A SYN flood is a form of
denial-of-service attack
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 users by temporarily or indefinitely disrupting services of a host co ...
on
data communication
Data communication, including data transmission and data reception, is the transfer of data, transmitted and received over a point-to-point or point-to-multipoint communication channel. Examples of such channels are copper wires, optic ...
s in which an attacker rapidly initiates a connection to a server without finalizing the connection. The server has to spend resources waiting for half-opened connections, which can consume enough resources to make the system unresponsive to legitimate traffic.
The
packet that the attacker sends is the
SYN packet, a part of
TCP's
three-way handshake
The Transmission Control Protocol (TCP) is one of the main protocols of the Internet protocol suite. It originated in the initial network implementation in which it complemented the Internet Protocol (IP). Therefore, the entire suite is common ...
used to establish a connection.
Technical details
When a client attempts to start a
TCP connection to a server, the
client
Client(s) or The Client may refer to:
* Client (business)
* Client (computing), hardware or software that accesses a remote service on another computer
* Customer or client, a recipient of goods or services in return for monetary or other valuable ...
and
server
Server may refer to:
Computing
*Server (computing), a computer program or a device that provides requested information for other programs or devices, called clients.
Role
* Waiting staff, those who work at a restaurant or a bar attending custome ...
exchange a series of messages which normally runs like this:
#The client requests a connection by sending a
SYN (''synchronize'') message to the server.
#The server ''acknowledges'' this request by sending
SYN-ACK back to the client.
#The client responds with an
ACK, and the connection is established.
This is called the
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
ACK code. The malicious client can either simply not send the expected
ACK, or by
spoofing the source
IP address
An Internet Protocol address (IP address) is a numerical label such as that is assigned to a device connected to a computer network that uses the Internet Protocol for communication. IP addresses serve two main functions: network interface i ...
in the
SYN, cause the server to send the
SYN-ACK to a falsified IP address – which will not send an
ACK because it "knows" that it never sent a
SYN.
The server will wait for the acknowledgement for some time, as simple network congestion could also be the cause of the missing
ACK. However, in an attack, the ''
half-open connections'' 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
half-open TCP
#SYN cache
#
SYN cookies
#Hybrid approaches
#Firewalls and proxies
See also
*
Fraggle attack
*
Internet Control Message Protocol
The Internet Control Message Protocol (ICMP) is a supporting protocol in the Internet protocol suite. It is used by network devices, including routers, to send error messages and operational information indicating success or failure when com ...
*
IP address spoofing
*
Ping flood
*
Smurf attack
A Smurf attack is a distributed denial-of-service attack in which large numbers of Internet Control Message Protocol (ICMP) packets with the intended victim's spoofed source IP are broadcast to a computer network using an IP broadcast address. ...
*
UDP flood attack
References
External links
Official CERT advisory on SYN Attacks
{{DEFAULTSORT:Syn Flood
Attacks against TCP
Denial-of-service attacks