A Guide to Implementing Rate Limiting with mod

Overview

In today’s digital age, where web services are integral to business operations, ensuring the availability and security of these services against potential threats has become paramount. One of the most prevalent threats to web service availability is the Denial of Service (DoS) attack, which aims to overwhelm a website with traffic, rendering it inaccessible to legitimate users. To combat this, web administrators must employ robust defense mechanisms. Among the various tools and techniques available, configuring rate limiting on web servers is an effective strategy to mitigate such attacks. Apache, one of the most widely used web servers, offers a module named mod_evasive specifically designed for this purpose.

This blog post aims to provide a comprehensive, step-by-step guide on how to leverage mod_evasive to protect your Apache server from DoS attacks, ensuring that your web services remain available and performant under adversarial conditions.

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Introduction

Configuring Apache to handle high traffic and prevent Denial of Service (DoS) attacks is crucial for maintaining the availability and performance of your web services. One effective tool for this purpose is mod_evasive, an Apache module designed to detect and mitigate excessive requests from a single IP address or range of IP addresses. In this blog post, I’ll guide you through installing, configuring, and testing mod_evasive on your Apache server.

Step-by-Step Guide

Step 1: Install mod_evasive

On Debian/Ubuntu Systems:

Update your package list:

sudo apt update

1	sudo apt update

2. Install mod_evasive:

sudo apt install libapache2-mod-evasive

1	sudo apt install libapache2-mod-evasive

3. Enable the module (if it’s not automatically enabled):

sudo a2enmod evasive

1	sudo a2enmod evasive

On CentOS/RHEL Systems:

You may need to add the EPEL repository:

sudo yum install epel-release

1	sudo yum install epel-release

2. Install mod_evasive:

sudo yum install mod_evasive

1	sudo yum install mod_evasive

Step 2: Configure mod_evasive

Locate the mod_evasive configuration file. This will typically be found at /etc/apache2/mods-available/evasive.conf on Debian/Ubuntu or /etc/httpd/conf.d/mod_evasive.conf on CentOS/RHEL.
Open the configuration file in your favorite editor (for example, using nano):

sudo nano /etc/apache2/mods-available/evasive.conf

1	sudo nano /etc/apache2/mods-available/evasive.conf

3. Configure the directives according to your needs. Here are some common settings:

DOSHashTableSize: The size of the hash table for tracking. A larger size can improve performance by spreading out the data. The default is 2048.
DOSPageCount: Threshold for the number of requests for the same page (or URI) per second. Crossing this limit could indicate an attack.
DOSSiteCount: Threshold for the total number of requests for any object by the same client on the same listener per second.
DOSPageInterval: The interval for the DOSPageCount threshold.
DOSSiteInterval: The interval for the DOSSiteCount threshold.
DOSBlockingPeriod: The time (in seconds) that a client will be blocked if they are added to the blacklist.

Example configuration:

<IfModule mod_evasive20.c>
    DOSHashTableSize    4096
    DOSPageCount        2
    DOSSiteCount        50
    DOSPageInterval     1
    DOSSiteInterval     1
    DOSBlockingPeriod   10
</IfModule>

DOSHashTableSize 4096

DOSPageCount 2

DOSSiteCount 50

DOSPageInterval 1

DOSSiteInterval 1

DOSBlockingPeriod 10

</IfModule>

4. Save and close the file.

Step 3: Restart Apache

To apply the changes, restart Apache:

On Debian/Ubuntu:

sudo systemctl restart apache2

1	sudo systemctl restart apache2

On CentOS/RHEL:

sudo systemctl restart httpd

1	sudo systemctl restart httpd

Step 4: Testing mod_evasive

To test if mod_evasive is working correctly, you can use a tool like siege or simply script multiple rapid requests to your server:

for i in {1..100}; do curl http://yourserver.com/; done

1	for i in {1..100}; do curl http://yourserver.com/; done

If mod_evasive is configured properly, you should receive a 403 Forbidden response after a certain number of requests, indicating that the IP has been temporarily blocked.

Conclusion

Safeguarding your Apache server from Denial of Service attacks is crucial for maintaining the integrity and availability of your web services. Through the implementation of mod_evasive, you have a powerful tool at your disposal to detect and prevent excessive requests, effectively mitigating potential DoS attacks.

This guide has walked you through the installation, configuration, and testing of mod_evasive, providing you with the knowledge to enhance your server’s defenses. Remember, the digital landscape is constantly evolving, and so are the tactics of adversaries. Therefore, continuously monitoring your server’s performance and adjusting your security measures accordingly is essential. Following the steps outlined in this guide and adopting a proactive approach to server security, you can ensure that your web services remain resilient against threats, providing a reliable and secure experience for your users.

Drop a query if you have any questions regarding Apache server and we will get back to you quickly.

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FAQs

1. What exactly is mod_evasive?

ANS: – Mod_evasive is an essential Apache module crafted to aid web administrators in safeguarding their servers from threats like Denial of Service (DoS), Distributed Denial of Service (DDoS), and brute force attacks. It achieves this by dynamically identifying and blocking the IP addresses of clients that send requests at a pace considered aggressive or potentially harmful.

2. Can you explain the operational mechanism of mod_evasive?

ANS: – The operational core of mod_evasive lies in its ability to vigilantly monitor the flow of incoming requests to an Apache web server. It enforces specific rules designed to cap the number of allowable requests from a single IP address within a predefined timeframe. Should a client surpass these established limits, mod_evasive interprets this behavior as aggressive, leading to a temporary blockade of further requests from the offending IP address.

WRITTEN BY Naman Jain

Naman works as a Research Intern at CloudThat. With a deep passion for Cloud Technology, Naman is committed to staying at the forefront of advancements in the field. Throughout his time at CloudThat, Naman has demonstrated a keen understanding of cloud computing and security, leveraging his knowledge to help clients optimize their cloud infrastructure and protect their data. His expertise in AWS Cloud and security has made him an invaluable team member, and he is constantly learning and refining his skills to stay up to date with the latest trends and technologies.