Masking a URL?

Nothing is imposible on F5 🙂

First of all, you need to configure your customer DNS to reach one F5's virtual server (subdomain.differentdomain.com).

This VS has to have a pool of nodes of the new domain (subdomain.companya.com).

After that, yo can replace the host header using an iRule like this.

when HTTP_REQUEST {
   # Check if requested host starts with subdomain.differentdomain.com
   if { [string tolower [HTTP::host]] eq "subdomain.differentdomain.com" } {
      # Replace the host header value with subdomain.companya.com
      HTTP::header replace Host "subdomain.companya.com"
   }
}

KR,

Dario.