The IT Detective agency: Excessive Requests for PAC file Crippling Web Server

Intro
Funny thing about infrastructure. You may have something running fine for years, and then suddenly it doesn’t. That is one of the many mysteries in the case of the excessive requests for PAC file.

The Details
We serve our Proxy Auto-config (PAC) file from a couple web servers which are load-balanced. It’s worked great for over 10 years. The PAC file is actually produced by a Perl script which can alter the content based on the user’s IP or other variables.

The web servers got bogged down last week. I literally observed the load average shoot up past 200 (on a 2-CPU server). This is not good.

I quickly noticed lots and lots of accesses for wpad.dat and proxy.pac. Some PCs were individually making hundreds of requests for these files in a day! Sometimes there were 15 or 20 requests in a minute. Since it is a script it takes some compute power to handle all those requests. So it was time to do one of two things: either learn the root cause and address it, or make a quick fix. The symptoms were clear enough, but I had no idea about the root cause. I also was fascinated by the requests for wpad.dat which I felt was serving no purpose whatsoever in our environment. So I went for the quick fix hopinG that understanding would come later.

To be continued…
As promised – three and a half years later! And we still have this problem. It’s probably worse than ever. I pretty much threw in the towel and learned how to scale up our apache web server to handle more PAC file requests simultaneously, see the references.

References
Scaling apache to handle more requests.