HTTP: The Definitive Guide

Relays

HTTP relays are simple HTTP proxies that do not fully adhere to the HTTP specifications. Relays process enough HTTP to establish connections, then blindly forward bytes.

Because HTTP is complicated, it’s sometimes useful to implement bare-bones proxies that just blindly forward traffic, without performing all of the header and method logic. Because blind relays are easy to implement, they sometimes are used to provide simple filtering, diagnostics, or content transformation. But they should be deployed with great caution, because of the serious potential for interoperability problems.

One of the more common (and infamous) problems with some implementations of simple blind relays relates to their potential to cause keep-alive connections to hang, because they don’t properly process the Connection header. This situation is depicted in Figure 8-14.

Figure 8-14. Simple blind relays can hang if they are single-tasking and don’t support the Connection header

Here’s what’s going on in this figure:

• In Figure 8-14a, a web client sends a message to the relay, including the Connection: Keep-Alive header, requesting a keep-alive connection if possible. The client waits for a response to learn if its request for a keep-alive channel was granted.

• The relay gets the HTTP request, but it doesn’t understand the Connection header, so it passes the message verbatim down the chain to the server ...