mod_proxy is the module that allows Lighttpd to relay requests to another web server. It is not to be confused with mod_proxy_core (of Lighttpd 1.5.0), which provides a basis for other interfaces such as CGI. Usually, we want to proxy only a specific subset of requests, for example, we might want to proxy requests for Java server pages to a Tomcat server. This could be done with the following proxy directive:

proxy.server = (
".jsp" => ( host => "127.0.0.1", port => "8080" )
# given our tomcat is on port 8080
)

Thus the tomcat server only serves JSPs, which is what it was built to do, whilst our Lighttpd does the rest.

Or we might have another server which we want to include in our Web presence at some given directory:

proxy.server = (
"/somepath" => ( host => "127.0.0.1", port => "8080" )
)

Assuming the server is on port 8080, this will do the trick. Now http://localhost/somepath/index.html will be the same as http://localhost:8080/index.html.