[SalesForce] How to get a debug log for the sites guest user/public profile? [updated for Spring 18]

Normally, you can put debug monitoring on a user by name to see the details on the error they're getting (especially for the non-descript Error Loading a Visualforce Page).

But when you are trying to debug a site page, how do you get a debug log for the guest user?

Update

As of Spring '18 Salesforce has dropped the requirement to include the debug_logs cookie to get debug logs for the force.com site requests.

In Winter ’17, we introduced a change that required guest users to set
browser cookies to activate debug logging. We had the best of
intentions: All your public site visitors share one guest user
license. Thus, when you enable logging for the guest user, the
visitors’ collective activities can fill your debug logs quickly,
hitting the log-size limit and causing log truncation. Sadly, people
didn’t like the Winter ’17 change. Setting cookies is complicated, and
the change made collecting debug logs for public users’ asynchronous
activity impossible. We’ve seen the error of our ways. Your public
users no longer need a debug_logs browser cookie to trigger logging.
This change applies to both Lightning Experience and Salesforce
Classic.

https://releasenotes.docs.salesforce.com/en-us/spring18/release-notes/rn_forcecom_debugging_guest_user.htm

Best Answer

Go to Monitoring>Debug log.

Where you would normally put the username, put the name of the Site.

For example: If your site is called MySite, enter MySite as the user name. Shockingly undocumented, but this gives you the debug you're looking for.

ATTENTION:

Winter17 is making a change to this--you'll now need to ALSO set a specific cookie in the guest user's browser to enable this.

https://releasenotes.docs.salesforce.com/en-us/winter17/release-notes/rn_forcecom_debugging_guest_user.htm#rn_forcecom_debugging_guest_user

Related Idea Exchange Idea for Integrations

The requirement for such a cookie is incompatible when building web-hook services for 3rd parties, there is an Idea Exchange post here to provide an IP whitelist as an alternative.

Update from Spring '18 release notes:

In Winter ’17, we introduced a change that required guest users to set browser cookies to activate debug logging. We had the best of intentions: All your public site visitors share one guest user license. Thus, when you enable logging for the guest user, the visitors’ collective activities can fill your debug logs quickly, hitting the log-size limit and causing log truncation. Sadly, people didn’t like the Winter ’17 change. Setting cookies is complicated, and the change made collecting debug logs for public users’ asynchronous activity impossible. We’ve seen the error of our ways. Your public users no longer need a debug_logs browser cookie to trigger logging. This change applies to both Lightning Experience and Salesforce Classic.