Comment 3 for bug 1829510

On the snapd side, what we do does not "know" about fastly at all: what the connectivity check does is
1. hit the store's "info" endpoint asking about "core",
2. HEAD the download url provided in (1)

The request done in (2) sets the cdn headers as usual (ie it looks at whether CDN is disabled by the environment, and at the device auth context to see if it's overridden by cloud info).

You can see this logic spelled out
https://github.com/snapcore/snapd/blob/master/store/store.go#L2404

if you turn on SNAPD_DEBUG and SNPAD_DEBUG_HTTP=7 you should be able to see the requests as they go.