Iptables – How to block tcp-reset with iptables to combat DDoS attacks

Essentially, you're outta luck if the DDoS attack manages to fill whatever pipe you have to the Internet (which is the purpose of any UDP reflection attack -- to fill the pipe). If your upstream link can take 1Gbps of traffic, and there's (say) 2Gbps of traffic total to go down the link, then half of it is going to be dropped by the router or switch that's putting the packets down the link. The attacker doesn't care that half of their attack traffic gets dropped, but your customers do: 50% packet loss in a TCP connection is going to do terrible, terrible things to the performance and reliability of those connections.

There are only two three ways to stop a volumetric DDoS attack:

1. Have a big enough pipe that the attack traffic doesn't fill it.
2. Stop the attack packets before they go down the pipe.
3. Shift to a different IP address that isn't under NTP reflection attack.

Blocking them in iptables won't do squat, because by then the attack traffic has already squeezed out the legit traffic and caused it to be dropped on the floor, so the attacker has won. Since you (presumably) don't control the upstream router or switch that is forwarding the attack traffic, yes, you'll have to get in touch with your upstream network provider and have them do something to stop the attack traffic from reaching your network link, whether that be

• block all traffic on the attack port (not something most ISPs are willing to do on their colo customer access routers, for $REASONS)

• filter out the source IP addresses of the attack (more plausible, with S/RTBH, but not something that every provider already has available)

• worst case, blackhole the destination IP address

Note that blackholing the IP only works if you've got other IP addresses that can continue operating -- if your provider blackholes your only IP address, the attacker has succeeded because you're off the Internet, which is what they were trying to achieve in the first place.

While blocking all incoming UDP is pretty simple:

iptables -A INPUT -p udp -s 0.0.0.0/0 -d your_ip_range -m state --state NEW -j DROP

you may want to consider giving a look to the:

Service Name and Transport Protocol Port Number Registry

to see which UDP services you may need to allow through.