RECOMMENDATIONS Block requests not directed at domain name

[Addisonb]

Hello,
On one of our PIAF installs, one of the system administrators put in a rule that will block requests directed at 12.34.56.78 (the public IP of our phone system) but accept requests to phone.system.com which is an A host that resolves to 12.34.56.78.
I need to duplicate this on another system. Any guesses on how this was done? I know IP tables doesn't have this functionality. Any recommendations on what would could accomplish this?
Thanks,
Addison

 

[Addisonb]

To follow up:
This is being accomplished using the "string matching" in IPtables.
You can read more about it here:
https://wiztelsys.com/blog/iptables-string-matching-for-advanced-firewalling/

In our case our command looks like this:
-A INPUT -i eth1 -p udp -m udp --dport 5060:5082 -m string --string "FQDN.domain.com" --algo bm --to 65535 -j ACCEPT

This allows for malicious attempts on ports 5060 to be dropped if they don't know our FQDN.