Configure mod_evasive to Help Survive DoS/DDoS Attacks

What is mod_evasive?

mod_evasive is a module for Apache that provides evasive action in the event of an HTTP Distributed Denial of Service (DDoS/DoS) attack or brute force attack. It is also designed to be a detection and network management tool, and can be easily configured to talk to ipchains, firewalls, routers, and more. mod_evasive presently reports abuse via email and syslog facilities.

This guide assumes you already have your LAMP server configured. Guides for setting up a LAMP stack can be found in our LAMP guides section.

How does mod_evasive work?

mod_evasive works on top of a dynamic IP table and URIs to issue or deny permissions to incoming requests. This enables executive evasive actions during sophisticated attacks on your systems and reports certain actions to your web servers.

To understand how mod_evasive handles both DDoS and brute force attacks, we must understand the attacks themselves. In a DDoS attack, your servers experience a high volume of requests. In a brute force attack, they receive a high number of log-in attempts using different username and password combinations. mod_evasive works by handling the common denominator between these two attacks: the surge in the number of requests to your servers.

For each listener in your web servers, mod_evasive uses your Apache web server’s own capacity to scale with request volumes. mod_evasive evaluates each incoming request and checks its IP against a hash table.

This hash table has URIs and IPs that are dynamically updated. Each request’s IP address is matched against a temporary blocking list of IPs that have potentially tried sending too many requests. If the request’s IP address is found in the blocking list, a 403 status code is returned in response to the request.

Prerequisites

mod_evasive has just one prerequisite beyond the standard LAMP install: the Apache Web Server Utility. To install this module, run the following as root:

Debian / Ubuntu:

apt-get install apache2-utils

CentOS / Fedora:

yum install httpd-devel

Install mod_evasive

1. Install the mod_evasive module:

   Debian / Ubuntu:

   apt install libapache2-mod-evasive
   

   CentOS / Fedora:

   yum install epel-release
   yum install mod_evasive
   

2. To add the mod_evasive configuration to your Apache configuration file, find the section appropriate to your Apache config:

   Debian / Ubuntu:

   File: /etc/apache2/apache2.conf

   1 2 3

   # Include module configuration: Include mods-enabled/*.load Include mods-enabled/*.conf

   CentOS / Fedora:

   File: /etc/httpd/conf/httpd.conf

   1 2	LoadModule evasive20_module /usr/lib/httpd/modules/mod_evasive20.so #

3. Below that section, add the mod_evasive configuration:

   File: /etc/apache2/apache2.conf

   1 2 3 4 5 6 7 8 9

   <IfModule mod_evasive20.c> DOSHashTableSize 3097 DOSPageCount 2 DOSSiteCount 50 DOSPageInterval 1 DOSSiteInterval 1 DOSBlockingPeriod 60 DOSEmailNotify <someone@somewhere.com> </IfModule>

4. Restart Apache for your changes to take effect:

   Debian / Ubuntu:

   /etc/init.d/apache2 restart
   

   CentOS / Fedora:

   /etc/init.d/httpd restart
   

mod_evasive Configuration Options

These configuration option descriptions were taken directly from the README file packaged with the mod_evasive tarball you downloaded during installation.

DOSHashTableSize

The hash table size defines the number of top-level nodes for each child’s hash table. Increasing this number provides faster performance by decreasing the number of iterations required to get to the record, but consume more memory for table space. You should increase this if you have a busy web server. The value you specify automatically tiers up to the next prime number in the primes list (see mod_evasive.c for a list of primes used).

DOSPageCount

This is the threshold for the number of requests for the same page (or URI) per page interval. Once the threshold for that interval has been exceeded, the IP address of the client is added to the blocking list.

DOSSiteCount

This is the threshold for the total number of requests for any object by the same client on the same listener per site interval. Once the threshold for that interval has been exceeded, the IP address of the client is added to the blocking list.

DOSPageInterval

The interval for the page count threshold; defaults to 1 second intervals.

DOSSiteInterval

The interval for the site count threshold; defaults to 1 second intervals.

DOSBlockingPeriod

The blocking period is the amount of time (in seconds) that a client is blocked for if they are added to the blocking list. During this time, all subsequent requests from the client result in a 403 (Forbidden) and the timer being reset (e.g. another 10 seconds). Since the timer is reset for every subsequent request, it is not necessary to have a long blocking period; in the event of a DoS attack, this timer keeps getting reset.

DOSEmailNotify

If this value is set, an email is sent to the address specified whenever an IP address becomes blacklisted. A locking mechanism using /tmp prevents continuous emails from being sent.

DOSSystemCommand

If this value is set, the system command specified is executed whenever an IP address becomes blocked. This is designed to enable system calls to ip filter or other tools. A locking mechanism using /tmp prevents continuous system calls. Use %s to denote the IP address of the blacklisted IP.

