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A CAPTCHA is a challenge-response test most often placed within web forms to determine whether the user is human. The purpose of the CAPTCHA module is to block form submissions by spam-bots, which are automated scripts that post spam content everywhere they can. The CAPTCHA module includes several options which allow you to add a challenge to virtually every form on the website.
Varnish will cache your page with the challenge. Only the first user to view a given form will have been served a unique challenge by the web server, therefore only the first user will actually be able to complete the form.
This will only stop spam-bots. It will not stop humans with ulterior motives.
The CAPTCHA module can be installed and enabled with the normal instructions in the Installation Guide.
The CAPTCHA module adds two new permissions to the permissions page, which can be accessed by going to admin/user/permissions in Drupal 6 or admin/people/permissions in Drupal 7. The two new permissions are 'administer CAPTCHA settings' and 'skip CAPTCHA'. The 'administer CAPTCHA settings' permission gives the user access to the pages admin/user/captcha in Drupal 6 or admin/config/people/captcha in Drupal 7.The 'Skip CAPTCHA' permission tells Drupal not to show or require a CAPTCHA for the specified role.