Apache includes modules that add functionality to your website. Apache comes with several standard modules, and you can purchase additional modules from software vendors or download them from the Internet. You can find information about available Apache modules at www.apache.org/docs/mod.
Before enabling or disabling modules, you should have a specific functionality goal and fully understand the implications.
Some web modules are mutually exclusive or are interdependent. Here are some examples:
auth_digest_module and digest_module must never be enabled simultaneously.
proxy_module must be enabled if proxy_connect_module, proxy_ftp_module, proxy_http_module, proxy_ajp_module, or proxy_balancer_module are enabled.
dav_module and dav_fs_module should be in the same state.
mod_dav_svn requires that mod_dav and mod_dav_fs are enabled.
encoding_module requires that headers_module, dav_module, and dav_fs_module are enabled.
cache_module is required for mem_cache_module and disk_cache_module.
mod_userdir is disabled by default.
mod_userdir_apple, a secure replacement for mod_userdir, doesn’t distinguish between nonexistent users and users who cannot access userdir. mod_userdir_ apple is also disabled by default.
When mod_userdir and mod_userdir_apple are disabled, a browser can’t access content from a user’s Sites folder. For example, if your server is named example.com and the user’s short name is refuser, the content of the Sites folder can no longer be accessed at http://example.com/~refuser.
mod_userdir and mod_userdir_apple must never be enabled simultaneously.
mod_bonjour is disabled by default, but requires at least one of the two mod_userdir modules for full functionality.