The client then negotiates with the proxy server in order to communicate with the destination service. When a client program establishes a connection to a destination service, it connects to an application gateway, or proxy. An ALG can interact with NAT to set up state, use NAT state information, modify application-specific payloads, and do anything else necessary to get the application running across different address ranges.Also known as application proxy or application-level proxy, an application gateway is an application program that runs on a firewall system between two networks. Legitimate application data can thus pass through the security checks of the firewall or NAT, which would otherwise have restricted traffic because it does not meet their limited filtering criteria.Īpplication Level Gateways (ALGs) are application-specific translation agents that allow an application (e.g., VOIP) on a host in one address range to transparently connect to its counterpart on a host in another range. It allows custom NAT traversal filters to be inserted into the gateway to support address and port translation for certain application layer “control/data” protocols such as FTP, BitTorrent, SIP, RTSP, and file transfer in IM applications.įor these protocols to work through NAT or a firewall, either the application must know a combination of address and port number that allows incoming packets, or the NAT must monitor control traffic and dynamically release port assignments (firewall pinholes) as needed. Such network applications include File Transfer Protocol (FTP), Telnet, Real Time Streaming Protocol (RTSP), and BitTorrent.Īn application-level gateway (ALG, also called an application layer gateway, application gateway, application proxy, or application-level proxy) is a security component that complements a firewall or NAT in a computer network. It filters incoming node traffic according to certain specifications, which means that only transmitted network application data is filtered. An application gateway or application level gateway (ALG) is a firewall proxy that provides network security.
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