Thursday, May 20, 2010

Rate Limiting by mod_evasive

When there is high traffic on your apache web server, chances are that - are that sooner or later it might come under a DoS attack. So as per my promise in my previous post here is one of the must install tools, implementation steps.

mod_evasive is an Apache module specifically designed to deal with this. From the author’s site:

"mod_evasive is an evasive maneuvers module for Apache to provide evasive action in the event of an HTTP DoS or DDoS 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 etcetera. mod_evasive presently reports abuses via email and syslog facilities."

So this is how you go about installing mod_evasive in a linux Box ...

  1. cd /usr/local/src

wget http://www.zdziarski.com/blog/wp-content/uploads/2010/02/mod_evasive_1.10.1.tar.gz

  1. tar -zxvf mod_evasive_1.10.1.tar.gz
  2. cd mod_evasive
  3. /usr/local/apache2/bin/apxs -cia mod_evasive20.c
  4. Edit httpd.conf file and add -

LoadModule evasive20_module /usr/local/apache2/modules/mod_evasive20.so


DOSHashTableSize 3097
DOSPageCount 2
DOSSiteCount 50
DOSPageInterval 1
DOSSiteInterval 1
DOSBlockingPeriod 120

It is highly recommended that you go through the README file that came with the source, and then keeping a sharp eye on what your webserver does, to see if you need to tweak any defaults. I’d also suggest adding the email alerting option inside the IfModule configuration:

DOSEmailNotify debajitkataki@gmail.com

Explanation Of Various Parameters:

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.

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

You can Choose an alternative temporary directory. By default “/tmp” will be used for locking mechanism.

HOW TO WHITE-LIST THE 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:

DOSWhitelist
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.

You will notice that I have a couple of different settings in there than the default. For ex. if you want to ban the IP for 1 hour, you need to tweak the value to 3600 seconds.

That’s it! Time To Catch Some Fish Guys ;-)

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