Indicates that a request came from Fastly.
Fastly writes this header into requests. It is proprietary to Fastly.
This header is attached to all requests that come from a Fastly server. In practice, this means that in any service that has clustering or shielding enabled,
Fastly-Client will be true on all but the first server that received the request. It will also be included on the request forwarded to the backend.