D-Link DFL-2500 User Guide - Page 166
Application Layer Gateway
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18 CHAPTER Application Layer Gateway (ALG) 18.1 Overview To complement the limitations of packet filtering, which only inspect in the packet headers, such as IP, TCP, UDP, and ICMP headers, D-Link firewalls embed an Application Layer Gateway (ALG) to support higher level protocols that have address information within the payload. The ALG acts as the user's representative to obtain most commonly used Internet applications outside the protected network, e.g. Web access, file transfer, and multimedia. Such application-aware agents provide higher security than packet-filtering-only firewalls, since they are capable of scrutinizing all traffic for specific service protocols to give protection at the top level of the TCP/IP stack. In this chapter, the following application standards supported by D-Link ALGs are described. • FTP • HTTP • H.323 147
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