HTTP/3 Checker

HTTP/3 carries HTTP semantics over QUIC, a secure transport that runs over UDP instead of TCP. The checker attempts a real connection so you can distinguish a working HTTP/3 endpoint from a domain that only appears to be configured for the protocol. The result reports the negotiated HTTP version, response status, and whether QUIC was detected.

A successful handshake is the strongest confirmation that the tested endpoint can serve traffic over HTTP/3. If it fails while discovery signals pass, investigate the path between the client and edge, including UDP port 443, firewall rules, load balancers, CDN settings, certificates, and supported QUIC versions.

Clients need a way to discover and negotiate the protocol. An Alt-Svc response header can advertise an HTTP/3 endpoint such as h3 on port 443. DNS HTTPS resource records can publish service parameters, including ALPN protocol identifiers, before an HTTP response is available. During TLS negotiation, ALPN helps the client and server agree on the application protocol.

The checker displays these signals separately because one passing check does not prove that the others work. It also exposes advertised versions, raw header values, and DNS records when available, giving developers and system administrators evidence they can compare with server or CDN configuration.

This result usually means the domain tells clients that HTTP/3 is available but the connection cannot be completed. Confirm that the hostname in Alt-Svc points to the intended service, the certificate covers that hostname, UDP traffic reaches the correct edge, and the advertised h3 version is supported. Cached Alt-Svc values can also outlive a configuration change, so review the header lifetime when rolling back or migrating.

Test both the apex domain and important subdomains. HTTP/3 configuration is often attached to a specific CDN distribution or virtual host, so support on one hostname does not prove support everywhere.

A direct HTTP/3 request can succeed even when an Alt-Svc header or DNS HTTPS record is absent. However, first-time clients still need an effective discovery path before they can choose the protocol. Review which browser, CDN, and deployment model you support, then confirm that the intended advertisement is present on production responses.

Keep HTTP/2 and HTTP/1.1 enabled as fallbacks. Some enterprise networks, VPNs, or middleboxes block UDP, and a resilient deployment should fall back cleanly without breaking requests.

QUIC combines transport and cryptographic setup, supports connection migration, and avoids TCP-level head-of-line blocking between independent streams. These properties can help on high-latency or unstable networks, especially when packets are lost or a mobile device changes networks. Benefits depend on connection reuse, geography, CDN reach, and the visitor's network conditions.

HTTP/3 does not remove origin processing time or optimize slow application code. Use a TTFB test to investigate server response and a HAR File Analyzer to inspect the request waterfall, connection phases, caching, and resource weight.

Test before enabling HTTP/3, immediately after the change, and again after DNS or CDN caches have expired. Check representative hostnames and regions, monitor fallback traffic, and compare real-user performance rather than relying on a single synthetic request.

When a result changes unexpectedly, capture the raw Alt-Svc value, advertised ALPN identifiers, DNS HTTPS records, status code, and timestamp. Those details make it easier to determine whether the change came from DNS, an edge configuration, an origin, or the network path.