ZyXEL Communications ZyWALL5UTM 4.0 User Manual

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ZyWALL 5/35/70 Series User’s Guide
Chapter 10 Firewalls
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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 91   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. 
• 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.