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Check HTTP/3 support via the Alt-Svc header

harmlessImages and performancedebian, ubuntu
Adjust the values, the command updates live
user@server
curl -sI https://example.com/ | grep -i '^alt-svc'

Browsers learn from the Alt-Svc header that a server speaks HTTP/3 and switch over on the next connection. If it contains something like h3=":443", HTTP/3 is advertised. No output means the server does not offer HTTP/3.

Note: The curl shipped with Debian 12 and Ubuntu 24.04 cannot do --http3 itself, hence this workaround. UDP port 443 must be open in the firewall, otherwise connections stay on HTTP/2. The entry of the same name in the nginx category checks HTTP/2.

Also searched as

  • check if server supports http/3
  • check if quic is enabled
  • test http3 command line
  • show alt-svc header

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