Fastly: Edge cloud and CDN

What Fastly is

Fastly is an edge cloud platform built around a smaller number of large points of presence rather than very wide placement. Customers configure cache and routing behaviour in VCL or in Fastly's Compute runtime, which makes Fastly common in front of APIs and dynamic applications as well as static assets.

How DNSTrace.dev detects it

Detection correlates independent public signals. A single match is reported as strong evidence; agreement between a response header and the announcing network is reported as confirmed.

x-fastly-request-id — Per-request identifier.
fastly-debug-path — Present when debugging output is enabled.
x-served-by — Names the cache node that answered, in the form cache-<pop>. DNSTrace.dev matches this pattern rather than the header alone, because other software reuses the same header name.
x-cache and x-cache-hits — Cache outcome for the object.
CNAME targets under fastly.net — The standard Fastly delivery mapping.

What a detection proves

The response came from a Fastly cache node, and the named point of presence handled the request.

What it does not prove

The origin behind the cache. A Fastly hit may be served entirely from cache without the origin being contacted at all.

Common questions

Why is x-served-by matched by pattern rather than presence?

Several other proxies emit an x-served-by header. DNSTrace.dev only treats it as Fastly evidence when the value matches Fastly's cache-node naming, which avoids a false positive.

Does a cache HIT mean the origin is down?

No. A hit only means the object was already stored at the edge. It says nothing about the current state of the origin.