Proxies Versus Gateways 代理与网关的区别

Web proxy servers are middlemen that fulfill transactions on the client's behalf.Without a web proxy, HTTP clients talk directly to HTTP servers. With a web proxy, the client instead talks to the proxy, which itself communicates with the server on the client's behalf. The client still completes the transaction, but through the good services of the proxy server.

HTTP proxy servers are both web servers and web clients. Because HTTP clients send request messages to proxies, the proxy server must properly handle the requests and the connections and return responses, just like a web server. At the same time, the proxy itself sends requests to servers, so it must also behave like a correct HTTP client, sending requests and receiving responses (see Figure 6-1).


Strictly speaking, proxies connect two or more applications that speak the same protocol, while gateways hook up two or more parties that speak different protocols. A gateway acts as a "protocol converter," allowing a client to complete a transaction with a server, even when the client and server speak different protocols.



In practice, the difference between proxies and gateways is blurry. Because browsers and servers implement different versions of HTTP, proxies often do some amount of protocol conversion. And commercial proxy servers implement gateway functionality to support SSL security protocols, SOCKS firewalls, FTP access, and web-based applications. We'll talk more about gateways in Chapter 8.