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cbj4074 23rd May 2012 16:49

Preventing dynamically-served files from being gzipped (with clean URLs)

I (and others) have come to realize that IE 6/7/8 will double-compress any already-compressed file -- and fail to double-de-compress it -- when gzip compression is enabled for, e.g., a ZIP file download.

To work around this limitation, I need to have some means by which to exclude certain URLs from compression.

On its face, this seems simple enough, but this particular application requires the use of so-called "clean URLs". As such, Apache's SetEnvIf module seems not to be able to access the "clean URL"; it always sees "/index.php" for the "Request_URI".

I have this mod_rewrite rule:


# Rewrite URLs of the form 'index.php?q=x':
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^(.*)$ index.php?q=$1 [QSA,L]

I was hoping to be able to force no-gzip with something like this:


SetEnvIfNoCase Request_URI (.*)(/|download/.*)$ no-gzip dont-vary
The idea is that anything that is in a "download" directory does not receive gzip compression (which is enabled site-wide).

The problem is that the SetEnvIf directive is never satisfied because, as stated above, Request_URI always equals the "true" URI, as it exists before the clean URL" rewriting is performed: "/index.php".

I considered using mod_rewrite to modify the CGI environment variables, but I don't like that solution because a) it very likely wouldn't work when PHP is running under Mod-PHP (as opposed to Fast-CGI), it wouldn't work if SuPHP is in use, and it may not even be possible to modify the "no-gzip" environment variable; the Apache docs state: "It is not possible to override or change the standard CGI variables using the environment manipulation directives."

Does anyone have any other ideas?

EDIT to add: This kind of thing doesn't work, for the reasons outlined above:


# Don't compress images (or other files that are already compressed)
SetEnvIfNoCase Request_URI \
\.(?:gif|jpe?g|png|zip|7z|rar|sit?x|mp3|flac|ogg|pdf)$ no-gzip dont-vary

falko 24th May 2012 14:33

You can try this:

BrowserMatch ^Mozilla/4 gzip-only-text/html
BrowserMatch ^Mozilla/4\.0[678] no-gzip
BrowserMatch \bMSIE !no-gzip !gzip-only-text/html


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