an unknown filter was not added: DEFLATE

I looked in Apache’s error_log file and noticed multiple lines like this:

an unknown filter was not added: DEFLATE

After thinking a bit I recalled that I added the following line in the .htaccess file in the root directory for on one of my domains, but never tested it properly:

AddOutputFilterByType DEFLATE text/html text/plain text/xml

The fix is easy: enable mod_deflate. On Ubuntu:

$ sudo a2enmod deflate
Module deflate installed; run /etc/init.d/apache2 force-reload to enable.
$ sudo /etc/init.d/apache2 force-reload
* Forcing reload of apache 2.0 web server...

Why did I add the above filter line in the first place? It was suggested by YSlow.

Update Another kind of fix is of course to remove AddOutputFilterByType.


  1. oh…my code did not appear in the last message…trying again…

    Thanks – i was in exactly same situation
    I read the YSlow book and forgot to add the mod_deflate.c after moving to a new server.

    Just another alternative fix…wrap that line within :

    (open bracket) IfModule mod_deflate.c (close bracket)

    like so:

    (open bracket) IfModule mod_deflate.c (close bracket)

    #Compress everything except images -note some images have the .php extension -start

    ..stuff that was to do with deflating…

    #Compress everything except images -end
    (open bracket)/IfModule(close bracket)

    This means you can stave the problem if the site migrates.

Leave a comment

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.