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Trailer header

files/en-us/web/http/reference/headers/trailer/index.md

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The HTTP Trailer {{glossary("request header", "request")}} and {{glossary("response 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] The {{HTTPHeader("TE")}} request header needs to be set to trailers to allow trailer fields.

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

<table class="properties"> <tbody> <tr> <th scope="row">Header type</th> <td> {{Glossary("Request header")}}, {{Glossary("Response header")}}, {{Glossary("Content header")}} </td> </tr> <tr> <th scope="row">{{Glossary("Forbidden request header")}}</th> <td>Yes</td> </tr> </tbody> </table>

Syntax

http
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")}}, {{HTTPHeader("Content-Type")}}, {{HTTPHeader("Content-Range")}}, and Trailer
      • Authentication headers (e.g., {{HTTPHeader("Authorization")}} or {{HTTPHeader("Set-Cookie")}})
      • Message framing headers (e.g., {{HTTPHeader("Transfer-Encoding")}} and {{HTTPHeader("Content-Length")}})
      • Routing headers (e.g., {{HTTPHeader("Host")}})
      • Request modifiers (e.g., controls and conditionals, like {{HTTPHeader("Cache-Control")}}, {{HTTPHeader("Max-Forwards")}}, or {{HTTPHeader("TE")}})

Examples

Server-Timing as HTTP trailer

Some browsers support showing server timing data in developer tools when the {{HTTPHeader("Server-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
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