User manual

DG-BG4300N User Manual
6.2.4 NAT ALG and Pass-Through
An application-level gateway (also known as ALG or application layer gateway) consists of a security
component that augments a firewall or NAT employed in a computer network. It allows customized NAT
traversal filters to be plugged into the gateway to support address and port translation for certain
application layer "control/data" protocols such as IPSec, L2TP, PPTP, FTP, SIP, RTSP etc. In order for
these protocols to work through NAT or a firewall, either the application has to know about an
address/port number combination that allows incoming packets, or the NAT has to monitor the control
traffic and open up port mappings (firewall pinhole) dynamically as required. Legitimate application data
can thus be passed through the security checks of the firewall or NAT that would have otherwise
restricted the traffic for not meeting its limited filter criteria.