Free HTTP/3 (QUIC) Checker

A site supports HTTP/3 when it advertises h3 in its Alt-Svc response header, publishes an h3 ALPN in an HTTPS DNS record, or answers a QUIC packet on UDP port 443. This checker tests all three and reports a pass if any confirms it. You can also verify manually with curl --http3 https://example.com or by opening your browser developer tools, the Network tab, and looking for the Protocol column showing h3.

Browsers cannot begin a request over HTTP/3 without first learning the server speaks QUIC. The first connection is made over HTTP/2 or HTTP/1.1, the server advertises HTTP/3 in the Alt-Svc header, and the browser upgrades subsequent connections. So the very first request of a fresh visit always uses an older protocol. Publishing an HTTPS DNS record with an h3 ALPN lets capable clients choose HTTP/3 before that first connection.

HTTP/2 multiplexes streams over a single TCP connection, so one lost packet stalls every stream until TCP recovers — transport-level head-of-line blocking. HTTP/3 runs over QUIC on UDP, tracks each stream independently so a loss only affects that stream, folds the TLS handshake into connection setup for lower latency, and supports connection migration so a device switching networks keeps the same connection. HTTP/3 is most beneficial on mobile, high-latency, and lossy networks.

QUIC runs exclusively over UDP port 443. If the Alt-Svc advertisement is present but a direct QUIC probe gets no response, the most likely cause is a firewall, load balancer, or network path that allows TCP 443 but blocks inbound UDP 443. Open UDP 443 end to end. A probe marked inconclusive is different: it means our scanner could not send UDP from its own network, which is a scanner-side limitation, not a problem with your server.

Usually yes, especially for real-world users on mobile or unreliable networks. HTTP/3 reduces connection setup latency, removes transport head-of-line blocking, and survives network changes without reconnecting. The gains are largest on high-latency or lossy links and smaller on a fast, stable wired connection. It is a safe upgrade because clients that do not support HTTP/3 simply continue over HTTP/2, so there is no downside to enabling it.

The HTTPS record (DNS type 65, also called SVCB) lets you advertise protocol support such as alpn="h3,h2" directly in DNS. It is optional but valuable: it lets capable clients discover HTTP/3 before the first connection, removing the initial HTTP/2 round trip that the Alt-Svc mechanism otherwise requires. This checker reports it as a separate detection method, and publishing one alongside your Alt-Svc header gives the fastest possible HTTP/3 negotiation.