Enabling mod_headers on apache2

I'm trying to enable the module "headers" into my apache2 installation on ubuntu 10.04 system, here's some info:

Server version: Apache/2.2.14 (Ubuntu)

when I run the command:

a2enmod headers

I get this output: ERROR: Module headers does not exist!

but under /usr/lib/apache2/modules/ I can see there is mod_headers.so

any suggestion? thanks in advance

EDIT: There is something wrong with my modules, when I access to the webmin's apache modules list it list me just some modules, not every module I can find on /usr/lib/apache2/modules/ example: I can't see mod_rewrite from the list but I have it in the /usr/lib/apache2/modules/

It seems like someone has deleted the include files for certain mods, instead of removing symlinks like the a2enmod/a2dismod tools do.

Reinstall the apache common files to get these files back in place:

aptitude reinstall apache2.2-common

I found a solution which worked for me

Add this line to /etc/apache2/httpd.conf:

LoadModule headers_module /usr/lib/apache2/modules/mod_headers.so

a2enmod headers

service apache2 restart
 * Restarting web server apache2                                                                                                                               [Fri Sep 11 17:04:33 2015] [warn] module headers_module is already loaded, skipping
 ... waiting [Fri Sep 11 17:04:35 2015] [warn] module headers_module is already loaded, skipping
                                                                                                                                                   [ OK ]

After the restart I could delete the LoadModule headers_module /usr/lib/apache2/modules/mod_headers.so from from /etc/apache2/httpd.conf and it's further working!