ZyXEL Communications ZYWALL USG 2000 Manual page 311

Unified security gateway

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Chapter 18 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

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

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.

H.323 is a standard teleconferencing protocol suite that provides audio, data and

video conferencing. It allows for real-time point-to-point and multipoint

communication between client computers over a packet-based network that does

not provide a guaranteed quality of service. NetMeeting uses H.323.

The Session Initiation Protocol (SIP) is an application-layer control (signaling)

protocol that handles the setting up, altering and tearing down of voice and

ZyWALL USG 2000 User's Guide

Troubleshooting