Linux Advisory Watch – January 6, 2006

5. The SYN Flooding Attack

When a normal TCP connection starts, a destination host receives a SYN (synchronize/start)
packet from a source host and sends back a SYN ACK (synchronize acknowledge).
The destination host must then hear an ACK (acknowledge) of the SYN ACK before
the connection is established. This is referred to as the “TCP three-way handshake.”

While waiting for the ACK to the SYN ACK, a connection queue of finite size
on the destination host keeps track of connections waiting to be completed.
This queue typically empties quickly since the ACK is expected to arrive a few
milliseconds after the SYN ACK.

The TCP SYN flood attack exploits this design by having an attacking source
host generate TCP SYN packets with random source addresses toward a victim host.
The victim destination host sends a SYN ACK back to the random source address
and adds an entry to the connection queue. Since the SYN ACK is destined for
an incorrect or nonexistent host, the last part of the “three-way handshake”
is never completed and the entry remains in the connection queue until a timer
expires, typically for about one minute.

By generating phony TCP SYN packets from random IP addresses at a rapid rate,
it is possible to fill up the connection queue and deny TCP services such as
e-mail, file Transfer or WWW to legitimate users.

There is no easy way to trace the originator of the attack because the IP
address of the source is forged.

5.1 TCP SYN flood

A TCP SYN flood is an attack based on bogus TCP connection requests, created
with a spoofed source IP address, sent to the attacked system. Connections are
not completed, thus soon it will fill up the connection request table of the
attacked system, preventing it from accepting any further valid connection request.

The source host for the attack sends a SYN packet to the target host. The
target hosts replies with a SYN/ACK back to the legitimate user of the forged
IP source address.

Since the spoofed source IP address is unreachable, the attacked system will
never receive the corresponding ACK packets in return, and the connection request
table on the

Attacked system will soon be filled up.The attack works if the spoofed source
IP address is not reachable by the attacked system. If the spoofed source IP
address where reachable by the attacked system, then the legitimate owner of
the source IP address would respond with a RST packet back to the target host,
closing the connection and defeating the attack.

TCP SYN flood is a denial of service attack that sends a host more TCP SYN
packets than the protocol implementation can handle.

This is a resource starvation DoS attack because once the connection table
is full; the server is unable to service legitimate requests.

5.2 TCP SYN flood protection

5.2.1 Apply Operating System fixes

Systems periodically check incomplete connection requests, and randomly clear
connections that have not completed a three-way handshake. This will reduce
the likelihood of a complete block due to a successful SYN attack, and allow
legitimate client connections to proceed.

• Configure TCP SYN traffic rate limiting
• Install IDS (Intrusion Detection Systems) capable of detecting TCP SYN
  flood attacks

5.2.2 Filter network traffic

Use circuit level firewalls (stateful inspection) to monitor the handshake
of each new connection and maintain the state of established TCP connections.
The filtering system must be able to distinguish harmful uses of a network service
from legitimate uses.

Static packet filtering (stateless) does not protect from TCP SYN flood attacks.

Read Article:
http://www.linuxsecurity.com/content/view/121169/49/