ZyXEL Communications 100 Series User Manual

Chapter 18 ALG

ZyWALL USG 100/200 Series User’s Guide

Here is more detailed information about the Application Layer Gateway.

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. 

If you send your ALG-managed traffic through an interface trunk and all of the interfaces are 
set to active, you can configure routing policies to specify which interface the ALG-managed 
traffic uses.

You could also have a trunk with one interface set to active and a second interface set to 
passive. The ZyWALL does not automatically change ALG-managed connections to the 
second (passive) interface when the active interface’s connection goes down. When the active 
interface’s connection fails, the client needs to re-initialize the connection through the second 
interface (that was set to passive) in order to have the connection go through the second 
interface. VoIP clients usually re-register automatically at set intervals or the users can 
manually force them to re-register.

File Transfer Protocol (FTP) is an Internet file transfer service that operates on the Internet and 
over TCP/IP networks. A system running the FTP server accepts commands from a system 
running an FTP client. The service allows users to send commands to the server for uploading 
and downloading files. 

Enable FTP 
Transformations

Turn on the FTP ALG to allow FTP sessions to pass through the ZyWALL. FTP (File 
Transfer Program) enables fast transfer of files, including large files that may not be 
possible by e-mail. 
Using the FTP ALG allows you to use bandwidth management on FTP traffic.

FTP Signaling 
Port 

If you are using a custom TCP port number (not 21) for FTP traffic, enter it here. 

Additional FTP 
Signaling port  
for 
transformations

If you are also using FTP on an additional TCP port number, enter it here. 

Apply

Click Apply to

 

save your changes back to the ZyWALL.

Reset

Click Reset to begin configuring this screen afresh.

Table 108   Network > ALG (continued)