Attack on server

[DaWe]

Hello killtube,
someone is sending millions of packets that are same as packets from real clients to my server.
When attack is active, I can see only "Hitch warning: X msec frame time" and "SV_AuthorizeIpPacket: challenge not found" in console and server is consuming so much CPU.
He is also sending the attack directly to port 28960, because servers on other ports work without any problem.
As I know it should be fixed by using libcod, I'm using latest libcod by voron, but looks like it doesn't help.

Can you please help me with setting up firewall or so?