Fastly's cache servers run an evolution of Varnish which diverged from the community project at version 2.1. Varnish Configuration Language (VCL) remains the primary way to configure our cache behaviors, and our VCL syntax derives from Varnish 2.1.5, but has been significantly upgraded and extended for Fastly-specific features. While it's possible that VCL written for the open source Varnish will