Sec-Purpose
The Sec-Purpose
fetch metadata request header indicates the purpose for which the requested resource will be used, when that purpose is something other than immediate use by the user-agent.
The only purpose that is currently defined is prefetch
, which indicates that the resource is being requested in anticipation that it will be needed by a page that is likely to be navigated to in the near future, such as a page linked in search results or a link that a user has hovered over.
The server can use this knowledge to: adjust the caching expiry for the request, disallow the request, or perhaps to treat it differently when counting page visits.
The header is sent when a page is loaded that has a <link>
element with attribute rel="prefetch"
.
Note that if this header is set then a Sec-Fetch-Dest
header in the request must be set to empty
(any value in the <link>
attribute as
is ignored) and the Accept
header should match the value used for normal navigation requests.
Header type | Fetch Metadata Request Header |
---|---|
Forbidden header name | yes (prefix Sec- ) |
CORS-safelisted request header | no |
Syntax
http
Sec-Purpose: prefetch
Directives
The allowed tokens are:
prefetch
-
The purpose is to prefetch a resource that may be needed in a probable future navigation.
Examples
A prefetch request
Consider the case where a browser loads a file with a <link>
element that has the attribute rel="prefetch"
and an href
attribute containing the address of an image file.
The resulting fetch()
should result in an HTTP request where Sec-Purpose: prefetch
, Sec-Fetch-Dest: empty
, and an Accept
value that is the same as the browser uses for page navigation.
An example of such a header (on Firefox) is given below:
http
GET /images/some_image.png HTTP/1.1
Host: example.com
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:109.0) Gecko/20100101 Firefox/116.0
Accept: text/html,application/xhtml+xml,application/xml;q=0.9,image/avif,image/webp,*/*;q=0.8
Accept-Language: en-US,en;q=0.5
Accept-Encoding: gzip, deflate, br
Sec-Purpose: prefetch
Connection: keep-alive
Sec-Fetch-Dest: empty
Sec-Fetch-Mode: no-cors
Sec-Fetch-Site: same-origin
Pragma: no-cache
Cache-Control: no-cache
Note: At time of writing FireFox incorrectly sets the Accept
header as Accept: */*
for prefetches.
The example has been modified to show what the Accept
value should be.
This issue can be tracked in Firefox bug 1836334.
Specifications
Specification |
---|
Fetch Standard # sec-purpose-header |
Browser compatibility
BCD tables only load in the browser
See also
- Prefetch (Glossary)
<link>
element with attributerel="prefetch"