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Patrick Debois

Simulate slow browsers on MacOSX using ipfw

As a developer when you are working on your local machine, it’s easy to forget that there are still a lot of people out there without sufficient broadband access.

Especially when executing long browser transactions like file uploads or running lenghty reports, timeout can have an impact on the way our code reacts to endusers.

This is also true when browsing web pages with a lot of images, javascript and css. On our laptops with lots of cpu power and memory it might take a few milliseconds to render the page, but if you are targeting a broad non-technical audience , you really need to test how the page loading experience works for them.

Usually this involves asking the network guys to setup a slow link to see this. Thanks to http://www.macosxhints.com/article.php?story=20080119112509736 I found an easy way to simulate this on my local Mac machine.

Enter the use of ipfw, the firewall system provided by Apple. A less know feature is that i can also shape traffic. The following example is a simulation of enduser with a bandwith of 15KByte/s.

Step 1: create a network pipe 1 of 15KByte/S

sudo ipfw pipe 1 config bw 15KByte/s

Step 2: attach the pipe to the src-port 80 (web traffic)

sudo ipfw add 1 pipe 1 src-port 80

Step 3: test your website via your browser

That should to the trick. Still there a few caveats, this does not work for localhost as this traffic does not pass through the ip filter. The other thing is that this will slow down all web traffic, so you might to specify destination in the pipe add command.

Step 4: remove the pipe again

sudo ipfw delete 1

A more advanced explanation can be found on http://www.afp548.com/article.php?story=20060214081244545