TCP reflective SYNs: blocking by the /24 / 2019-10-28

It seems the TCP reflective SYN attacks are continuing. In researching my options I saw the option to use a netmask with the iptables recent module.

This helps a bit with the attacks trying to flood an entire block. I've updated the filtering to work by the /24, start a check on a SYN from such a block, end when an ACK flies by and start dropping when the rate is over 10 per 2 minutes.

iptables -A INPUT -p tcp -m tcp --tcp-flags SYN,ACK SYN -m recent --update --seconds 120 --hitcount 10 --name tcpsyn --mask 255.255.255.0 --rsource -j LOGDROP
iptables -A INPUT -p tcp -m tcp --tcp-flags SYN,ACK SYN -m recent --set --name tcpsyn --mask 255.255.255.0 --rsource
iptables -A INPUT -p tcp -m tcp --tcp-flags SYN,ACK ACK -m recent --remove --name tcpsyn --mask 255.255.255.0 --rsource

LOGDROP is a rule to drop packets and ratelimit the logging of dropped packets, to avoid turning a network attack into a disk attack.

But I have to be careful not to make services hard to reach for legitimate clients. The above is working, and during attacks I don't see a single SYN_RECV socket.