDOS is a pure hardware targeted attack which can be much faster, and waste less resources then using a botnet in a DDOS attack, because these features and the potential and high probability of security exploits on Network Enabled Embedded Devices (NEEDs) this technique has come to the attention of numerous hacker communities such as Hack A Day. 
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. 
On IRC, IRC floods are a common electronic warfare weapon.
Various DoS-causing exploits such as buffer overflow can cause server-running software to get confused and fill the disk space or consume all available memory or CPU time. An exploit (from the same word in the French language, meaning "achievement" or "accomplishment" is a piece of Software, a chunk of data or In Computer security and programming, a buffer overflow, or buffer overrun, is an anomalous condition where a process attempts to
Other kinds of DoS rely primarily on brute force, flooding the target with an overwhelming flux of packets, oversaturating its connection bandwidth or depleting the target's system resources. Bandwidth-saturating floods rely on the attacker having higher bandwidth available than the victim; a common way of achieving this today is via Distributed Denial of Service, employing a botnet. Botnet is a Jargon term for a collection of Software robots or bots that run autonomously and automatically Other floods may use specific packet types or connection requests to saturate finite resources by, for example, occupying the maximum number of open connections or filling the victim's disk space with logs.
A "banana attack" is another particular type of DoS. It involves redirecting outgoing messages from the client back onto the client, preventing outside access, as well as flooding the client with the sent packets.
An attacker with access to a victim's computer may slow it until it is unusable or crash it by using a fork bomb. In Computing, the fork bomb, a form of denial-of-service attack against a Computer system, implements the fork operation (or equivalent
A 'pulsing zombie' is a term referring to a special denial-of-service attack. A network is subjected to hostile pinging by different attacker computers over an extended amount of time. Computer networking is the Engineering Discipline concerned with communication between Computer systems or devices Networking routers Ping is a Computer network tool used to test whether a particular host is reachable across an IP network it is also used to self test the network interface This results in a degraded quality of service and increased workload for the network's resources. This type of attack is more difficult to detect than traditional denial-of-service attacks due to their surreptitious nature.
A Nuke is an old denial-of-service attack against computer networks consisting of fragmented or otherwise invalid ICMP packets sent to the target, achieved by using a modified ping utility to repeatedly send this corrupt data, thus slowing down the affected computer until it comes to a complete stop. The Internet Control Message Protocol ( ICMP) is one of the core protocols of the Internet Protocol Suite. Ping is a Computer network tool used to test whether a particular host is reachable across an IP network it is also used to self test the network interface
In online gaming, nuking is used by spamming another user, or all other users, with random repeated messages in quick succession. Such techniques are also seen in instant messaging programs as repeatedly sending text can be assigned to a macro or AppleScript. A macro (from the Greek 'μάκρο' for long or far in Computer science is a rule or Pattern that specifies how a certain input sequence (often a sequence AppleScript is a Scripting language devised by Apple Inc, and built into Mac OS. Modern operating systems are usually resistant to these nuke attacks, and online games now have third party "Flood control. An operating system (commonly abbreviated OS and O/S) is the software component of a Computer system that is responsible for the management and coordination "
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. The term WinNuke refers to a remote Denial-of-service attack (DoS that affected the Microsoft Windows 95, Microsoft Windows NT and NetBIOS ( is an acronym for Network Basic Input/Output System. Windows 95 is a consumer-oriented Graphical user interface -based Operating system. A string of out-of-band data was sent to TCP port 139 of the victim's machine, causing it to lock up and display a Blue Screen of Death. The Transmission Control Protocol (TCP is one of the core protocols of the Internet Protocol Suite. The Blue Screen of Death (also known as a stop error, BSoD, bluescreen, or Blue Screen of Doom) is an Error screen displayed
A distributed denial of service attack (DDoS) occurs when multiple compromised systems flood the bandwidth or resources of a targeted system, usually one or more web servers. These systems are compromised by attackers using a variety of methods.
