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P-662H/HW-D Series User’s Guide
Chapter 10 Firewalls 173
Under normal circumstances, the application that initiates a session sends a SYN
(synchronize) packet to the receiving server. The receiver sends back an ACK
(acknowledgment) packet and its own SYN, and then the initiator responds with an ACK
(acknowledgment). After this handshake, a connection is established.
• SYN Attack floods a targeted system with a series of SYN packets. Each packet causes
the targeted system to issue a SYN-ACK response. While the targeted system waits for
the ACK that follows the SYN-ACK, it queues up all outstanding SYN-ACK responses
on what is known as a backlog queue. SYN-ACKs are moved off the queue only when an
ACK comes back or when an internal timer (which is set at relatively long intervals)
terminates the three-way handshake. Once the queue is full, the system will ignore all
incoming SYN requests, making the system unavailable for legitimate users.
Figure 92 SYN Flood
•In a LAND Attack, hackers flood SYN packets into the network with a spoofed source
IP address of the targeted system. This makes it appear as if the host computer sent the
packets to itself, making the system unavailable while the target system tries to respond
to itself.
7 A brute-force attack, such as a "Smurf" attack, targets a feature in the IP specification
known as directed or subnet broadcasting, to quickly flood the target network with
useless data. A Smurf hacker floods a router with Internet Control Message Protocol
(ICMP) echo request packets (pings). Since the destination IP address of each packet is
the broadcast address of the network, the router will broadcast the ICMP echo request
packet to all hosts on the network. If there are numerous hosts, this will create a large
amount of ICMP echo request and response traffic. If a hacker chooses to spoof the
source IP address of the ICMP echo request packet, the resulting ICMP traffic will not
only clog up the "intermediary" network, but will also congest the network of the spoofed
source IP address, known as the "victim" network. This flood of broadcast traffic
consumes all available bandwidth, making communications impossible.