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'{{refimprove|date=January 2017}} [[Image:Tcp normal.svg|thumb|A normal connection between a user ([[Alice and Bob|Alice]]) and a server. The three-way handshake is correctly performed.]] [[Image:Tcp synflood.png|thumb|SYN Flood. The attacker ([[Alice and Bob|Mallory]]) sends several packets but does not send the "ACK" back to the server. The connections are hence half-opened and consuming server resources. Alice, a legitimate user, tries to connect but the server refuses to open a connection resulting in a denial of service.]] A '''SYN flood''' is a form of [[denial-of-service attack]] in which an attacker sends a succession of <code>[[SYN (TCP)|SYN]]</code> requests to a target's system in an attempt to consume enough server resources to make the system unresponsive to legitimate traffic.<ref>[https://www.cert.org/historical/advisories/CA-1996-21.cfm TCP SYN Flooding and IP Spoofing Attacks], 1996 Advisory, Software Engineering Institute, Carnegie-Mellon University</ref><ref>[https://partners.nytimes.com/library/cyber/week/0914panix.html New York's Panix Service Is Crippled by Hacker Attack], New York Times, September 14, 1996</ref> ==Technical details== Normally when a client attempts to start a [[Transmission Control Protocol|TCP]] connection to a server, the [[client (computing)|client]] and [[Server (computing)|server]] exchange a series of messages which normally runs like this: #The client requests a connection by sending a <code>SYN</code> (''synchronize'') message to the server. #The server ''acknowledges'' this request by sending <code>SYN-ACK</code> back to the client. #The client responds with an <code>ACK</code>, and the connection is established. This is called the [[Transmission Control Protocol#Connection establishment|TCP three-way handshake]], and is the foundation for every connection established using the TCP protocol. A SYN flood attack works by not responding to the server with the expected <code>ACK</code> code. The malicious client can either simply not send the expected <code>ACK</code>, or by [[IP address spoofing|spoofing]] the source [[IP address]] in the <code>SYN</code>, causing the server to send the <code>SYN-ACK</code> to a falsified IP address - which will not send an <code>ACK</code> because it "knows" that it never sent a <code>SYN</code>. The server will wait for the acknowledgement for some time, as simple network congestion could also be the cause of the missing <code>ACK</code>. However, in an attack, the ''[[half-open connection]]s'' created by the malicious client bind resources on the server and may eventually exceed the resources available on the server. At that point, the server cannot connect to any clients, whether legitimate or otherwise. This effectively denies service to legitimate clients. Some systems may also malfunction or crash when other operating system functions are starved of resources in this way. ==Countermeasures== There are a number of well-known countermeasures listed in RFC 4987 including: #Filtering #Increasing Backlog #Reducing SYN-RECEIVED Timer #Recycling the Oldest [[TCP half-open|Half-Open TCP]] #SYN Cache #[[SYN cookies]] #Hybrid Approaches #Firewalls and Proxies ==See also==<!-- PLEASE RESPECT ALPHABETICAL ORDER --> * [[Denial-of-service attack]] * [[Fraggle attack]] * [[Internet Control Message Protocol]] * [[IP address spoofing]] * [[Ping flood]] * [[Smurf attack]] * [[UDP flood attack]] ==References== <references/> ==External links== *[http://www.cert.org/advisories/CA-1996-21.html Official CERT advisory on SYN Attacks] {{DEFAULTSORT:Syn Flood}} [[Category:Denial-of-service attacks]]'