What is a Trailer header?
Lists which header fields will appear in the trailer of a message sent with chunked transfer encoding.
Trailer lets a website announce, up front, which headers will only be known once the body has finished streaming - useful for things like a checksum that can't be computed until all the data has passed through. It's a niche header in practice, mostly seen alongside chunked transfer encoding on large or generated responses, and rarely something a general-purpose uptime monitor needs to inspect directly.
Common use case
A server streaming a large generated file announces a trailing checksum header via Trailer, so the client can verify integrity only after the whole body has arrived.
Example
Trailer: Content-MD5History
Part of the original HTTP/1.1 specification, RFC 2068 (1997).
Did you know?
Trailers got a second life in gRPC, which uses HTTP/2 trailers to carry a call's final status code after the response body has already started streaming.