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/HTTP

Alt-Svc

The Alt-Svc HTTP response header is used to advertise alternative services through which the same resource can be reached. An alternative service is defined by a protocol/host/port combination.

Syntax

Alt-Svc: clear
Alt-Svc: <protocol-id>=<alt-authority>; ma=<max-age>
Alt-Svc: <protocol-id>=<alt-authority>; ma=<max-age>; persist=1
clear
The special value ''clear" indicates that the origin requests all alternatives for that origin to be invalidated.
<protocol-id>
The ALPN protocol identifier. Examples include h2 for HTTP/2 and h3-25 for draft 25 of the HTTP/3 protocol.
<alt-authority>
The quoted string specifying the alternative authority which consists of an optional host override, a colon, and a mandatory port number.
ma=<max-age>Optional
The number of seconds for which the alternative service is considered fresh. If omitted, it defaults to 24 hours. Alternative service entries can be cached for up to <max-age> seconds, minus the age of the response (from the Age header). If the cached entry expires, the client can no longer use this alternative service for new connections.
persist=1Optional
Usually cached alternative service entries are cleared on network configuration changes. Use of the persist=1 parameter ensures that the entry is not deleted through such changes.

Multiple entries can be specified in a single Alt-Svc header using comma as separator. In that case, early entries are considered more preferable.

Example

Alt-Svc: h2=":443"; ma=2592000;
Alt-Svc: h2=":443"; ma=2592000; persist=1
Alt-Svc: h2="alt.example.com:443", h2=":443"
Alt-Svc: h3-25=":443"; ma=3600, h2=":443"; ma=3600

Specifications

Specification Status Comment
RFC 7838 IETF RFC Initial definition.

Browser compatibilityUpdate compatibility data on GitHub

Desktop
Chrome Edge Firefox Internet Explorer Opera Safari
Alt-Svc Yes ≤79 38
38
37 — 38
Only supports draft-04
35 — 37
Disabled
Only supports draft-03
Disabled From version 35 until version 37 (exclusive): this feature is behind the network.http.altsvc.enabled preference (needs to be set to true) and the altsvc.oe preference (needs to be set to true). To change preferences in Firefox, visit about:config.
? Yes ?
Mobile
Android webview Chrome for Android Firefox for Android Opera for Android Safari on iOS Samsung Internet
Alt-Svc Yes Yes 38
38
37 — 38
Only supports draft-04
35 — 37
Disabled
Only supports draft-03
Disabled From version 35 until version 37 (exclusive): this feature is behind the network.http.altsvc.enabled preference (needs to be set to true) and the altsvc.oe preference (needs to be set to true). To change preferences in Firefox, visit about:config.
Yes ? Yes

See also

© 2005–2020 Mozilla and individual contributors.
Licensed under the Creative Commons Attribution-ShareAlike License v2.5 or later.
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Alt-Svc