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What might be causing bursts of 10 - 20 proxy auto config file requests?

I am seeing bursts of requests for our PAC file, which is hosted on an internal Apache server. Some bursts are up to 20 per second from the same host. Looking in on the /var/log/apache2/access.log shows that these request bursts are being made by something that does not supply its user agent. It appears most browsers are nice about making requests and supply an ETag and user agent, and result in a 304. However all of the bursting requests result in 200s.

I've checked one of the bursting hosts for the registry key listed in How Do I Prevent MSIE 8 From Reloading PAC File? to make sure caching is not disabled. I've confirmed the key does not exist, so caching should be enabled by default.

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Identify the host and dump traffic on it to identify the application. – mgorven Feb 20 '13 at 17:17

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