Chapter 14 ALG
USG20(W)-VPN Series User’s Guide
272
14.3 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 USG examines and uses IP address
and port number information embedded in the VoIP traffic’s data stream. When a device behind the
USG uses an application for which the USG has VoIP pass through enabled, the USG 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 security policy 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 USG 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.
FTP
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
Select this option to have the USG modify IP addresses and port numbers
embedded in the FTP data payload to match the USG’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 USG’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 USG.
Reset
Click
Reset
to return the screen to its last-saved settings.
Table 108
Configuration > Network > ALG (continued)
LABEL
DESCRIPTION
Summary of Contents for ZyWall USG20-VPN
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