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Configure mod_evasive to Help Survive DoS/DDoS Attacks

Updated by Linode Written by Chris Ciufo

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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.

mod_evasive on Apache

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.

Prerequisites

mod_evasive has just one prerequisite beyond the standard LAMP install. 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. Download the mod_evasive package, uncompress it, and install it using apxs:

    cd /usr/src
wget https://www.zdziarski.com/blog/wp-content/uploads/2010/02/mod_evasive_1.10.1.tar.gz
tar xzf mod_evasive_1.10.1.tar.gz
cd mod_evasive
apxs2 -cia mod_evasive20.c

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

    Debian / Ubuntu:

    /etc/apache2/apache2.conf 
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    # Include module configuration:
Include mods-enabled/*.load
Include mods-enabled/*.conf

    CentOS / Fedora:

    /etc/httpd/conf/httpd.conf 
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    LoadModule evasive20_module /usr/lib/httpd/modules/mod_evasive20.so
#

  3. Below that section, add the mod_evasive configuration:

    mod_evasive configuration 
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    <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 will provide 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 will automatically be tiered 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 will be 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 will be 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 will be blocked for if they are added to the blocking list. During this time, all subsequent requests from the client will 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 will keep getting reset.

DOSEmailNotify

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

Note
Be sure MAILER is set correctly in mod_evasive.c (or mod_evasive20.c). The default is “/bin/mail -t %s” where %s is used to denote the destination email address set in the configuration. If you are running on linux or some other operating system with a different type of mailer, you’ll need to change this.

DOSSystemCommand

If this value is set, the system command specified will be executed whenever an IP address becomes blacklisted. 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

By default “/tmp” will be 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’ll 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 whitelisted 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 will open 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 whitelist an address (or range) add an entry to the Apache configuration in the following fashion:

/etc/apache2/apache2.conf 
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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.

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.

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This guide is published under a CC BY-ND 4.0 license.