Linux: Turn On TCP SYN Cookie Protection

I am under DoS attack. My cloud based server hosting company asked me to enable TCP SYN cookie protection to save my domain from SYN Attack. How do I turn on TCP Syn cookie protection under Ubuntu or CentOS Linux based server?

The TCP Syn is DoS (Denial of Service) attack. It consumes resources on your Linux server. The attacker begin with the TCP connection handshake sending the SYN packet, and then never completing the process to open the connection. This results into massive half-open connections. The Linux kernel can block such attacks easily.

See the current settings

Use sysctl command to configure or see kernel parameters at runtime. To see the current settings for net.ipv4.tcp_syncookies kernel parameter, enter:
# sysctl -n net.ipv4.tcp_syncookies
# cat /proc/sys/net/ipv4/tcp_syncookies
Sample outputs:

Fig.01: View current TCP SYN cookie protection

Enable TCP SYN cookie protection

Edit the file /etc/sysctl.conf, run:
# vi /etc/sysctl.conf
Append the following entry:

net.ipv4.tcp_syncookies = 1

Save and close the file. To reload the change, type:
# sysctl -p

Recommended readings

  1. How to set advanced security options of the TCP/IP stack and virtual memory to improve security and performance of your Linux based system.
  2. Twenty Linux server hardening security tips
  3. Linux firewall tutorial and shorewall firewall tutorial for more information.

Posted by: SXI ADMIN

The author is the creator of SXI LLC and a seasoned sysadmin, DevOps engineer, and a trainer for the Linux operating system/Unix shell scripting. Get the latest tutorials on SysAdmin, Linux/Unix and open source topics via RSS/XML feed or weekly email newsletter.