Thursday, June 02, 2005

PAC issue in Safari on Tiger (MacOS 10.4)

The Proxy Automatic Configuration URL feature, as implemented in Safari, is broken.

Instead of one HTTP request for the PAC file at the start of a session, Safari 2.0(412) makes a HTTP request to the PAC server once for each object requested -- for each HTTP request out to the internet, a corresponding request is made for the PAC.

This leads to one host generating PAC request rates exceeding 57 GETs/second, totalling over ten thousand hits in a day from a single Tiger workstation, and hundreds of thousands of extra PAC hits per day.

For comparison, the average MS-Windows client hits the PAC URL a mere 7 times per day. As the number of Tiger installations grow, the load on the server hosting PAC grows, until critical mass is reached, the server falls over, and suddenly nobody can reach the internet because nobody can load the proxy settings.

When using a local PAC file (a file::/localhost/... URL), the network problems are avoided, but browser performance is poor, with many broken images and general slowness in loading pages.

Well-written PAC-aware browsers will retrieve a fresh copy of PAC when first launched, and then cache this copy, refreshing the contents either based on the Expires header or using an internal refresh timeout (Under MSIE, the refresh time can be set using the IEAK). Safari drops the ball.

In Mac OSX Tiger, the option to configure proxy settings is under System Preferences/Network. This menu gives the user the option to set a "PAC File URL", but no option to control how/whether this file is cached and refreshed. Also, Safari does not respect Expires headers sent by the PAC server.

For each site accessed, Safari makes a new TCP connection to the PAC URL (specifying Connection: close) and sends a HTTP/1.0 request for the URL as set in the System Preferences. If they'd at least implement keepalives (as Firefox does), the problem would be greatly reduced.


Installing 10.4.1 does not resolve this issue.

As a temporary workaround, switching to Firefox eliminates this problem. Firefox will only download the PAC at the start of the session or when the user manually chooses to reload PAC.


Blogger Nonesuch said...

Just this week has Apple finally gotten around to offering us access to a pre-release fix for this issue.


Tue Aug 02, 12:22:00 AM CDT  
Blogger Nonesuch said...

And come the first week of November, Apple releases a patch cluster which finally fixes the bug.

Almost makes me nostalgic for Microsoft's bug handling process.

Wed Nov 09, 12:03:00 AM CST  

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