Lists entities that have forwarded the request, to detect loops.

Fastly writes this header into requests. It is defined by an external standard.

The CDN-Loop header is appended by Fastly (and other CDN providers) when a request transits the network. It is similar to the Fastly-FF header, in that it can be used to detect infinite loops in edge logic.

Unlike Fastly-FF, the CDN-Loop header is recognized and modified by other cloud platform providers.


If a user visits a domain that resolves to Fastly, and we forward that request to your origin server, the request will have an added CDN-Loop header with up to four "Fastly" tokens listed depending on whether your service is configured to use Clustering and Shielding:

CDN-Loop: Fastly, Fastly

In this example, a user has visited a domain that resolves to OtherProvider, and that provider has forwarded the request to Fastly:

CDN-Loop: OtherProvider, Fastly