Rewrite Uri rule doesn't work

First recommendation, don't patch the application with an irule. The challenge will be getting the correct event, probably HTTP_REQUEST_RELEASE since you need to know the pool member selected (that will not be available on the initial http request.) when HTTP_REQUEST_RELEASE { if { [ getfield [HTTP::host] : 1 ] equals "xyz.com" ] } { HTTP::header replace Host [ LB::server addr ] } } 

Also, uri is irrelevant since it is not changing. The host header is the only place the FQDN would appear. 

iRule was tested and is functional on a lab system                          Run Your Pool App