Boost Cache, External CDN? I have been thrown into a Drupal 6 environment, helping a client whose site has been running terribly for some time now. Among other issues, they had a ton of traffic and were serving it all locally, so we aim to implement a CDN.

Our CDN of choice is Incapsula, who operate much like Cloudflare if you're familiar with them. At their most basic, they can cache static content. Should be simple.

What I'm finding is that when Incapsula's caching is enabled, the cache never updates. If you go through the public address to the site, you will see old content. It's not just old images and CSS, entire pages will be missing if they are added after the cache is enabled. If you bypass the proxy, you see the correct content. Manually clearing the cache from within the Incapsula control panel does not resolve the problem, I can only do that by actually disabling the cache. Of course, while it is enabled, the site flies!

Complicating this, they have Boost installed and enabled. I am new to Drupal, newer to Boost, and I am attempting to understand how the caching works but have not made much progress. Are there specific settings within Boost that might affect this? It feels like a header is misconfigured. Any thoughts and advice are appreciated!
If the CDN caches whole pages you can most likely disable boost.
https://drupal.org/project/advagg - Needed for D6 CSS/JS
https://drupal.org/project/imageinfo_cache - Should help with imagecache generation