ZyXEL Communications Corporation VSG1435B101 Manuel D’Utilisation

Chapter 16 Firewall

VSG1435-B101 Series User’s Guide

A 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 a backlog queue. SYN-ACKs are moved off the queue only 
when an ACK comes back or when an internal timer 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.

Denials of Service (DoS) attacks are aimed at devices and networks with a 
connection to the Internet. Their goal is not to steal information, but to disable a 
device or network so users no longer have access to network resources. The 
ZyXEL Device is pre-configured to automatically detect and thwart all known DoS 
attacks.

A DDoS attack is one in which multiple compromised systems attack a single 
target, thereby causing denial of service for users of the targeted system.

In a LAND attack, hackers flood SYN packets into the network with a spoofed 
source IP address of the target 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.

Ping of Death uses a "ping" utility to create and send an IP packet that exceeds 
the maximum 65,536 bytes of data allowed by the IP specification. This may 
cause systems to crash, hang or reboot.

Stateful Packet Inspection (SPI) tracks each connection crossing the firewall and 
makes sure it is valid. Filtering decisions are based not only on rules but also 
context. For example, traffic from the WAN may only be allowed to cross the 
firewall in response to a request from the LAN.