VCL Redirect Hack

gistfile1.pl

sub vcl_recv {
    # ...

    # detect reader's device, redirect if necessary
    if (req.http.user-agent ~ "iPhone|iPod|Android") {
        if(req.http.host ~ "^example.com") {
            # mobile reader is on desktop site. 
            # raise an error with status=700 and use correct url as response body.
            # vcl_error will use this response body as the Location: header.
            error 700 "http://m.example.com" + req.url;
        }
    }
}

sub vcl_error {
    # catch custom 700 error, deliver a redirect as response
    if(obj.status == 700) {
        set obj.status = 302;
        # use the response body (the correct url, set in vcl_recv) as the new location
        set obj.http.Location = obj.response;
        # change the response to a friendly message about the redirect
        set obj.response = "Redirecting you to mobile site";
    }
}