Can Joomla site use an external site for sign on?

I'm trying to setup a "Single sign on" environment, where one of the sites is a Joomla site. I want all lo开发者_如何学JAVAgins to happen at an external site (not the Joomla site).

The Joomla site does have some guest access. I'm having a problem when the guest on the Joomla site clicks a link that requires them to be logged in. Joomla automatically redirects to it's own login page when I want it to go to an external page.

Is there an existing plugin that can redirect to a custom, external login page? Or, is there a Joomla function that can be overridden to intercept the login redirect?

Basically, I want to override what happens in /includes/application.php - method "Authorize"

Yes it can be done. You can do it through cURL, details here -

Logging In To Joomla 1.5 Using External Form (not within joomla folder, but on same server)

In order to get the users to an external login page when Joomla tries to send the user to it's own login page, I would use a simple redirect in htaccess.

If you are not using SEF URLs then the redirect will need to look something like this -

RewriteCond %{QUERY_STRING} (^|&)option=com_user(&|$)
RewriteCond %{QUERY_STRING} (^|&)view=login(&|$)
RewriteRule ^$ http://mySSOsite.com? [R=301,L]

Joomla 1.5 comes with plugins for LDAP, OpenID and Gmail authentication. The plugins can be configured in the Joomla backend (Extensions > Plugin Manager). If you do not use any of these authentication methods you might check out the plugin code to create a new one fitting your needs.