Understanding Linux Network Internals

Proxying the Neighboring Protocol

When a host intercepts traffic addressed to another host and processes it in place of the latter, it is said to act as a proxy. The term does not, of course, cover a malicious host that launches a man-in-the-middle attack. Rather, a common example of a proxy is a caching HTTP server that cuts down on network traffic by intercepting requests directed to popular web servers and serving up pages from those web servers that are stored in its own cache.

If hosts and applications do not need to be explicitly configured to benefit from the services provided by a proxy, this proxy is said to be transparent. The caching HTTP server just mentioned is an example of a transparent proxy . But as Figure 26-7 shows, a service could be provided by either a transparent proxy or a nontransparent proxy. The figure shows two examples of an HTTP proxy in use:

• (a) The proxy is installed on the router used by a local network to access the Internet. All browser requests from hosts on the network go through the router, so the administrator can configure the router to intercept and proxy all HTTP requests. This is considered transparent proxying because no configuration or specially programmed browser is needed on Host B.

• (b) The browser of Host B is configured to use the proxy on the host named Proxy to browse the Internet. The host Proxy uses the router when it is needed (that is, when there is a cache miss).

Of course, several other options are possible. For instance, the ...