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Website and domain checks

HTTP/3 and QUIC test

Check for a real HTTP/3 connection and see whether the site advertises h3 through Alt-Svc.

  • QUIC connection
  • Alt-Svc check
  • No silent fallback

Free, no sign-up. Lookups and results are not saved by this site.

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HTTP/3 test

HTTP/3 runs over QUIC, which uses UDP instead of TCP. This test tries a real HTTP/3 connection and never counts an Alt-Svc header alone as proof.

Technical references: curl HTTP version options (opens in a new tab), HTTP/3 specification (RFC 9114) (opens in a new tab) and Alt-Svc header reference (opens in a new tab).

What you can do

  • Strict HTTP/3 negotiation where the checker supports it

  • h3 Alt-Svc advertisement check

  • A clear result when the checker cannot test HTTP/3

  • Network and UDP failures reported as inconclusive

  • No connections to other advertised hosts or ports

How to use it

  1. Enter the address

    Enter a public HTTPS address without a username, password or other private details.

  2. Run the test

    Start the test. A quick Cloudflare security check runs first, then this site checks the address and its redirects before connecting.

  3. Read the result

    Read the result, the evidence and the limits. If you change a server or CDN setting, run the test again.

How the HTTP/3 test works

HTTP/3 runs over QUIC, which uses UDP. This test tells an h3 advertisement apart from a connection that really used HTTP/3. Where the checker supports it, the HTTP/3 request is strict, with no fallback to an older protocol.

Know the limits

The checker's PHP and libcurl build must support HTTP/3 and QUIC; without it the result says the test is unavailable, even if h3 is advertised. UDP filtering, network problems or server policy can stop the test. No Alt-Svc header does not prove there is no HTTP/3, and DNS HTTPS records are not checked. Only the address's host on port 443 is tested.

Frequently asked questions

Does an Alt-Svc h3 header prove HTTP/3 works?

No. It only advertises an alternative service. Proof needs a connection that actually negotiated HTTP/3.

Why does the result say unavailable?

The server running this checker may not have HTTP/3 support in PHP or libcurl. That is a limit of the checker, not a fault in the website you entered.

Why can an HTTP/3 test be inconclusive?

HTTP/3 uses UDP. A firewall, a blocked UDP path, a timeout or a handshake problem can stop the test even when the website supports HTTP/3 for other visitors.

Can this silently fall back to HTTP/2?

No. The HTTP/3 test uses curl's strict no-fallback setting, so any other protocol is never reported as verified HTTP/3. The first redirect check is a separate HTTP/1.1 request.

Are these results for my origin server or my CDN?

The checker connects to the public URL. A CDN, reverse proxy or load balancer may answer instead, so the result does not show how your origin server itself is set up.

What is sent when I run a test?

Your URL and a fresh Cloudflare Turnstile response are sent to this site. After the security check, our server makes a few limited public requests. Page contents and cookies are not returned or saved.

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