add more examples of use

INSTALL

+=====================
 Install and configure
 =====================
 
-
-
 Serve the different content on the same URL
 -------------------------------------------
 
@@ -71,12 +70,91 @@ sub vcl_recv {
 sub vcl_error {
     if (obj.status == 750) {
         set obj.http.Location = "http://m.example.com/";
-	set obj.status = 302;
+        set obj.status = 302;
         return(deliver);
     }
 }
 """
 
+Signaling device type to the backend
+------------------------------------
+
+Except where redirection is used, the backend needs to be told what kind of 
+client the content is meant to be served to.
+
+** Example 1: 
+The basic case is that Varnish add the X-UA-Device HTTP header on the 
+backend requests, and the backend mentions in the response Vary header that the
+content is dependant on this header. Everything works out of the box from 
+Varnish's perspective.
+
+** Example 2: Override the User-Agent string sent
+
+If you do not have full control, but can access the User-Agent string (think
+the basic set of headers available by default for CGI scripts), you can change it
+into the device type.
+
+To make sure that any caches out on the Internet doesn't cache it, a Vary header
+on User-Agent must be added on the way out.
+
+VCL code::
+
+"""
+# override the header before it is sent to the backend
+sub vcl_miss { if (req.http.X-UA-Device) { set bereq.http.User-Agent = req.http.X-UA-Device; } }
+sub vcl_pass { if (req.http.X-UA-Device) { set bereq.http.User-Agent = req.http.X-UA-Device; } }
+
+# rewrite the response from the backend
+sub vcl_fetch {
+    if (req.http.X-UA-Device) {
+        if (beresp.http.Vary) { set beresp.http.Vary = beresp.http.Vary + ", User-Agent"; }
+        else { set beresp.http.Vary = "User-Agent"; }
+    }
+}
+"""
+
+** Example 3: Add the device class as a GET query parameter
+
+If everything else fails, you can add the device type as a GET argument. 
+http://example.com/article/1234.html -> http://example.com/article/1234.html?devicetype=mobile-iphone
+
+The same Vary trickery from Example 2 must be added here also.
+
+VCL::
+
+"""
+# override the header before it is sent to the backend
+sub add_get_devicetype { 
+    if (req.http.X-UA-Device && req.method == "GET") {
+        unset req.http.X-get-devicetype;
+        if (bereq.url !~ "\?") {
+            set req.http.X-get-devicetype = "&devicetype=" + req.http.X-UA-Device;
+        } else { 
+            set req.http.X-get-devicetype = "?devicetype=" + req.http.X-UA-Device;
+        }
+        set bereq.url = bereq.url + req.http.X-get-devicetype;
+    }
+}
+sub vcl_miss { call add_get_devicetype; }
+sub vcl_pass { call add_get_devicetype; }
+
+# rewrite the response from the backend
+sub vcl_fetch {
+    if (req.http.X-UA-Device) {
+        if (beresp.http.Vary) { set beresp.http.Vary = beresp.http.Vary + ", User-Agent"; }
+        else { set beresp.http.Vary = "User-Agent"; }
+        # if the backend returns a redirect (think missing trailing slash), we
+        # will potentially show the extra address to the client. we don't want
+        # that.
+        # if the backend reorders the get parameters, you may need to be smarter here. (? and & ordering)
+        if (beresp.status == 301 || beresp.status == 302 || beresp.status == 303) {
+            set beresp.http.location = regsub(beresp.http.location, req.http.X-get-devicetype, "");
+        }
+    }
+    unset req.http.X-get-devicetype;
+}
+"""
+
 
 
 Testing tools