SYN attack
Specifically, a SYN attack, or SYN flood attack, is a well-known TCP/IP attack in which a
malicious attacker targets a vulnerable device and effectively denies it from establishing new
TCP connections.
The SYN attack is characterized by the following pattern:
Using a spoofed IP address, an attacker sends multiple SYN packets to a listening TCP port
on the target machine (the victim). For each SYN packet received, the target machine allocates
resources and sends an acknowledgement (SYN-ACK) to the source IP address. The TCP
connection is called a “half-open” connection at this point since the initiating side did not yet
send back an acknowledgment (termed the third ACK).
Because the target machine does not receive a response from the attacking machine, it
attempts to resend the SYN-ACK, typically five times, at 3-, 6-, 12-, 24-, and 48-second
intervals, before de-allocating the resources, 96 seconds after attempting the last resend.
Altogether, the target machine typically allocates resources for over three minutes to respond
to a single SYN attack.
When an attacker uses this technique repeatedly, the target machine eventually runs out of
memory resources since it holds numerous half-open connections. It is unable to handle any
more connections, thereby denying service to legitimate users.
Moreover, flooding the victim with TCP SYN at a high rate can cause the internal queues to
fill up, also causing a denial of service.
SYN cookies
SYN cookies refers to a well-known method of protection against a SYN attack.
SYN cookies protect against SYN attacks by employing the following strategies:
• Not maintaining any state for half-open inbound TCP sessions, thus preventing the SYN
attack from depleting memory resources.
SYN cookies are able to maintain no state for half-open connections by responding to
SYN requests with a SYN-ACK that contains a specially crafted initial sequence number
(ISN), called a cookie. The value of the cookie is not a pseudo-random number generated
by the system, but the result of a hash function. The hash result is generated from the
source IP, source port, destination IP, destination port, and some secret values. The
cookie can be verified when receiving a valid third ACK that establishes the connection.
Special security features
Administering Avaya G430 Branch Gateway October 2013 53
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