I am a little concerned about auto-generating Varnish conf, since the VCL used by Zero is shared with mobile web more broadly. Perhaps the VCL should be refactored to pull the Zero-specific bits out and capture them in their own separate file for inclusion in the VCL. There's still potential for auto-generation to do something that will cause some sort of conflict, but at least the auto-generated stuff will be contained.
Topic on Talk:Requests for comment/Zero architecture
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Updated section to clarify - we are not generating VCL, only a simple text file with IP blocks => Carrier ID mapping. VCL will pull it in and set X-CS header to the found ID, same way as geoIP calculates the country.