Malware can carry DDoS attack mechanisms; one of the more well known examples of this was MyDoom. Mydoom, also known as W32MyDoom@mm, Novarg, MimailR and Shimgapi, is a Computer virus affecting Microsoft Windows. Its DoS mechanism was triggered on a specific date and time. This type of DDoS involved hardcoding the target IP address prior to release of the malware and no further interaction was necessary to launch the attack.
A system may also be compromised with a trojan, allowing the attacker to download a zombie agent (or the trojan may contain one). Attackers can also break into systems using automated tools that exploit flaws in programs that listen for connections from remote hosts. This scenario primarily concerns systems acting as servers on the web.
Stacheldraht is a classic example of a DDoS tool. Stacheldraht ( German for Barbed wire) is a piece of Software written by Random for Linux and Solaris systems which acts as a It utilizes a layered structure where the attacker uses a client program to connect to handlers, which are compromised systems that issue commands to the zombie agents, which in turn facilitate the DDoS attack. A zombie computer (often shortened as zombie) is a computer attached to the Internet that has been compromised by a hacker, a Computer virus Agents are compromised via the handlers by the attacker, using automated routines to exploit vulnerabilities in programs that accept remote connections running on the targeted remote hosts. Each handler can control up to a thousand agents. 
These collections of compromised systems are known as botnets. DDoS tools like stacheldraht still use classic DoS attack methods centered around IP spoofing and amplification like smurf attacks and fraggle attacks (these are also known as bandwidth consumption attacks). In Computer networking, the term IP ( Internet Protocol) address spoofing refers to the creation of IP packets with a forged (spoofed source The Smurf attack is a way of generating a lot of Computer network traffic to a victim host In Computer security a fraggle attack is a type of Denial-of-service attack where an attacker sends a large amount of UDP echo traffic to IP Broadcast SYN floods (also known as resource starvation attacks) may also be used. Newer tools can use DNS servers for DoS purposes. (see next section)
Unlike MyDoom's DDoS mechanism, botnets can be turned against any IP address. Script kiddies use them to deny the availability of well known websites to legitimate users. In Hacker culture, a script kiddie (as opposed to " Speed kiddie " occasionally script bunny, skiddie, script kitty  More sophisticated attackers use DDoS tools for the purposes of extortion — even against their business rivals. Extortion, outwresting, or exaction is a criminal offense, which occurs when a person Unlawfully obtains either money property or services 
It is important to note the difference between a DDoS and DoS attack. If an attacker mounts a smurf attack from a single host it would be classified as a DoS attack. In fact, any attack against availability would be classed as a Denial of Service attack. On the other hand, if an attacker uses a thousand zombie systems to simultaneously launch smurf attacks against a remote host, this would be classified as a DDoS attack.
The major advantages to an attacker of using a distributed denial-of-service attack are that multiple machines can generate more attack traffic than one machine, multiple attack machines are harder to turn off than one attack machine, and that the behavior of each attack machine can be stealthier, making it harder to track down and shut down. These attacker advantages cause challenges for defense mechanisms. For example, merely purchasing more incoming bandwidth than the current volume of the attack might not help, because the attacker might be able to simply add more attack machines.
