ZyWALL Series CLI Reference Guide
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CHAPTER 25
ALG
This chapter covers how to use the Zyxel Device’s ALG feature to allow certain applications to pass
through the Zyxel Device.
25.1 ALG Introduction
The Zyxel Device can function as an Application Layer Gateway (ALG) to allow certain NAT un-friendly
applications (such as SIP) to operate properly through the Zyxel Device’s NAT.
Some applications cannot operate through NAT (are NAT un-friendly) because they embed IP
addresses and port numbers in their packets’ data payload. The Zyxel Device examines and uses IP
address and port number information embedded in the VoIP traffic’s data stream. When a device
behind the Zyxel Device uses an application for which the Zyxel Device has VoIP pass through enabled,
the Zyxel Device translates the device’s private IP address inside the data stream to a public IP address.
It also records session port numbers and allows the related sessions to go through the firewall so the
application’s traffic can come in from the WAN to the LAN.
The Zyxel Device only needs to use the ALG feature for traffic that goes through the Zyxel Device’s NAT.
The firewall allows related sessions for VoIP applications that register with a server. The firewall allows or
blocks peer to peer VoIP traffic based on the firewall rules.
You do not need to use a TURN (Traversal Using Relay NAT) server for VoIP devices behind the Zyxel
Device when you enable the SIP ALG.
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