Chapter 17 Firewall
EX5501-B0 / AX7501-B0 / PX7501-B0 User’s Guide
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17.1.2 What You Need to Know
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.
DoS
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.
DDoS
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.
LAND Attack
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
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.
SPI
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.
17.2 Firewall Settings
Use this screen to set the security level of the firewall on the Zyxel Device. Firewall rules are grouped
based on the direction of travel of packets to which they apply. A higher firewall level means more
restrictions on the Internet activities you can perform.
Note: LAN to WAN is your access to all Internet services. WAN to LAN is the access of other
computers on the Internet to devices behind the Zyxel Device.
