Protecting Your Website From Application Layer DOS Attacks With mod_evasive

This guide explains how to protect your website from simple DDOS attacks by installing and configuring mod_evasive for the Apache web server.

There exist many ways of maliciously taking a website offline. The more complicated methods involve technical knowledge of databases and programming. A far simpler method is known as a “Denial Of Service”, or “DOS” attack. This attack derives its name from its goal which is to deny your regular clients or site visitors normal website service.

There are, generally speaking, two forms of DOS attack;

  1. Layer 3,4 or Network-Layer attacks.
  2. Layer 7 or Application-Layer attacks.

The first type of DOS attack, network-layer, is when a huge quantity of junk traffic is directed at the web server. When the quantity of junk traffic exceeds the capacity of the network infrastructure the website is taken offline.

The second type of DOS attack, application-layer, is where instead of junk traffic legitimate looking page requests are made. When the number of page requests exceeds the capacity of the web server to serve pages legitimate visitors will not be able to use the site.

This guide will look at mitigating application-layer attacks. This is because mitigating networking-layer attacks requires huge quantities of available bandwidth and the co-operation of upstream providers. This is usually not something that can be protected against through configuration of the web server.

An application-layer attack, at least a modest one, can be protected against through the configuration of a normal web server. Protecting against this form of attack is important because Cloudflare have recently reported that the number of network-layer attacks is diminishing while the number of application-layer attacks is increasing.

This guide will explain using the Apache2 module mod_evasive by zdziarski.

In addition, mod_evasive will stop an attacker trying to guess a username/password combination by attempting hundreds of combinations i.e. a brute force attack.

Mod_evasive works by keeping a record of the number of requests arriving from each IP address. When this number exceeds one of the several thresholds that IP is served an error page. Error pages require far fewer resources than a site page keeping the site online for legitimate visitors.

Installing mod_evasive on Ubuntu 16.04

Mod_evasive is contained in the default Ubuntu 16.04 repositories with the package name “libapache2-mod-evasive”. A simple apt-get will get it installed:

apt-get update
apt-get upgrade
apt-get install libapache2-mod-evasive

We now need to configure mod_evasive.

It’s configuration file is located at /etc/apache2/mods-available/evasive.conf. By default, all the modules settings are commented after installation. Therefore, the module won’t interfere with site traffic until the configuration file has been edited.

<IfModule mod_evasive20.c>
   #DOSHashTableSize    3097
   #DOSPageCount        2
   #DOSSiteCount        50
   #DOSPageInterval     1
   #DOSSiteInterval     1
   #DOSBlockingPeriod   10

   #DOSEmailNotify      you@yourdomain.com
   #DOSSystemCommand    "su - someuser -c '/sbin/... %s ...'"
   #DOSLogDir           "/var/log/mod_evasive"
</IfModule>

The first block of directives mean as follows:

If the default configuration shown above is used then an IP will be blocked if it:

If an IP exceeds these thresholds it is blocked for 10 seconds.

This may not seem like a lot, however, mod_evasive will continue monitoring the page requests even for blocked IP’s and reset their block period. As long as an IP is attempting to DOS the site it will remain blocked.

The remaining directives are:

Configuring mod_evasive

The default configuration is a good place to start as it should not block any legitimate users. The configuration file with all directives (apart from DOSSystemCommand) uncommented looks like the following:

<IfModule mod_evasive20.c>
   DOSHashTableSize   3097
   DOSPageCount       2
   DOSSiteCount       50
   DOSPageInterval    1
   DOSSiteInterval    1
   DOSBlockingPeriod  10

   DOSEmailNotify       JohnW@example.com
   #DOSSystemCommand    "su - someuser -c '/sbin/... %s ...'"
   DOSLogDir            "/var/log/mod_evasive"
</IfModule>

The log directory must be created and given the same owner as the apache process. Here it is created at /var/log/mod_evasive and given the owner and group of the Apache web server on Ubuntu www-data:

mkdir /var/log/mod_evasive
chown www-data:www-data /var/log/mod_evasive

After editing Apache’s configuration, especially on a live website, it is always a good idea to check the syntax of the edits before restarting or reloading. This is because a syntax error will stop Apache from re-starting and taking your site offline.

Apache comes packaged with a helper command that has a configuration syntax checker. Simply run the following command to check your edits:

apachectl configtest

If your configuration is correct you will get the response:

Syntax OK

However, if there is a problem you will be told where it occurred and what it was, e.g.:

AH00526: Syntax error on line 6 of /etc/apache2/mods-enabled/evasive.conf:
DOSSiteInterval takes one argument, Set site interval
Action 'configtest' failed.
The Apache error log may have more information.

If your configuration passes the configtest then the module can be safely enabled and Apache reloaded:

a2enmod evasive
systemctl reload apache2.service

Mod_evasive is now configured and running.

Testing

In order to test mod_evasive, we simply need to make enough web requests to the server that we exceed the threshold and record the response codes from Apache.

A normal, successful page request will receive the response:

HTTP/1.1 200 OK

However, one that has been denied by mod_evasive will return the following:

HTTP/1.1 403 Forbidden

The following script will make HTTP requests to 127.0.0.1:80, that is localhost on port 80, as rapidly as possible and print out the response code of every request.

All you need to do is to copy the following bash script into a file e.g. mod_evasive_test.sh:

#!/bin/bash
set -e

for i in {1..50}; do
        curl -s -I 127.0.0.1 | head -n 1
done

The parts of this script mean as follows:

Then make it executable:

chmod 755 mod_evasive_test.sh

When the script is run before mod_evasive is enabled you will see 50 lines of HTTP/1.1 200 OK returned.

However, after mod_evasive is enabled you will see the following:

HTTP/1.1 200 OK
HTTP/1.1 200 OK
HTTP/1.1 403 Forbidden
HTTP/1.1 403 Forbidden
HTTP/1.1 403 Forbidden
HTTP/1.1 403 Forbidden
HTTP/1.1 403 Forbidden
...

The first two requests were allowed, but then once a third in the same second was made mod_evasive denied any further requests. You will also receive an email letting you know that a DOS attempt was detected to the address you set with the DOSEmailNotify option.

Mod_evasive is now protecting your site!