F5 iRule to reject/allow outbound access based on URI, DNS

Yes, you can certainly do this as long as your webservers are routing through the BIG-IP. This would just require a simple iRule on whatever internal VIP the servers are routing through to inspect the HTTP request and deny based on host.

Something like:

when HTTP_REQUEST {
  if {[HTTP::host] ne "www.yahoo.com" } {
    reject
  }
}

Obviously that's very simple, but that's the basic idea.

Colin