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Accepted
Jay
Asked: 2011-11-26 17:00:37 +0800 CST

How to tell if mod_deflate is actually working?

  • 772

I have put the following in my http.conf file:

# mod_deflate configuration
<IfModule mod_deflate.c>

# Restrict compression to these MIME types
AddOutputFilterByType DEFLATE text/plain
AddOutputFilterByType DEFLATE text/html
AddOutputFilterByType DEFLATE application/xhtml+xml
AddOutputFilterByType DEFLATE text/xml
AddOutputFilterByType DEFLATE application/xml
AddOutputFilterByType DEFLATE application/x-javascript
AddOutputFilterByType DEFLATE text/javascript
AddOutputFilterByType DEFLATE text/css

# Level of compression (Highest 9 - Lowest 1)
DeflateCompressionLevel 9

# Netscape 4.x has some problems.
BrowserMatch ^Mozilla/4 gzip-only-text/html

# Netscape 4.06-4.08 have some more problems
BrowserMatch ^Mozilla/4\.0[678] no-gzip

# MSIE masquerades as Netscape, but it is fine
BrowserMatch \bMSI[E] !no-gzip !gzip-only-text/html

<IfModule mod_headers.c>
# Make sure proxies don't deliver the wrong content
Header append Vary User-Agent env=!dont-vary
</IfModule>

</IfModule>

My content doesn't return with a Content-Encoding of type gzip, but I find myself getting a lot more 304s and the Etag is appended with a +gzip suffix. Is mod_deflate actually doing its job? (Sorry about the n00b-ishness)

apache-2.2 gzip mod-deflate

4 Answers

  1. Best Answer

    What do the headers tell you, if it is not returning "content-encoding: gzip" it's probably not working.. you can test as follows:

    curl -I -H 'Accept-Encoding: gzip,deflate' http://yoursite.com/somefile
    • 10

  2. The apache docs for AddOutputFilterByType indicate this directive is deprecated in Apache httpd 2.1 and later and it doesn't always work well if Apache can not determine the mime type.

    I would suggest enabling compressing by using something like the following as a starting point and then add all the browser tweaks and compression levels back in. Obviously, you may want to double check httpd.conf to make sure it's actually loading mod_deflate.so as well:

    <Location />
    SetOutputFilter DEFLATE
    # Don't compress images
    SetEnvIfNoCase Request_URI \.(?:gif|png|jpg|jpeg)$ no-gzip dont-vary
    Header append Vary User-Agent env=!dont-vary
</Location>

    Use cURL as mentioned by Michael Steinfeld to verify.

    • 2

  3. Can't add acomment as I don't have enough reputation therefore I am writing this as an answer.

    The original answer by Michael Steinfeld was outputting the following error (on Windows Command Prompt)

    curl: (6) Could not resolve host: gzip,deflate'

    I solved it by removing the space after the colon:

    curl -I -H 'Accept-Encoding:gzip,deflate' http://yoursite.com/somefile

    • 2

  4. With LogLevel set to debug, mod_deflate will log its debug messages to ErrorLog. Every resource that is being compressed gets logged out there. You can see if mod_deflate is working in real-time with

    tail -f /path/to/error-log | grep deflate

    e.g.

    [Wed Apr 22 05:37:49.742615 2015] [deflate:debug] [pid 22627] mod_deflate.c(849): [client 127.0.0.1:55667] AH01384: Zlib: Compressed 10700 to 2637 : URL /web/js/common.js, referer: http://localhost/index.php

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