Although most DDoS attacks are malicious in nature, the same technique can be used to aid the Internet community. Internet fraud schemes, such as Nigerian 419 scams or phishing, commonly involve fraudulent websites that either impersonate a real website for purposes of stealing the victim's identity, or lend credibility to a scammer's fictional business venture to lure the victim into a false sense of confidence. An advance-fee fraud is a Confidence trick in which the target is persuaded to advance sums of money in the hope of realizing a very much larger gain In the field of computer security phishing is the Criminally Fraudulent process of attempting to acquire sensitive information such as usernames Passwords Scam baiters, who combat these scams by posing as victims for the purpose of wasting the scammer's time and money and obtaining information that can be used by authorities, will forward sites they encounter during the course of their conversations to groups that specialize in site-killing. Scam baiting is the practice of pretending interest in a fraudulent scheme in order to manipulate a Scammer The purpose of scam baiting might be to waste the scammers' time The group will first try to have a site taken down by informing the host of said site that the site is being used fraudulently. In the case where that approach fails, the group will organize a "takedown" of the site by encouraging its members to visit the site en masse and continually refresh its content (an intentional form of the Slashdot effect sometimes referred to as flash mobbing, although that term is technically reserved for real-world gatherings). The Slashdot effect, also known as slashdotting, is the phenomenon of a popular Website linking to a smaller site causing the smaller site to slow down or even temporarily A flash mob is a large group of people Alternately, some groups have special web pages that link to images hosted by these fake sites and show the images to visitors (usually members or supporters of the site-killing group) while constantly reloading them, which is known as intentional bandwidth hogging. Bandwidth hogs are otherwise legitimate users of a paid or free service who use so much bandwidth that it adversely affects other users or the company's ability to make a The purpose, similar to malicious DoS attacks, is to (a. ) rapidly consume all of the website's allocated monthly bandwidth, after which requests for the site's content are refused, (b. ) draw the attention of the site's host, who when faced with the constant onslaught on the entire hosting network's resources, will usually remove the site, and/or (c. ) take up all available connections and maximum throughput of the host so that would-be victims cannot access the site. In Communication networks, such as Ethernet or Packet radio, throughput is the average rate of successful message delivery over a communication channel
A distributed reflected denial of service attack (DRDoS) involves sending forged requests of some type to a very large number of computers that will reply to the requests. Using Internet protocol spoofing, the source address is set to that of the targeted victim, which means all the replies will go to (and flood) the target. In Computer networking, the term IP ( Internet Protocol) address spoofing refers to the creation of IP packets with a forged (spoofed source
ICMP Echo Request attacks (Smurf Attack) can be considered one form of reflected attack, as the flooding host(s) send Echo Requests to the broadcast addresses of mis-configured networks, thereby enticing many hosts to send Echo Reply packets to the victim. The Smurf attack is a way of generating a lot of Computer network traffic to a victim host Some early DDoS programs implemented a distributed form of this attack.
Many services can be exploited to act as reflectors, some harder to block than others.  DNS amplification attacks involve a new mechanism that increased the amplification effect, using a much larger list of DNS servers than seen earlier. 
This describes a situation where a website ends up denied, not due to a deliberate attack by a single individual or group of individuals, but simply due to a sudden enormous spike in popularity. This can happen when an extremely popular website posts a prominent link to a second, less well-prepared site, for example, as part of a news story. The result is that a significant proportion of the primary site's regular users — potentially hundreds of thousands of people — click that link in the space of a few hours, having the same effect on the target website as a DDoS attack.
News sites and link sites — sites whose primary function is to provide links to interesting content elsewhere on the Internet — are most likely to cause this phenomenon. The canonical example is the Slashdot effect. The Slashdot effect, also known as slashdotting, is the phenomenon of a popular Website linking to a smaller site causing the smaller site to slow down or even temporarily Sites such as Digg, Fark, Something Awful, and the webcomic Penny Arcade have their own corresponding "effects", known as "the Digg effect", "farking", "goonrushing" and "wanging"; respectively. Digg is a Website made for people to discover and share content from anywhere on the Internet, by submitting links and stories and voting and commenting on submitted Something Awful, often abbreviated to Penny Arcade is a Webcomic written by Jerry Holkins and illustrated by Mike Krahulik.