DOSLogDir

Choose an alternative temp directory for the mod_evasive locking mechanism.

By default “/tmp” is used for locking mechanism, which opens some security issues if your system is open to shell users. In the event you have nonprivileged shell users, you want to create a directory writable only to the user Apache is running as (usually root), then set this in your httpd.conf.

Whitelisting IP Addresses

IP addresses of trusted clients can be allowed to insure they are never denied. The purpose of whitelisting is to protect software, scripts, local searchbots, or other automated tools from being denied for requesting large amounts of data from the server. Whitelisting should not be used to add customer lists or anything of the sort, as this opens the server to abuse. This module is very difficult to trigger without performing some type of malicious attack, and for that reason it is more appropriate to allow the module to decide on its own whether or not an individual customer should be blocked.

To allow an address (or range) add an entry to the Apache configuration in the following fashion:

File: /etc/apache2/apache2.conf

1 2	DOSWhitelist 127.0.0.1 DOSWhitelist 127.0.0.*

Wildcards can be used on up to the last 3 octets if necessary. Multiple DOSWhitelist commands may be used in the configuration.

How to Restrict mod_evasive for Only One Virtual Host on Apache

When you define a mod_evasive configuration on Apache, you by default set it to work at a global level. To enable it for specific hosts, you can apply two different methods:

1. Put your default mod_evasive configuration to work with a large range of numbers. You can define these numbers in a way that a ban is never triggered.

2. Leave mod_evasive set at the global level, but set DOSBlockingPeriod to 0 for certain vhosts.

Adding numbers to avoid a mod_evasive ban for a specific host is highly likely to create performance issues for the host’s Apache web server. These performance issues are also the reason why this is not a recommended practice.

To exclude certain vhosts from mod_evasive bans or triggers without compromising the system level performance, we can use the DOSBlockingPeriod. We can add the following to exclude vhosts:

File: default.conf

1 2 3

<IfModule mod_evasive24.c> DOSBlockingPeriod 0 </IfModule>

How to Check if mod_evasive is Working

To test if our mod_evasive configuration is working as intended, we can use test.pl, which is created by mod_evasive’s developers. The example test.pl file is located here:

/usr/share/doc/libapache2-mod-evasive/examples/test.pl

However, this example file requires modification to properly test on a Linode. Open the file and edit line 13 so that the entire file contains the following:

File: /usr/share/doc/libapache2-mod-evasive/examples/test.pl

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

#!/usr/bin/perl # test.pl: a small perl script that test's mod_dosevasive's effectiveness use IO::Socket; use strict; for(0..100) { my($response); my($SOCKET) = new IO::Socket::INET( Proto => "tcp", PeerAddr=> "127.0.0.1:80"); if (! defined $SOCKET) { die $!; } print $SOCKET "GET /?$_ HTTP/1.0\r\nHost: 127.0.0.1\r\n\r\n"; $response = <$SOCKET>; print $response; close($SOCKET); }

If you don’t have Perl installed on your system, you can install it by running the following command on your terminal:

Debian / Ubuntu:

apt install perl

CentOS / Fedora:

yum install -y perl

Let’s run a test of our mod_evasive configuration to see if our mod_evasive is working well with the Apache web server. Our test is to send 100 requests to trigger mod_evasive.

We can run test.pl using the following command:

perl /usr/share/doc/libapache2-mod-evasive/examples/test.pl

You should see the output of these requests as shown below:

…
HTTP/1.1 200 OK
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
…

Apart from the terminal’s output, mod_evasive activities are logged to /var/log/syslog, which we can check to understand actions associated with IPs.

We can run the following command on our logs to check the last five lines to see possible mod_evasive actions:

tail -5 /var/log/syslog

Sep 28 15:11:10 li248-37 systemd[1]: apache2.service: Succeeded.
Sep 28 15:11:10 li248-37 systemd[1]: Stopped The Apache HTTP Server.
Sep 28 15:11:10 li248-37 systemd[1]: Starting The Apache HTTP Server...
Sep 28 15:11:10 li248-37 systemd[1]: Started The Apache HTTP Server.
Sep 28 15:11:13 li248-37 mod_evasive[21073]: Blacklisting address 127.0.0.1: possible DoS attack.

In the output above, the part that says Blacklisting address 127.0.0.1: possible DoS attack highlights a mod_evasive action. Here the IP address 127.0.0.1 is blocked by mod_evasive as a result of the test we ran earlier.

How to Load Test mod_evasive

Refer to our guide on Load Testing with Siege to test your site’s performance. Before you attempt to DDoS yourself, be aware that you risk banning your own IP. Linode does not recommend testing any server that isn’t your own.

More Information

You may wish to consult the following resources for additional information on this topic. While these are provided in the hope that they will be useful, please note that we cannot vouch for the accuracy or timeliness of externally hosted materials.

• mod_evasive on GitHub