mod_limitipconn.c
David Jao <djao@dominia.org>

This is an Apache 2.4/2.2/2.0 C module whose purpose is to limit the
maximum number of simultaneous connections per IP address. It supports
IPv4 and IPv6 connections. The module allows inclusion and exclusion of
files based on MIME type.

This module is not designed to prevent denial-of-service attacks. Its
function is only to deny users the ability to use large numbers of
simultaneous connections.

Tested with httpd-2.4.2 and httpd-2.2.17. Should work with httpd-2.0 but
has not been tested.

Example configuration:

---------------------------------------------------------------------------

# This command is always needed
ExtendedStatus On

# Only needed if the module is compiled as a DSO
LoadModule limitipconn_module lib/apache/mod_limitipconn.so

<IfModule mod_limitipconn.c>

    # Set a server-wide limit of 10 simultaneous downloads per IP,
    # no matter what.
    MaxConnPerIP 10
    <Location /somewhere>
	# This section affects all files under http://your.server/somewhere
	MaxConnPerIP 3
	# exempting images from the connection limit is often a good
	# idea if your web page has lots of inline images, since these
	# pages often generate a flurry of concurrent image requests
	NoIPLimit image/*
    </Location>

    <Directory /home/*/public_html>
	# This section affects all files under /home/*/public_html
	MaxConnPerIP 1
	# In this case, all MIME types other than audio/mpeg and video*
	# are exempt from the limit check
	OnlyIPLimit audio/mpeg video
    </Directory>
</IfModule>

---------------------------------------------------------------------------

Notes:

1) This module will not function unless mod_status is loaded and the
   "ExtendedStatus On" directive is set.

2) Server-wide access restrictions and per-directory access restrictions
   are computed separately.  In the above example, if someone is
   downloading 11 images from http://your.server/somewhere
   simultaneously, they WILL be denied on the 11th download, because the
   server-wide limit of 10 downloads is not affected by the per-directory
   NoIPLimit.  If you want to set global settings which can be overruled
   by per-directory settings, you will need something like

	<Location />
	# global per-directory settings here

		<Location /somewhere>
		# local per-directory settings here

		</Location>

	</Location>

3) If you are using any module based upon a quick handler hook (such as
   mod_cache), mod_limitipconn will not be able to process any
   per-directory configuration directives in time to affect the return
   result of the other module.  This is a technical limitation imposed
   by Apache.  In such a situation, you will have to use server-wide
   configuration directives only.

   Versions of mod_limitipconn prior to 0.23 did not allow any
   server-wide configuration directives, and hence could not be used
   with mod_cache at all.

4) The limits defined by mod_limitipconn.c apply to all IP addresses
   connecting to your Apache server. Currently there is no way to set
   different limits for different IP addresses. One workaround is to set
   up two different URIs for the same content and use access control to
   restrict which IP addresses can access which URIs.

5) Connections in excess of the limit result in a stock 503 Service
   Temporarily Unavailable response. The job of returning a more useful
   error message to the client is left as an exercise for the reader.

6) mod_limitipconn sets the LIMITIP environment variable to 1 whenever a
   download is denied on the basis of too high an IP count. You can use
   this variable to distinguish accesses that have been denied by this
   module. For example, a line like

      CustomLog /var/log/httpd/access_log common env=!LIMITIP

   in httpd.conf can be used to suppress logging of denied connections
   from /var/log/httpd/access_log. (Note that, if you really want to
   suppress logging, you'll probably also want to comment out the
   ap_log_rerror lines from mod_limitipconn.c as well.)

7) By default, all clients behind a proxy are treated as coming from the
   proxy server's IP address. If you wish to alter this behavior,
   use the mod_remoteip module included in Apache 2.4.