...
Although there will be at any time one or more specific network configurations for accomplishing this result, there are lots of rules and tricks and software that can accomplish the same thing in various ways. The thing to remember is that if you try to login to a Service Provider with a test Shibboleth, and instead of getting a CAS login you get a message saying that the request did not meet security restrictions, and the Shibboleth log contains an error message saying that the message was addressed to "https://auth.yale.edu/idp" but Shibboleth has decided that its own network address is "http://auth.yale.edu/idp" (http not https), or "https://auth.yale.edu:8080/idp" (a port number got added), or "http://localhost:8080/idp" (you didn't fake anything out at all), then you have not set the HTTP headers and the network path right and you have to map out all the intermediate boxes and tunnels through which the request is passing to figure out what is wrong or missing.
The problem is that some of this path may not be under the control of the developer or tester, and if you try and create a path that is completely under your control you now have to learn more about the configuration of Tomcat and HTTP proxy tools, which are normally a problem for someone else.
Some successful recipes will be provided in the Testing Setup document, but if you decide to deviate from them you need to understand the problem described above and figure out new solutions.