What is Mod_evasive and how to install it on server in order to prevent DDOS Attack.

What is Mod_evasive:

One way to stop one of the more basic attacks on a server is mod_evasive.This article will assist you the process of installing and configuring mod_evasive. This apache module will help protect against people sending too many requests to the webserver in an attempt to flood it. If it detects too many connections the offending ip will be blocked from the accessing apache for This is especially useful when the server is continuously getting attacked. With this default configuration it will block the offending ip for 10 minutes. If it continues to try and flood mod_evasive will automatically add more time to this.

Installation :

Follow this commands for Apache 1.3.x.
cd /usr/local/src
wget http://www.zdziarski.com/projects/mod_evasive/mod_evasive_1.10.1.tar.gz
tar -zxf mod_evasive_1.10.1.tar.gz
cd mod_evasive
/usr/local/apache/bin/apxs -cia mod_evasive.c

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Follow this commands for Apache 2.0.x.
cd /usr/local/src
wget http://www.zdziarski.com/projects/mod_evas/mod_evasive_1.10.1.tar.gz
tar -zxf mod_evasive_1.10.1.tar.gz
cd mod_evasive
/usr/sbin/apxs -cia mod_evasive20.c

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Configuration:

If you are adding the is module to apache 1.3.x the following lines need to be added to the httpd.conf below the AddModule section.

<IfModule mod_evasive.c>
DOSHashTableSize 3097
DOSPageCount 5
DOSSiteCount 100
DOSPageInterval 2
DOSSiteInterval 2
DOSBlockingPeriod 600
</IfModule>

If you are using apache 2.0.x you need to scroll to below the LoadModule section in the httpd.conf and add the following:

<IfModule mod_evasive20.c>
DOSHashTableSize 3097
DOSPageCount 5
DOSSiteCount 100
DOSPageInterval 2
DOSSiteInterval 2
DOSBlockingPeriod 10
DOSBlockingPeriod 600
</IfModule>

Exit and save out of the httpd.conf

Now it should be ready to go. Exit out of vi and restart apache.

service httpd restart

It is installed 🙂

Note:

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

DOSSiteCount

This is the threshhold for the total number of requests for any object by the same client on the same listener per site interval. Once the threshhold 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 threshhold; defaults to 1 second intervals.

DOSSiteInterval

The interval for the site count threshhold; 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.