Trailer header

The HTTP Trailer {{glossary("request header", "request")}} and {{glossary("response header")}}header allows the sender to include additional fields at the end of chunked messages in order to supply metadata that might be dynamically generated while the message body is sent.

[!NOTE]NOTE: The {{HTTPHeader("TE")}}TE request header needs to be set to trailers to allow trailer fields.

[!WARNING]WARNING: Developers cannot access HTTP trailers via the Fetch API or XHR. Additionally, browsers ignore HTTP trailers, with the exception of {{HTTPHeader("Server-Timing")}}Timing. See Browser compatibility for more information.

Header type	{{Glossary("[Request header")}}header](https://developer.mozilla.org/en-US/docs/Glossary/Request%20header), {{Glossary("[Response header")}}header](https://developer.mozilla.org/en-US/docs/Glossary/Response%20header), {{Glossary("[Content header")}}header](https://developer.mozilla.org/en-US/docs/Glossary/Content%20header)
{{Glossary("[Forbidden request header")}}header](https://developer.mozilla.org/en-US/docs/Glossary/Forbidden%20request%20header)	Yes

Syntax

Trailer: header-names

Directives

• header-names
  • : HTTP header fields which will be present in the trailer part of chunked messages. The following header names are disallowed:
    • {{HTTPHeader("Content-Encoding")}}Encoding, {{HTTPHeader("Content-Type")}}Type, {{HTTPHeader("Content-Range")}}Range, and Trailer
    • Authentication headers (e.g., {{HTTPHeader("Authorization")}}Authorization or {{HTTPHeader("Set-Cookie")}}Cookie)
    • Message framing headers (e.g., {{HTTPHeader("Transfer-Encoding")}}Encoding and {{HTTPHeader("Content-Length")}}Length)
    • Routing headers (e.g., {{HTTPHeader("Host")}}Host)
    • Request modifiers (e.g., controls and conditionals, like {{HTTPHeader("Cache-Control")}}Control, {{HTTPHeader("Max-Forwards")}}Forwards, or {{HTTPHeader("TE")}}TE)

Examples

Server-Timing as HTTP trailer

Some browsers support showing server timing data in developer tools when the {{HTTPHeader("Server-Timing")}}Timing header is sent as a trailer. In the following response, the Trailer header is used to indicate that a Server-Timing header will follow the response body. A metric custom-metric with a duration of 123.4 milliseconds is sent:

HTTP/1.1 200 OK
Transfer-Encoding: chunked
Trailer: Server-Timing

--- response body ---
Server-Timing: custom-metric;dur=123.4

Specifications

{{Specifications}}

Browser compatibility

{{Compat}}

See also

• {{HTTPHeader("Server-Timing")}}Timing
• {{HTTPHeader("Transfer-Encoding")}}Encoding
• {{HTTPHeader("TE")}}TE
• Chunked transfer encoding