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Chapter 21 ALG
ZyWALL USG 300 User’s Guide
433
21.3 ALG Technical Reference
Here is more detailed information about the Application Layer Gateway.
ALG
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.
ALG and Trunks
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
Enable FTP ALG Turn on the FTP ALG to detect FTP (File Transfer Program) traffic and
help build FTP sessions through the ZyWALL’s NAT. Enabling the FTP
ALG also allows you to use the application patrol to detect FTP traffic
and manage the FTP traffic’s bandwidth (see Chapter 32 on page 547).
Enable FTP
Transformations
Select this option to have the ZyWALL modify IP addresses and port
numbers embedded in the FTP data payload to match the ZyWALL’s
NAT environment.
Clear this option if you have an FTP device or server that will modify IP
addresses and port numbers embedded in the FTP data payload to
match the ZyWALL’s NAT environment.
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 return the screen to its last-saved settings.
Table 109 Configuration > Network > ALG (continued)
LABEL DESCRIPTION