Routers have also been known to create unintentional DoS attacks, as both D-Link and Netgear routers have created NTP vandalism by flooding NTP servers without respecting the restrictions of client types or geographical limitations. D-Link Corporation was founded in 1986 in Taipei as Datex Systems Inc Netgear, founded in 1996 is a US manufacturer of Computer networking equipment and other Computer hardware. NTP server misuse and abuse covers a number of practices which cause damage or degradation to an NTP server ranging from flooding it with traffic (effectively a DDoS
Similar unintentional attacks can also occur via other media, e. g. when a URL is mentioned on television. If a server is being indexed by Google or another search engine during peak periods of activity, or does not have a lot of available bandwidth while being indexed, it can also experience the effects of a DoS attack. Google Inc is an American public corporation, earning revenue from advertising related to its Internet search, e-mail, online
The first major attack involving DNS servers as reflectors occurred in January 2001. The target was Register. com.  This attack, which forged requests for the MX records of AOL. com (to amplify the attack) lasted about a week before it could be traced back to all attacking hosts and shut off. It used a list of tens of thousands of DNS records that were a year old at the time of the attack.
In February, 2001, the Irish Government's Department of Finance server was hit by a denial of service attack carried out as part of a student campaign from NUI Maynooth. The Department officially complained to the University authorities and a number of students were disciplined.
In July 2002, the Honeynet Project Reverse Challenge was issued.  The binary that was analyzed turned out to be yet another DDoS agent, which implemented several DNS related attacks, including an optimized form of a reflection attack.
On two occasions to date, attackers have performed DNS Backbone DDoS Attacks on the DNS root servers. The DNS Backbone DDoS Attacks have been several significant Internet events in which distributed denial of service attacks (DDoS have targeted one or more of the Since these machines are intended to provide service to all Internet users, these two denial of service attacks might be classified as attempts to take down the entire Internet, though it is unclear what the attackers' true motivations were. The first occurred in October 2002 and disrupted service at 9 of the 13 root servers. The second occurred in February 2007 and caused disruptions at two of the root servers.
In February 2007, more than 10,000 online game servers like Return to Castle Wolfenstein, Halo, Counter-Strike and many others were attacked by "RUS" hacker group. Return to Castle Wolfenstein is a First person shooter Computer game published by Activision and originally released on November 19 Halo is a Science fiction Video game franchise created by Bungie and owned and published by Microsoft Game Studios. Counter-Strike (commonly abbreviated to CS) is a tactical First-person shooter Video game which originated from a The DDoS attack was made from more than a thousand computer units located in the republics of the former Soviet Union, mostly from Russia, Uzbekistan and Belarus. The Union of Soviet Socialist Republics (USSR was a constitutionally Socialist state that existed in Eurasia from 1922 to 1991 Russia (Россия Rossiya) or the Russian Federation ( Rossiyskaya Federatsiya) is a transcontinental Country extending Uzbekistan, officially the Republic of Uzbekistan ( Uzbek: O‘zbekiston Respublikasi or Ўзбекистон Республикаси is a doubly Belarus ( Belarusian Беларусь / Biełaruś is a Landlocked country in Eastern Europe, bordered by Russia to the north and east Minor attacks are still continuing to be made today.
In late January 2008, a group calling themselves Anonymous began a DDoS attack on Scientology web sites as a part of an Anti-Scientology campaign called Project Chanology. The self-styled Anonymous (used as a Mass noun) is a label and Internet meme adopted within Internet culture to represent the actions of many online community Scientology is a body of beliefs and related practices initially created by American Science fiction author L Project Chanology, also called Operation Chanology, In the video music from Cruise's Mission Impossible films plays in the background and Cruise makes 
The investigative process should begin immediately after the DoS attack begins. There will be multiple phone calls, callbacks, emails, pages and faxes between the victim organization, one's provider, and others involved. In Telecommunications, a callback (also written as call-back) occurs when the originator of a call is immediately called back in a second call as a response Electronic mail, often abbreviated to e-mail, email, or originally eMail, is a Store-and-forward method of writing sending receiving A pager (sometimes called a beeper) is a simple personal Telecommunications device for short messages Fax (short for facsimile, from Latin fac simile, "make similar" i This can be a very time consuming process. It has taken some very large networks with plenty of resources several hours to halt a DoS attack.
