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Gzip/mod_deflate not Working? Check your Proxy Server

After scratching my head for a while, I finally figured out the problem, hinted at by a comment on a question on Stack Overflow. Our work’s proxy server was blocking the ‘Accept-Encoding’ http header that is sent along with every file request; this prevented a gzipped transfer of any file, thus Yslow gave an F.

I set up a secure tunnel (using SSH) from my computer to the web server directly, and then reloaded the page in FireFox, and re-ran YSlow:

Much happier now. I’ve contacted our IT department to see if it’s possible to allow the proxy server to pass through the Accept-Encoding headers, but for now, I’ll know to watch out for false positives on the YSlow test, and check from multiple locations.