Security Expert Help Needed - CentOS

We have what seems to be DOS attacks on our CentOS server every night.? The attacker seem to flood the port 80but we do not know much more for the moment.? The server mainly acts as a telephony server Asterisk) so the expert will have to be in communication with our Asterisk expert in order to make sure the service runs smoothly whild adjusting firewall or other strategy? **We will only hire someone with proven and extensive experience in security matters. **This person will 1) Fix the issue we have now 2) Help us on an ongoing basis to keep the server secure. I? don't want to be inundated with form-letter messages from IT shops and will not hire a company where I cannot see the exact qualifications of the expert. Thanks