iRule for URL Re-write

I'm not sure I caught the part about needing to forward them to a proxy server. Are you saying you need to forward the traffic two places? Or are you just wanting to redirect a request from https://theirsite.externaldomain.com to http://oursite.internaldomain.com?

If the latter is the case, then all you'd need is the redirect command, like this:

when HTTP_REQUEST {
  if { [HTTP::host] eq "theirsite.externaldomain.com" } {
    HTTP::redirect http://oursite.internaldomain.com
  }
}

You would, of course, have to be sure that the BIG-IP is decrypting the SSL traffic by way of a client-ssl profile before inspecting things.

Colin