How to mitigate TCP SYN Flood attack and resolve it on Linux

TCP SYN flood is a one type of DDoS (Distributed Denial of Service) attack that exploits part of the normal TCP three-way handshake to consume resources on the targeted server and render it unresponsive.

With SYN flood DDoS, the attacker sends TCP connection requests faster than the targeted machine can process them.

A normal TCP between a client and server establish three-way handshake, the process is looks like this:

1. On first connection, client request connection by sending SYN (synchronize) packet to the server
2. Then server send responds to that initial packet with a SYN/ACK packet, in order to acknowledge client and server communication
3. Client responds with an ACK (acknowledge) message, and the connection is established.

You can read more about TCP SYN Flood on https://www.cloudflare.com/learning/ddos/syn-flood-ddos-attack/

How to mitigate your server is under SYN flood attact:

# netstat -tuna | grep SYN_RECV

Command below is counting how much SYN_RECV connection:

# netstat -tuna | grep SYN_RECV | wc -l

Below is sample our server is under “small” SYN flood attact

Many people don’t use optimized kernel settings to better mitigate the effects of DDoS attacks.

We have tried many settings of sysctl.conf , but not one work or not lucky yet until found this settings.

This is CentOS 7 kernel settings that we working.

Edit /etc/sysctl.conf file with code below.

Please note on vm.swappiness=0, we don’t need to use Swap, because our server has 256 GB.

vm.swappiness=0 #This server have 256 GB of RAM, we don't need swap
vm.vfs_cache_pressure=50
vm.dirty_background_ratio = 5
vm.dirty_ratio = 80
vm.max_map_count = 655300
fs.file-max = 2097152
fs.nr_open = 2097152

kernel.printk = 4 4 1 7 
kernel.panic = 10 
kernel.sysrq = 0 
kernel.shmmax = 4294967296 
kernel.shmall = 4194304 
kernel.core_uses_pid = 1 
kernel.msgmnb = 65536 
kernel.msgmax = 65536 
net.core.netdev_max_backlog = 262144 
net.core.rmem_default = 31457280 
net.core.rmem_max = 67108864 
net.core.wmem_default = 31457280 
net.core.wmem_max = 67108864 
net.core.somaxconn = 65535 
net.core.optmem_max = 25165824 
net.ipv4.neigh.default.gc_thresh1 = 4096 
net.ipv4.neigh.default.gc_thresh2 = 8192 
net.ipv4.neigh.default.gc_thresh3 = 16384 
net.ipv4.neigh.default.gc_interval = 5 
net.ipv4.neigh.default.gc_stale_time = 120 
net.netfilter.nf_conntrack_max = 10000000 
net.netfilter.nf_conntrack_tcp_loose = 0 
net.netfilter.nf_conntrack_tcp_timeout_established = 1800 
net.netfilter.nf_conntrack_tcp_timeout_close = 10 
net.netfilter.nf_conntrack_tcp_timeout_close_wait = 10 
net.netfilter.nf_conntrack_tcp_timeout_fin_wait = 20 
net.netfilter.nf_conntrack_tcp_timeout_last_ack = 20 
net.netfilter.nf_conntrack_tcp_timeout_syn_recv = 20 
net.netfilter.nf_conntrack_tcp_timeout_syn_sent = 20 
net.netfilter.nf_conntrack_tcp_timeout_time_wait = 10 
net.ipv4.tcp_slow_start_after_idle = 0 
net.ipv4.ip_local_port_range = 1024 65000 
net.ipv4.ip_no_pmtu_disc = 1 
net.ipv4.route.flush = 1 
net.ipv4.route.max_size = 8048576 
net.ipv4.icmp_echo_ignore_broadcasts = 1 
net.ipv4.icmp_ignore_bogus_error_responses = 1 
net.ipv4.tcp_congestion_control = htcp 
net.ipv4.tcp_mem = 65536 131072 262144 
net.ipv4.udp_mem = 65536 131072 262144 
net.ipv4.tcp_rmem = 4096 87380 33554432 
net.ipv4.udp_rmem_min = 16384 
net.ipv4.tcp_wmem = 4096 87380 33554432 
net.ipv4.udp_wmem_min = 16384 
net.ipv4.tcp_max_tw_buckets = 1440000 
net.ipv4.tcp_tw_recycle = 0 
net.ipv4.tcp_tw_reuse = 1 
net.ipv4.tcp_max_orphans = 400000 
net.ipv4.tcp_window_scaling = 1 
net.ipv4.tcp_rfc1337 = 1 
net.ipv4.tcp_syncookies = 1 
net.ipv4.tcp_synack_retries = 1 
net.ipv4.tcp_syn_retries = 2 
net.ipv4.tcp_max_syn_backlog = 16384 
net.ipv4.tcp_timestamps = 1 
net.ipv4.tcp_sack = 1 
net.ipv4.tcp_fack = 1 
net.ipv4.tcp_ecn = 2 
net.ipv4.tcp_fin_timeout = 10 
net.ipv4.tcp_keepalive_time = 600 
net.ipv4.tcp_keepalive_intvl = 60 
net.ipv4.tcp_keepalive_probes = 10 
net.ipv4.tcp_no_metrics_save = 1 
net.ipv4.ip_forward = 0 
net.ipv4.conf.all.accept_redirects = 0 
net.ipv4.conf.all.send_redirects = 0 
net.ipv4.conf.all.accept_source_route = 0 
net.ipv4.conf.all.rp_filter = 1

To apply setting on /etc/sysctl.conf without restart, use command below.

# sysctl -p

When we under TCP SYN flood attact and we apply /etc/sysctl.conf setting, SYN Flood attact number decrease.

TCP SYN Flood connection will make very high connection on Nginx.

We can check on netdata, about 7.226 active connections on Nginx, with only 90 connection per second.

This condition make slow down Nginx respons.

After we apply sysctl.conf, active connection decrease.