6. Application-level gateway (proxy and reverse-proxy)

An Application Level Gateway (ALG) is a software module that provides more or less fine-grained filtering for a set of specific TCP/IP services. More precisely, this type of filtering is based on a client-server model, where the gateway acts as a server (or proxy) that retrieves a service request from a client and decides whether or not to forward it to the server. In this model, the client interacting with the gateway actually believes it is interacting directly with the server, while the server believes it is communicating with the client while interacting with the gateway. So, instead of having a single connection between the client and the server, there are two, one between the client and the gateway, and another between the gateway and the server. Note that if a connection is authorized, the gateway relays traffic between client and server and can continue to filter all exchanges....