Some time ago, I posted a forum topic asking if any one knew about remote authentication via XML-RPC. Ultimately, I learned it myself and am now writing this page.
To enable the remote login feature in Drupal5, enable the Drupal and Ping modules (both are core Drupal modules). For Drupal 6, you'll need the site_network module instead of the Drupal module.
The Drupal module will allow your site to communicate with the directory server you specify on the configuration page, which will be covered shortly. The ping module will announce any changes or update to other sites.
After enabling the Drupal module, go to admin> settings > site registry.
You will see options for the following...
1) Register with a Drupal server: select 'enable'
2) Drupal XML-RPC server: enter the directory server name, e.g. "http://drupal.org/xmlrpc.php", with no quotes.
3) Send system information:
If you want your site to send the system information which its running on, you should select "enable", otherwise select "disable".
4) Send statistics:
If you select "enable" for this option, your site will send the directory server the summary site statistics about number of registered users and number of posts on your site.
5) Allow other Drupal sites to register:
This option allows other sites to authenticate their users on your site, just as you do with the server in the second option.
Save your configuration options and turn the page over(!)
on the admin > settings, find the link for "Distributed authentication".
on the following page, you will find following options.
1) Authentication service: select "enable".
2) Default authentication server: Name of the server which you want your users authenticated with. Do not write "http://"
3) Only allow authentication from default server:
If you want to allow others site to use your site as authenticated server, select "disable". Then your site will be authenticating users for others sites too.
Save your settings, run the cron and log in with the username and password of the server in the second option.