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The Alt-Svc header is used to list alternate ways to reach this website.

Syntax

Alt-Svc: clear
Alt-Svc: <service-list>; ma=<max-age>
Alt-Svc: <service-list>; ma=<max-age>; persist=1
<service-list>
Comma separated list of service definitions in the format of <service-name>=​"<host-name>:<port-number>", where <service-name> is a valid ALPN identifier.
<max-age>Optional
The time in seconds for which the alternative service is considered valid for establishing new connections. Defaults to 24 hours.
persistOptional
Optional parameter specifying that the alternative service can be remembered between sessions and network changes.

Specifications

Specification Status Comment
RFC 7838 IETF RFC Initial definition.

Browser compatibility

Update compatibility data on GitHub
DesktopMobile
ChromeEdgeFirefoxInternet ExplorerOperaSafariAndroid webviewChrome for AndroidFirefox for AndroidOpera for AndroidSafari on iOSSamsung Internet
Alt-SvcChrome Full support YesEdge ? Firefox Full support 38
Full support 38
No support 37 — 38
Notes
Notes Only supports draft-04
No support 35 — 37
Notes Disabled
Notes 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.
IE ? Opera Full support YesSafari ? WebView Android Full support YesChrome Android Full support YesFirefox Android Full support 38
Full support 38
No support 37 — 38
Notes
Notes Only supports draft-04
No support 35 — 37
Notes Disabled
Notes 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.
Opera Android Full support YesSafari iOS ? Samsung Internet Android Full support Yes

Legend

Full support  
Full support
Compatibility unknown  
Compatibility unknown
See implementation notes.
See implementation notes.
User must explicitly enable this feature.
User must explicitly enable this feature.

See also

Document Tags and Contributors

Contributors to this page: ExE-Boss, mdnwebdocs-bot
Last updated by: ExE-Boss,