![]() O’Reilly members experience books, live events, courses curated by job role, and more from O’Reilly and nearly 200 top publishers. Get ScreenOS Cookbook now with the O’Reilly learning platform. For instance, a policy that references the FTP service on its default TCP port will automatically use the FTP ALG as long as the FTP ALG is enabled globally. An enabled ALG automatically kicks in and performs application layer inspection and the dynamic opening/closing of TCP/UDP ports as well as the associated network/ port address translation when a ScreenOS security policy that uses its associated service is referenced with matching traffic. Copy the command below, paste it into the command window and press ENTER: sc config ALG start demand 3. The dynamic TCP, UDP, or other ports that are opened by the ScreenOS gateway to permit these data or secondary channels are referred to as pinholes, and are active strictly for the duration of activity on the data channel.Īn ALG implementation requires a ScreenOS gateway to inspect the application layer payload of a packet and understand the application control messages. It intends to prevent some of the problems caused by router firewalls. Such applications include the File Transfer Protocol (FTP) and various IP telephony protocols. SIP ALG stands for Application Layer Gateway and is common in many commercial routers. Although there are other ScreenOS features, such as deep inspection, in which the gateway inspects traffic at the application layer, ALGs are typically employed to support applications that use the application layer payload to communicate the dynamic Transmission Control Protocol (TCP) or User Datagram Protocol (UDP) ports on which the applications open data connections. 3) Inspection and logging of VoIP traffic (using ALG/Proxy instead of session-helper). ![]() An application layer gateway (ALG) is a feature on ScreenOS gateways that enables the gateway to parse application layer payloads and take decisions on them. Otherwise, firewall policies need to statically open a wide range of ports.
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