How do I check if my website supports HTTP/3 (QUIC)?

HTTP/3 is the third version of HTTP, and the first that does not run over TCP. It runs over QUIC (RFC 9000), a UDP-based transport that folds the TLS 1.3 handshake into connection setup, so connections open in fewer round trips, a single lost packet no longer stalls every stream (no transport-level head-of-line blocking), and a connection survives a device switching networks. The catch when testing for it: a browser never uses HTTP/3 on the very first request, so "I loaded the page and DevTools said h2" does not mean HTTP/3 is off.

The signal that matters: Alt-Svc

A browser can't start a request over HTTP/3 cold, because it doesn't yet know the server speaks QUIC. The first connection is made over HTTP/2 (or HTTP/1.1), and the server advertises HTTP/3 in the Alt-Svc response header:

Alt-Svc: h3=":443"; ma=86400

The browser caches that for the ma= (max-age) seconds and upgrades subsequent connections to HTTP/3. So the presence of an h3 entry in Alt-Svc is the primary "this site supports HTTP/3" signal — even though the request you read it on was HTTP/2. A newer mechanism, the HTTPS DNS record (type 65 / SVCB), can advertise alpn="h3,h2" in DNS so capable clients pick HTTP/3 before the first connection, skipping that initial round trip.

Three ways to test

1. The IntoDNS.ai HTTP/3 checker. Enter your domain at the HTTP/3 checker. It runs three independent detections — it reads the Alt-Svc header for h3, queries the HTTPS DNS record for an h3 ALPN, and sends a real QUIC Version Negotiation packet to UDP/443 to confirm a QUIC server actually answers. A pass means at least one method confirmed it, and the detail panel shows which. This is the same HTTP/3 detection the full IntoDNS.ai security scan uses, so the result won't contradict your full report.

2. curl on the command line.

curl -sI --http3 https://example.com

If curl was built with HTTP/3 support and the server answers over QUIC, the status line reads HTTP/3 200. You can also run curl -sI https://example.com over HTTP/2 and look for an alt-svc: h3=... line in the response headers.

3. Browser DevTools. Open DevTools, go to the Network tab, reload the page twice (the second load lets the cached Alt-Svc upgrade take effect), right-click the column header, enable Protocol, and look for h3. The two-load step is the part people miss.

Reading a "not supported" result

If no h3 is advertised and a QUIC probe gets no response, HTTP/3 is genuinely off — enable it (below). But two near-misses are worth knowing. If Alt-Svc advertises h3 yet the QUIC probe is silent, suspect a firewall blocking inbound UDP 443: QUIC is UDP-only, and many networks allow TCP 443 while filtering UDP 443. And if the site is simply unreachable over HTTPS, support can't be confirmed at all — that's reported as unconfirmed, not a failure.

How to enable HTTP/3

• nginx 1.25+ (QUIC module): add listen 443 quic reuseport; beside listen 443 ssl;, set http3 on;, and add_header Alt-Svc 'h3=":443"; ma=86400';.
• Cloudflare: one toggle — Speed → Optimization → Protocol Optimization → HTTP/3 (with QUIC).
• Caddy: HTTP/3 is on automatically, no configuration.
• LiteSpeed / OpenLiteSpeed: enable QUIC in the server tuning panel.

Two things are non-negotiable regardless of server: UDP port 443 must be open through every firewall and load balancer (QUIC doesn't use TCP), and the Alt-Svc header must actually reach the browser — a proxy that strips it silently keeps clients on HTTP/2. For the fastest negotiation, also publish an HTTPS DNS record: example.com. IN HTTPS 1 . alpn="h3,h2". After any change, re-test with the HTTP/3 checker to confirm Alt-Svc, the HTTPS record, and the live QUIC probe all line up.