curl/docs/cmdline-opts/alt-svc.md

c	SPDX-License-Identifier	Long	Arg	Protocols	Help	Added	Category	Multi	See-also	Example
Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.	curl	alt-svc	<filename>	HTTPS	Enable alt-svc with this cache file	7.64.1	http	append	resolve connect-to	--alt-svc svc.txt $URL

--alt-svc

Enable the alt-svc parser. If the filename points to an existing alt-svc cache file, that gets used. After a completed transfer, the cache is saved to the filename again if it has been modified.

Specify a "" filename (zero length) to avoid loading/saving and make curl just handle the cache in memory.

You may want to restrict your umask to prevent other users on the same system to access the created file.

If this option is used several times, curl loads contents from all the files but the last one is used for saving.