curl features EXPERIMENTAL support for the Alt-Svc: HTTP header.

Enable Alt-Svc in build

./configure --enable-alt-svc


RFC 7838

Alt-Svc cache file format

This a text based file with one line per entry and each line consists of nine space separated fields.


h2 8443 h3-22 8443 "20190808 06:18:37" 0 0


  1. The ALPN id for the source origin
  2. The host name for the source origin
  3. The port number for the source origin
  4. The ALPN id for the destination host
  5. The host name for the destination host
  6. The host number for the destination host
  7. The expiration date and time of this entry within double quotes. The date format is “YYYYMMDD HH:MM:SS” and the time zone is GMT.
  8. Boolean (1 or 0) if “persist” was set for this entry
  9. Integer priority value (not currently used)


  • handle multiple response headers, when one of them says clear (should override them all)
  • using Age: value for caching age as per spec