Emulating a slow internet link for http

Emulating a slow internet link for http

When developing a Web Application, many times you need to test how will it react then operating over a slow internet connection, like 56k modem, GPRS or any other situation where the bandwidth is tight. This is important not only to test how long will those funky pictures take to load, but as AJAX gets more and more used everywhere, you really need to guarantee that everything works acceptably on slow network links, including AJAX calls, timeouts, animations, and everything else that might depend on the server.

There are some graphical applications for Mac OS X that try to do that, but many don’t work as promised. Well, Mac OS X has the solution built-in, you just have to configure it, and that it the internal firewall (ipfw). Since the introduction of Tiger, ipfw can now do QoS (Quality of Service) stuff, and that means you can create channels (or pipes, in ipfw terminology) and control the priority, bandwith, delay and some other settings of those channels. Than, you simply have to pump the traffic you want to see slowed down thru those channels.

I have done two simple scripts to configure my firewall automatically. They will slow down the http port (80) to a crawl. I do not handle the SSL port for https traffic, but it’s easily achieved by duplicating the script content and adjusting the parameters.

Here are the scripts. The first one, that slows down everything:


/sbin/ipfw add 100 pipe 1 ip from any 80 to any out
/sbin/ipfw add 200 pipe 2 ip from any 80 to any in
/sbin/ipfw pipe 1 config bw 128Kbit/s queue 64Kbytes delay 250ms
/sbin/ipfw pipe 2 config bw 128Kbit/s queue 64Kbytes delay 250ms

And the one that brings your network back to normally:


/sbin/ipfw delete 100
/sbin/ipfw delete 200

As you can see, this slows down all your http traffic to 128 Kbps, and introduces a delay of 250ms (which is roughly what you have on slow, modem-like or wireless connections). The delay can really impact the performance of your application, specially if you do a lot of requests to get information to display on a web page. You can change the values accoring to your needs, and even bring the 128 Kbps down to 56 or even less if you have the guts! I still don’t know exactly what infuence is caused bu changing the queue size, but the 64 KB value produces a nice result.

Don’t forget, if you are just making a static web page, for this to work, you have to turn on the apache web server (the easiest way if to turn on Web Sharing in the System Preferences “Sharing” panel) and browse your pages thru it. Direct file access won’t naturally be affected by the firewall. Also, this slows down ALL your http traffic. Not only the traffic that comes from your computer, but also the traffic that comes from the Internet. So, if you are wondering why is your net really slow, the solution is simple: you forgot to run the second script after finishing your tests! 🙂 If for some reason you cannot recover the normal speed, just reboot and your Mac will be back to normal.

To run this scripts, just copy them to two text files (using any text editor that saves in TXT – RTF won’t do it!) and then make then executable (using the “chmod u-x ” command on the shell). Also, you have to run then as root (using “sudo” or switching to root before running them).