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ZyWALL (ZLD) CLI Reference Guide 127
CHAPTER 14
ALG
This chapter covers how to use the ZyWALL’s ALG feature to allow certain applications to pass
through the ZyWALL.
14.1 ALG Introduction
The ZyWALL can function as an Application Layer Gateway (ALG) to allow certain NAT un-friendly
applications (such as SIP) to operate properly through the ZyWALL’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 ZyWALL examines and uses IP
address and port number information embedded in the VoIP traffic’s data stream. When a device
behind the ZyWALL uses an application for which the ZyWALL has VoIP pass through 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 allows the related sessions to go through the firewall so the
application’s traffic can come in from the WAN to the LAN.
The ZyWALL only needs to use the ALG feature for traffic that goes through the ZyWALL’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
ZyWALL when you enable the SIP ALG.