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What authentication mechanisms can I use?

Any authentication mechanism which sets the REMOTE_USER (or REDIRECT_REMOTE_USER, in the case of ScriptAlias'd PHP-as-CGI) environment variable can be used in conjunction with this plugin. Examples include Apache's mod_auth and mod_auth_ldap.

How should I set up external authentication?

This depends on your hosting environment and your means of authentication.

Many Apache installations allow configuration of authentication via .htaccess files, while some do not. Try adding the following to your blog's top-level .htaccess file:

<Files wp-login.php>
AuthName "WordPress"
AuthType Basic
AuthUserFile /path/to/passwords
Require user dwc
</Files>

(You may also want to protect your xmlrpc.php file, which uses separate authentication code.)

Then, create another .htaccess file in your wp-admin directory with the following contents:

AuthName "WordPress"
AuthType Basic
AuthUserFile /path/to/passwords
Require user dwc

In both files, be sure to set /path/to/passwords to the location of your password file. For more information on creating this file, see below.

Where can I find more information on configuring Apache authentication?

See Apache's HOWTO: Authentication, Authorization, and Access Control.

How does this plugin authenticate users?

This plugin doesn't actually authenticate users. It simply feeds WordPress the name of a user who has successfully authenticated through Apache.

To determine the username, this plugin uses the REMOTE_USER or the REDIRECT_REMOTE_USER environment variable, which is set by many Apache authentication modules. If someone can find a way to spoof this value, this plugin is not guaranteed to be secure.

This plugin generates a random password each time you create a user or edit an existing user's profile. However, since this plugin requires an external authentication mechanism, this password is not requested by WordPress. Generating a random password helps protect accounts, preventing one authorized user from pretending to be another.

If I disable this plugin, how will I login?

Because this plugin generates a random password when you create a new user or edit an existing user's profile, you will most likely have to reset each user's password if you disable this plugin. WordPress provides a link for requesting a new password on the login screen.

Also, you should leave the admin user as a fallback, i.e. create a new account to use with this plugin. As long as you don't edit the admin profile, WordPress will store the password set when you installed WordPress.

In the worst case scenario, you may have to use phpMyAdmin or the MySQL command line to reset a user's password.