The easiest way to survive an attack is to have planned for the attack. Having a separate emergency block of IP addresses for critical servers with a separate route can be invaluable. An Internet Protocol ( IP) address is a numerical identification ( Logical address) that is assigned to devices participating in a Computer network A separate route (perhaps a DSL) is not that extravagant, and it can be used for load balancing or sharing under normal circumstances and switched to emergency mode in the event of an attack.
Filtering is often ineffective, as the route to the filter will normally be swamped so only a trickle of traffic will survive. However, by using an extremely resilient stateful packet filter that will inexpensively drop any unwanted packets, surviving a DoS attack becomes much easier. A firewall is an integrated collection of security measures designed to prevent unauthorized electronic access to a networked computer system When such a high performance packet filtering server is attached to an ultra-high bandwidth connection (preferably an internet backbone), communication with the outside world will be unimpaired so long as not all of the available bandwidth is saturated, and performance behind the packet filter will remain normal as long as the packet filter drops all DoS packets. A firewall is an integrated collection of security measures designed to prevent unauthorized electronic access to a networked computer system The Internet backbone refers to the main "trunk" connections of the Internet.  It should be noted however, that in this case the victim of the DoS attack still would need to pay for the excessive bandwidth. The price of service unavailability thus needs to be weighed against the price of truly exorbitant bandwidth/traffic.
Firewalls have simple rules such as to allow or deny protocols, ports or IP addresses. A firewall is an integrated collection of security measures designed to prevent unauthorized electronic access to a networked computer system Some DoS attacks are too complex for today's firewalls, e. g. if there is an attack on port 80 (web service), firewalls cannot prevent that attack because they cannot distinguish good traffic from DoS attack traffic. Additionally, firewalls are too deep in the network hierarchy. Your router may be affected even before the firewall gets the traffic. Nonetheless, firewalls can effectively prevent users from launching simple flooding type attacks from machines behind the firewall.
Modern stateful firewalls like Check Point FW1 NGX & Cisco PIX have a built-in capability to differentiate good traffic from DoS attack traffic. In Computing, a stateful firewall (any firewall that performs stateful packet inspection ( SPI) or stateful inspection) is a firewall Check Point Software Technologies Ltd ( is a hardware and software company that is best known for its firewall and VPN products Cisco PIX ( Private Internet eXchange) is a popular IP firewall and Network address translation (NAT appliance. This capability is known as a "Defender", as it confirms TCP connections are valid before proxying TCP packets to service networks (including border routers). A similar ability is present in OpenBSD's pF, which is available for other BSDs as well. In that context, it is called "synproxy".
Comodo Firewall Pro has a built-in Emergency Mode which is activated when the number of incoming packets per seconds exceed a set value for more than the specified time, for example, more than 20 packets/sec for more than 20 seconds. If this happens, the firewall classifies it as a DoS attack and switches to Emergency Mode. In this mode, all inbound traffic is blocked except previously established and active connections, but outbound traffic is allowed. The packet number threshold and the time needed for verifying an attack can be adjusted by the user separately for TCP, UDP and ICMP. The firewall also has some other attack prevention mechanisms, like protocol analysis, checksum verification (so that the packets are not altered since transmission) and NDIS protocol monitoring for attempts at making a DoS attack by using own protocols, thus outmaneuvering older firewalls.
Most switches have some rate-limiting and ACL capability. In Computer security, an access control list ( ACL) is a list of permissions attached to an object Some switches provide automatic and or system-wide rate limiting, traffic shaping, delayed binding (TCP splicing), deep packet inspection and Bogon filtering (bogus IP filtering) to detect and remediate denial of service attacks through automatic rate filtering and WAN Link failover and balancing. In Computer networks rate limiting is used to control the rate of traffic sent or received on a Network interface. Traffic shaping (also known as "packet shaping" is the control of Computer network traffic in order to optimize or guarantee performance lower latency, Delayed binding, also called TCP splicing, is the postponement of the connection between the client and the server in order to obtain sufficient information to make a routing Delayed binding, also called TCP splicing, is the postponement of the connection between the client and the server in order to obtain sufficient information to make a routing Deep packet inspection ( DPI) (or sometimes complete packet inspection) is a form of Computer network Packet filtering that examines the Data " Bogon " is an informal name for an IP packet on the public Internet that claims to be from an area of the IP Address space reserved
These schemes will work as long as the DoS attacks are something that can be prevented by using them. For example SYN flood can be prevented using delayed binding or TCP splicing. Similarly content based DoS can be prevented using deep packet inspection. Attacks originating from dark addresses or going to dark addresses can be prevented using Bogon filtering. Automatic rate filtering can work as long as you have set rate-thresholds correctly and granularly. Wan-link failover will work as long as both links have DoS/DDoS prevention mechanism.
