Syn Flood

I've been getting a syn flood for the last week or so.

I've pretty much tried everything I could online but have been unsuccessful in stopping them.

I talked to the data center techs and they basically can't put a stop to it either.

Here's a very small portion of my netstat

tcp 0 0 xxx.xxx.xxx.xxx:80 86.50.121.144:8540 SYN_RECV -
tcp 0 0 xxx.xxx.xxx.xxx:80 41.100.16.152:7824 SYN_RECV -
tcp 0 0 xxx.xxx.xxx.xxx:80 52.53.22.7:3146 SYN_RECV -
tcp 0 0 xxx.xxx.xxx.xxx:80 77.217.49.124:1659 SYN_RECV -
tcp 0 0 xxx.xxx.xxx.xxx:80 75.162.93.151:3230 SYN_RECV -
tcp 0 0 xxx.xxx.xxx.xxx:80 113.85.63.249:1656 SYN_RECV -
tcp 0 0 xxx.xxx.xxx.xxx:80 15.253.35.29:8849 SYN_RECV -
tcp 0 0 xxx.xxx.xxx.xxx:80 24.56.59.180:6911 SYN_RECV -
tcp 0 0 xxx.xxx.xxx.xxx:80 33.185.99.83:1917 SYN_RECV -
tcp 0 0 xxx.xxx.xxx.xxx:80 103.5.8.249:4782 SYN_RECV -

root@xxx [/]# netstat -nap |grep SYN |wc -l
2008

The IP's change often and it's not possible to narrow it down.

So far the things I have done;

syn cookies enabled

reduced time out
echo "1" > /proc/sys/net/ipv4/netfilter/ip_conntrack_tcp_timeout_syn_recv

increased
echo "150000" > /proc/sys/net/ipv4/ip_conntrack_max

installed apf but it slowed down the server to a crawl which made my clients really unhappy so had to remove it.

The bandwidth is constantly staying at 30Mbps with slight bumps here and there but every day around 7pm it drops completely to normal levels and the flood stops. It starts back up around 7 in the morning.