Drupal 6.17 版本起有个不兼容的修改

Incompatible changes
Drupal 6.17 introduces the following incompatible changes.
Session handling

Drupal 6.17 changes the way session cookies are handled. Most people don't need to have this setting set, but if you have an explicit $cookie_domain set in settings.php, verify that it is set to a sensible value:

• 'example.com' if you want sessions to apply to the example.com domain, and none of its sub-domains (especially not www.example.com),
• 'www.example.com' if you want sessions to apply to the www.example.com domain, and none of its sub-domains nor parent domains (especially not example.com),
• '.example.com' if you want sessions to apply to the example.com domain and all its subdomains (www.example.com, mydomain.example.com, etc.).