ZyXEL 2WG 사용자 가이드

ZyWALL 2WG User’s Guide

This chapter covers how to use the ZyWALL’s ALG feature to allow certain applications to 
pass through the ZyWALL.
An Application Layer Gateway (ALG) manages a specific protocol (such as SIP, H.323 or 
FTP) at the application layer. The ZyWALL can function as an ALG to allow certain NAT un-
friendly applications (such as SIP) to operate properly through the ZyWALL. 
Some applications cannot operate through NAT (are NAT un-friendly) because they embed IP 
addresses and port numbers in their packets’ data payload. The ZyWALL examines and uses 
IP address and port number information embedded in the data stream. When a device behind 
the ZyWALL uses an application for which the ZyWALL has ALG service enabled, the 
ZyWALL translates the device’s private IP address inside the data stream to a public IP 
address. It also records session port numbers and dynamically creates implicit NAT port 
forwarding and firewall rules for the application’s traffic to come in from the WAN to the 
LAN. 
To configure the ALG screen proceed to 

The ZyWALL dynamically creates an implicit NAT session for the application’s traffic from 
the WAN to the LAN.
The ALG on the ZyWALL supports all NAT mapping types, including One to One, Many to 
One, Many to Many Overload and Many One to One.

The ZyWALL uses the dynamic port that the session uses for data transfer in creating an 
implicit temporary firewall rule for the session’s traffic. The firewall rule only allows the 
session’s traffic to go through in the direction that the ZyWALL determines from its inspection 
of the data payload of the application’s packets. The firewall rule is automatically deleted after 
the application’s traffic has gone through.

When the ZyWALL has two WAN interfaces and uses the second highest priority WAN 
interfaces as a back up, traffic cannot pass through when the primary WAN connection fails. 
The ZyWALL does not automatically change the connection to the secondary WAN interfaces.