AX/DX/EX/PX Series User’s Guide
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CHAPTER 19
Firewall
19.1 Firewall Overview
This chapter shows you how to enable the Zyxel Device firewall. Use the firewall to protect your Zyxel
Device and network from attacks by hackers on the Internet and control access to it. The firewall:
• allows traffic that originates from your LAN computers to go to all other networks.
• blocks traffic that originates on other networks from going to the LAN.
By default, the Zyxel Device blocks DoS attacks whether the firewall is enabled or disabled.
The following figure illustrates the firewall action. User A can initiate an IM (Instant Messaging) session
from the LAN to the WAN (1). Return traffic for this session is also allowed (2). However other traffic
initiated from the WAN is blocked (3 and 4).
Figure 201 Default Firewall Action
19.1.1 What You Need to Know About Firewall
SYN Attack
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.
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