Similar to switches, routers have some rate-limiting and ACL capability. In Computer security, an access control list ( ACL) is a list of permissions attached to an object They, too, are manually set. Most routers can be easily overwhelmed under DoS attack. If you add rules to take flow statistics out of the router during the DoS attacks, they further slow down and complicate the matter. Cisco IOS has features that prevents flooding, i. Cisco IOS (originally Internetwork Operating System) is the software used on the vast majority of Cisco Systems Routers and all current Cisco Network e. example settings. 
Application front end hardware is intelligent hardware placed on the network before traffic reaches the servers. It can be used on networks in conjunction with routers and switches. Application front end hardware analyzes data packets as they enter the system, and then identifies them as priority, regular, or dangerous. There are more than 25 bandwidth management vendors. Hardware acceleration is key to bandwidth management. Look for granularity of bandwidth management, hardware acceleration, and automation while selecting an appliance.
Intrusion-prevention systems (IPS) are effective if the attacks have signatures associated with them. An intrusion prevention system is a Network security device that monitors network and/or system activities for malicious or unwanted behavior and can react in real-time to However, the trend among the attacks is to have legitimate content but bad intent. Intrusion-prevention systems which work on content recognition cannot block behavior based DoS attacks.
An ASIC based IPS can detect and block denial of service attacks because they have the processing power and the granularity to analyze the attacks and act like a circuit breaker in an automated way.
A rate-based IPS (RBIPS) must analyze traffic granularly and continuously monitor the traffic pattern and determine if there is traffic anomaly. An intrusion prevention system is a Network security device that monitors network and/or system activities for malicious or unwanted behavior and can react in real-time to It must let the legitimate traffic flow while blocking the DoS attack traffic.
In computer network security, backscatter is a side-effect of a spoofed denial of service (DoS) attack. In this kind of attack, the attacker spoofs (or forges) the source address in IP packets sent to the victim. The Internet Protocol ( IP) is a protocol used for communicating data across a Packet-switched Internetwork using the Internet Protocol In Information technology, a packet is a formatted unit of Data carried by a Packet mode Computer network. In general, the victim machine can not distinguish between the spoofed packets and legitimate packets, so the victim responds to the spoofed packets as it normally would. These response packets are known as backscatter.
If the attacker is spoofing source addresses randomly, the backscatter response packets from the victim will be sent back to random destinations. This effect can be used by network telescopes as an indirect evidence of such attacks. A network telescope (also known as a darknet, internet motion sensor or black hole) is an Internet system that allows one to observe different
The term "backscatter analysis" refers to observing backscatter packets arriving at a statistically significant portion of the IP address space to determine characteristics of DoS attacks and victims. An Internet Protocol ( IP) address is a numerical identification ( Logical address) that is assigned to devices participating in a Computer network
An educational animation describing such backscatter can be found on the animations page maintained by CAIDA, the Cooperative Association for Internet Data Analysis. The Cooperative Association for Internet Data Analysis (CAIDA ˈkeɪdə is a collaborative undertaking among organizations in the commercial government and research sectors aimed at The Cooperative Association for Internet Data Analysis (CAIDA ˈkeɪdə is a collaborative undertaking among organizations in the commercial government and research sectors aimed at