Prerequisites

HTTP/3 operates over QUIC (UDP-based) instead of TCP, making the setup more demanding than HTTP/2.

HTPS Certificate

HTTP/3 requires a valid security certificate. Unlike HTTP/2, insecure certificates will not work at all.

Install mkcert for local development:

winget install mkcert

Initialize the local certificate authority:

mkcert -install

Generate a certificate for localhost:

mkcert localhost 127.0.0.1 ::1

Configure Caddyfile:

tls ./localhost+2.pem ./localhost+2-key.pem

Alt-Svc Header

The server must announce the UDP port via the Alt-Svc response header:

header Alt-Svc `h3=":8443"; ma=2592000, h3-29=":8443"; ma=2592000`

UDP Port Accesibility

Caddy may open only IPv6 UDP ports by default. Verify open UDP endpoints using PowerShell:

Get-NetUDPEndpoint | Select-Object LocalAddress, LocalPort, OwningProcess

Filter by specific port:

PS D:\www> Get-NetUDPEndpoint -LocalPort 8443 | Select-Object LocalAddress, LocalPort, OwningProcess

LocalAddress LocalPort OwningProcess
------------ --------- -------------
::                8443         81976

Browser Configuration

Chrome requires explicit flags to enable HTTP/3. Launch Chrome with QUIC forcing enabled:

& "C:\Users\32956\AppData\Local\Google\Chrome\Application\chrome.exe" `
    --user-data-dir="$env:TEMP\chrome_h3_test" `
    --origin-to-force-quic-on=127.0.0.1:8443 `
    --force-quic `
    --ignore-certificate-errors `
    "https://127.0.0.1:8443/image-hosts"

If HTTP/3 fails, the browser will not fallback to HTTP/2.

Testing HTTP/3 Connectivity

Use the aioquic library to verify QUIC handshake:

# /// script
# dependencies = ["aioquic"]
# ///
import asyncio
from aioquic.quic.configuration import QuicConfiguration
from aioquic.asyncio import connect

async def test_quic_connection():
    config = QuicConfiguration(is_client=True, alpn_protocols=["h3"])
    config.verify_mode = 0
    
    try:
        async with connect("127.0.0.1", 8443, configuration=config) as client:
            print("QUIC handshake successful - HTTP/3 is working")
    except Exception as err:
        print(f"Connection failed: {err}")

if __name__ == "__main__":
    asyncio.run(test_quic_connection())

Protocol Compatibility

Protocol	Transport	Requirements
HTTP/1.1	TCP	Minimal
HTTP/2	TCP	HTTPS recommended
HTTP/3	UDP	Valid certificate + Alt-Svc header

HTTP/3 has the strictest requirements among all HTTP verisons.

Network Considerations

When using reverse proxy services or tunnel forwarding, separate tunnels are needed for TCP and UDP traffic. This adds complexity to the configuration compared to traditional HTTP setups.