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


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)


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
  • like I said, it doesn't return a Content-Encoding header at all – Jay Nov 27 '11 at 7:11

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

Use cURL as mentioned by Michael Steinfeld to verify.

  • 1
    apache 2.4 upgrade doc seems to indicate that it will automagically determine whether or not to gzip a given resource: "mod_deflate will now skip compression if it knows that the size overhead added by the compression is larger than the data to be compressed" -- httpd.apache.org/docs/2.4/upgrading.html – virtualeyes Jan 15 '13 at 0:27
  • 2
    In httpd-2.4 this AddOutputFilterByType directive is corrected, un-deprecated and moved to filter_module. – Tometzky May 6 '13 at 12:42
  • Header append Vary User-Agent env=!dont-vary i think you should not do that. googve vary header, quite long disccusions about that. – T.Todua Aug 22 '16 at 17:37

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


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


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

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