Block SYNACK response with iptables

Summary

– Use iptables to block SYNACK packets and protect against DDoS attacks.
– Configure iptables rules to drop incoming SYNACK packets.
– Verify that the rules are working properly.

Introduction

A SYN flood, or SYN attack, is a type of Denial-of-Service (DoS) attack where an attacker sends a large number of TCP connection requests to a server, consuming its resources and making it unavailable for legitimate users. One way to mitigate the effects of a SYN flood is by blocking SYNACK packets using iptables. This article provides a comprehensive solution on how to block SYNACK response with iptables.

Blocking SYNACK Packets with Iptables

To block SYNACK packets, we need to configure iptables to drop incoming SYNACK packets. The following steps can be used:

Step 1: Install iptables

Before configuring iptables, ensure that it is installed on the server. You can install iptables using the following command:

sudo apt-get update && sudo apt-get install iptables

Step 2: Create a New Chain

Create a new chain to handle SYNACK packets. Use the following command to create a new chain named SYNACK:

sudo iptables -N SYNACK

Step 3: Add Rules to the Chain

Add rules to the newly created chain to drop incoming SYNACK packets. The following command can be used:

sudo iptables -A INPUT -m tcp –tcp-flags SYN,SYN-ACK SYN -j SYNACK
sudo iptables -A OUTPUT -m tcp –tcp-flags SYN,SYN-ACK SYN -j SYNACK

The above commands add two rules to the INPUT and OUTPUT chains to drop incoming and outgoing SYN packets respectively.

Step 4: Add Chain to Policy

Add the newly created chain to the policy of both the INPUT and FORWARD chains. Use the following command to add it to the INPUT chain:

sudo iptables -P INPUT ACCEPT
sudo iptables -A INPUT -j SYNACK

Similarly, use the following command to add it to the FORWARD chain:

sudo iptables -P FORWARD ACCEPT
sudo iptables -A FORWARD -j SYNACK

Step 5: Verify that the Rules are Working Properly

To ensure that the rules are working properly, use the following command to check the status of the iptables rules:

sudo iptables –L -n

The above command will display a list of active chains and their corresponding rules. If the SYNACK chain has been added successfully, it should be displayed in the output along with its rules.

Conclusion

In conclusion, blocking SYNACK response using iptables can help mitigate the effects of a SYN flood attack on a server. The steps outlined above provide a comprehensive solution on how to configure iptables to drop incoming and outgoing SYNACK packets.

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