IRULE Template for URL Masking (Request and Response)

Based on your description it looks like you may not even need to do any kind of transformation.

The request from the client specifies the path (e.g. "/abc") separately from the hostname (which is stored in the HTTP-header "host").

Depending on how the node (pool member) handles the hostname you may not have to worry about it.

It is possible to re-write the request, but it looks like you may not need to do this.

If you do need to change the host you can swap out the value of the HTTP-header "host":

when HTTP_REQUEST {

    HTTP::header replace host server1

}