Linux – iptables drop not working with udp flood, causing high ping

If your firewall isn't already configured this way, default deny is the safer way to operate. This means that anything that isn't explicitly allowed is denied. For iptables, this means:

 iptables -P INPUT DROP

You can do this for the OUTPUT and FORWARD chains as well but OUTPUT in particular requires a little more discipline in managing what your server is allowed to connect to.

After disallowing everything, you allow what you want. Does your game listen for UDP traffic on ports 27015 and 27018? If so, add rules that allow that traffic in. Does the client always use a small range of source ports or even a single source port? If so, restrict the allowed packets to just those source ports. The first example you gave where the source port is 80 would be automatically blocked in that case. The other one might be depending on what source ports your client uses.

How are you determining that the firewall rules are not working? tcpdump attaches outside the firewall and hence will see packets that are destined to be dropped (or in the outbound direction, that have already passed the firewall). You can use a -j LOG rule to monitor specific parts of your chains or use in-application logging.

I'm not quite sure what you mean by "overload the udp ports". Does your application need to use a separate UDP port for each connecting IP address? Is there some shared resource that is acting as a bottleneck? Is all this UDP traffic just filling the networking hardware's buffers, causing the kind of latency associated with bufferbloat? If this last one is true, blocking using iptables won't even solve your problem.

The only iptable rules that would help you even remotely would be dropping all traffic coming in on those ports. The problem with that is that your service will go down as well. The only thing you can do is talk to your ISP and ask them to drop all incoming udp traffic before it reaches you.