FreeBSD and apache 2.2 band width limiting

Jos Chrispijn jos at
Mon Jun 15 16:31:26 UTC 2009

Dave wrote:
 > amound of band width, the other group gets another set amount. I'm 
 > if this is doable?


The traditional solution to handling traffic surges is to throttle 
traffic on the network -- often at the router. While effective, 
reconfiguring a router typically requires you to get in touch with your 
ISP or hosting provider and convince them to implement the necessary 
throttling. All the while, your bill grows larger and larger. Even 
worse, if you hit a bandwidth cap, your entire site can effectively be 
taken off-line.

Rather than depending on someone else to solve your traffic problems, 
you can take matters into your own hands and install the Apache module 
mod_throttle. mod_throttle allows you to build custom bandwidth and 
connection rate policies for individual files and directories and entire 

To understand why mod_throttle is so powerful, consider the traditional 
solution of throttling at the router (or firewall). This simple-minded 
approach configures a router to limit the rate at which packets go to 
and from your server. It's a unilateral constraint: all traffic is 
effected. Throttle at the router and all of your services are punished 
for the bad behavior of just one. In a virtual hosting environment, that 
means all customers are punished for the popularity of a single, 
unrelated site.

If the content you'd like to throttle is in one particular directory, 
there's no need to punish the users of the rest of your site. With 
mod_throttle, you can make that decision. Even the most sophisticated 
router-based throttling isn't granular enough to handle this. Using 
mod_throttle, you can control the average maximum transfer rate for a 
given client.

Modus Operandi

As you can see, throttling isn't as simple as it first might seem. 
Luckily, mod_throttle provides policies to combat most of the throttling 
problems you're likely to encounter. Let's take a quick look at them.

* The None policy doesn't provide any throttling. It's used to test 
throttling rules before deploying them.
* Using the Concurrent policy you can control the number of concurrent 
requests that can be made. This policy can keep clients from 
monopolizing your server with too many simultaneous requests.
* The Document policy limits the number of "document" requests per time 
period. This policy applies to "pages" rather than "hits" since a single 
page may cause many requests to retrieve referenced images, CSS, and 
other media files.
* The Idle policy forces a minimum idle time (or delay) between 
requests. This can be used to counteract web spiders that try to suck 
down pages from your site as fast as possible.
* The Original policy is a volume-based policy that is inherited from an 
earlier version of mod_throttle. It's a bit complicated, so check the 
mod_throttle docs if you're curious.
* The Random policy randomly accepts a percentage of incoming requests. 
If the percentage is 100, all requests are accepted. If it is 0, none 
are. By using a value between 0 and 100, you can effectively refuse a 
percentage of your requests.
* The Speed policy imposes a limit on how fast data is sent per period. 
The Volume policy imposes a limit on how much data is sent per period. 
And the Request policy limits the number of requests per time period. 
The Request policy is also effective in slowing web spiders.

Each of the policies are controlled by parameters, including a limit and 
a period. If you want to throttle something to 10 KB/s, then the limit 
is 10 KB and the period is 1 second.

The Original, Speed, and Volume policies all work on the amount of data 
sent. In other words, they count bytes. Because you can configure the 
period for those policies, you can be tolerant of short bursts of 
traffic while still capping potential abusers. Concurrent, Document, 
Idle, and Request deal with requests, regardless of size.

Adding the Throttle

Installing mod_throttle is simple. The easiest method is to build it as 
a Dynamic Shared Object (DSO) and then add it to your Apache 
configuration. First, download and unpack the code:

First get mod_throttle from ports

Next, build and install it:

$ make; sudo make install

Add a LoadModule line to your httpd.conf:

LoadModule throttle_module /usr/lib/apache/1.3/

And then add an initial throttle configuration:

<IfModule mod_throttle.c>
ThrottleClientIP 100 None
ThrottlePolicy None

<Location /throttle-status>
SetHandler throttle-status

<Location /throttle-me>
SetHandler throttle-me

Next, test and restart Apache:

$ sudo apachectl configtest
$ sudo apachectl restart

Figure One: The mod_throttle status page

Finally, visit the throttle-status page to make sure the module is 
working. Use a URL similar to http://www.example .com/throttle-status. 
The page should look roughly like Figure One.


With mod_throttle installed and working, let's look at a couple of real 
configurations to get you started.

First, let's consider serving a directory of large and popular content, 
such as a collection of parodies of Apple's "switch" commercials 

<Location /switch/>
<IfModule mod_throttle.c>
ThrottlePolicy Speed 100K 1s

The ThrottlePolicy directive, as you might guess, specifies the policy 
you'd like to implement for this location. The specifications always 
follow the form:

ThrottlePolicy Policy Limit Period

If you put a collection of big movies on the server and then try to 
fetch a movie from another machine, you might find that the file is 
transmitted at a very high speed -- much higher than 100 KB/s. What's 

Unlike the granular rate-limiting that is possible at the router or 
kernel level, mod_throttle works with HTTP requests (rather than network 
packets). All of the bytes are counted, but they can't be counted until 
after a request is serviced. mod_throttle keeps track of how many bytes 
were sent and how long it took.

Next, let's consider the problem of a Web robot that hits your site too 
quickly. The following entry limits all clients (identified by unique IP 
address) to five requests per second. That rule is applied server-wide 
unless the entry is enclosed in a VirtualHost, Location, or Directory block.

<IfModule mod_throttle.c>
ThrottlePolicy Request 5 1

Watch it
As you add new policies to your server, the mod_throttle statistics page 
reflects each one, along with various statistics, including number of 
hits refused and bytes sent. By clicking on a policy name, you can 
discover which clients are currently in violation of the policy. You can 
also use the Web interface to reset those clients.

That's all there is to getting started. mod_throttle can keep your 
bandwidth costs under control, and can also help prevent a Denial of 
Service attack from crippling your web site.

hope this helps,
Jos Chrispijn*

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