http://linuxtoday.com/news_story.php3?ltsn=2001-11-03-002-20-SC-RH


Syncookies, while not enabled in default installations of Red Hat Linux,
are used to protect an Internet server against a certain type of DoS
attack--the so called "synflood"--by using a cryptographic challenge
protocol which ensures legitimate users can keep using the server. Under an
attack, the TCP/IP layer will, instead of just accepting new connections,
send back the challenge and only accept the connections in the
second phase ("syn ack") of the TCP/IP handshake (where the other party
returns the challenge value). The DoS attack, which consists of sending as
many first phase ("syn") packets as possible will be neutralized because
system resources are only used as part of the second phase.

Certain firewall configurations only filter the first phase ("syn") packets
to prevent connections to specific services. These systems are vulnerable
when an attacker can both force a system into flood protection state (by
starting a synflood attack on a non-firewalled port) and guess the
cryptographic challenge of a firewalled port.

While the cryptographic hash used is strong, the number of bits available
is restricted by the TCP protocol header design. With a high speed link and
a lot of time, an attacker can eventually succeed in faking a valid cookie
and making a connection that a syn only firewall rule might have
prohibited.

The updated kernels have a modified synflood protection algorithm that now
uses a per port "under attack" state so that ports with only a first-phase
firewall rule will not use the "under attack" regime even when other,
non-firewalled, ports are under attack.


In addition, these packages fix a remote denial of service attack against
the TUX web server.  This attack can only succeed if the TUX web server
has been explicitly enabled; it is disabled by default.  Thanks to
Aidan O'Rawe for finding this bug.

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