Securing server from SYN flooding DOS attack

As the internet is becoming larger day by day, therefore security risks for your servers are also increasing. If your server is connected to internet and is hosting some good high traffic and popular websites, then there are high chances that you get SYN flooding. SYN flooding is a kind of DOS attack, a.k.a “Deniel of service” attack.


SYN packet are specific packes used to establish tcp connection, a packet which send a message to host, something like “Hello, is there anyone on this port?”
If a server is alive, it will give response, “yes, a service is listening”
Now the final step ACK packet, sent back to service and tell “OK, lets begin our talk”

And it start the exchange of information betweek client and server.

SYN flooding

SYN flooding is kind of DOS, which contains large amount of SYN packets, until the capacity of server gets full and it may become not able to receive any more packets.
The basic purpose of SYN flood is to send as large amount of SYN requests, that they consume all the capacity of server and the server may not establish any more connections and give “timed out” error.

Be aware, that there is no way to discard such kind of attacks, but THERE are ways to reduce the impact of such attacks.

Edit /etc/sysctl.conf

and add

Edit iptables firewall

You can run those iptables rules in your server one by one, or just download file from here, chmod it to 755 and just run it.

Keep smiling 🙂

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About shk

shk is a DevOps engineer with more than 12 years of experience in different organizations. He enthusiastic about learning new technologies and shares his knowledge through his blogs.


  1. Hello there,
    Good post although I have not yet tried it.

    How can someone implement those iptables rules if SHOREWALL is in place?
    Do you know?

    Thank you

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