Application-level Gateway - ALG Service in Microsoft Windows

ALG Service in Microsoft Windows

The Application Layer Gateway service in Microsoft Windows provides support for third-party plugins that allow network protocols to pass through the Windows Firewall and work behind it and Internet Connection Sharing. ALG plugins can open ports and change data that is embedded in packets, such as ports and IP addresses. Windows Server 2003 also includes an ALG FTP plugin. The ALG FTP plugin is designed to support active FTP sessions through the NAT engine in Windows. To do this, the ALG FTP plugin redirects all traffic that passes through the NAT and that is destined for port 21 (FTP control port) to a private listening port in the 3000-5000 range on the Microsoft loopback adapter. The ALG FTP plugin then monitors/updates traffic on the FTP control channel so that the FTP plugin can plumb port mappings through the NAT for the FTP data channels.

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