Alt-Svc
The Alt-Svc header tells browsers that the site offers an alternative service, such as HTTP/3 on port 443.
Website · HTTP/3 · QUIC
HTTP/3 and QUIC check
Check whether a website advertises HTTP/3 support with the Alt-Svcheader and whether a real QUIC connection to port 443 succeeds.
HTTP/3 runs on top of QUIC over UDP port 443. An Alt-Svcheader alone does not guarantee that the actual QUIC connection works.
Enter a domain name and start the check.
Guide · HTTP/3 and QUIC
What does this HTTP/3 and QUIC check do?
HTTP/3 is a newer version of the HTTP protocol. It runs on top of QUIC over UDP port 443 instead of using TCP like HTTP/1.1 and HTTP/2.
The test first checks that the site responds over HTTPS. It then looks for HTTP/3 support in the Alt-Svc header and finally attempts a real HTTP/3 connection.
QUIC
QUIC is a transport protocol that runs over UDP. HTTP/3 uses QUIC instead of TCP.
UDP 443
HTTP/3 requires UDP port 443 to be reachable. TCP port 443 alone is not enough.
Real connection test
NetTools.fi does not only check the header. It also attempts a real HTTP/3 connection.
Good result
A good result means the site returns an Alt-Svc: h3=":443"style header and the actual HTTP/3 connection succeeds.
Common causes of failure
- UDP port 443 is not open in the firewall.
- The server advertises HTTP/3, but QUIC does not actually respond.
- An upstream firewall or hosting provider security layer blocks UDP traffic.
- Nginx, Caddy, LiteSpeed or CDN configuration is only partially enabled.
Common issues
- The Alt-Svc header is missing.
- The HTTP/3 connection cannot be established.
- The server advertises HTTP/3 but the connection fails.
- A firewall or proxy blocks QUIC traffic.
Related tools
HTTP/3 is connected to performance, TLS configuration and browser-level security.