Chrome: This site can’t be reached — ERR_QUIC_PROTOCOL_ERROR (HTTP/3 over UDP blocked)
Chrome tried HTTP/3 (QUIC over UDP) and the connection failed — usually a firewall, VPN or proxy that blocks or mangles UDP port 443.
QUIC protocol error Meaning
Chrome should fall back to TCP automatically, but middleboxes sometimes break the switch. Disabling QUIC in Chrome flags is a quick test; the permanent fix is to allow UDP 443 or to turn off HTTP/3 on the server/CDN if you cannot.
Common causes
- Firewall or router dropping UDP 443
- VPN/proxy not carrying UDP traffic
- Security software inspecting QUIC
- Server/CDN advertising HTTP/3 (Alt-Svc) but UDP not reachable
- Outdated Chrome with a QUIC bug
⚡ Quick fix
- Test with QUIC disabled:
chrome://flags/#enable-quic→ Disabled - Allow outbound UDP 443 on the firewall
- Update Chrome
Detailed fix by platform
Chrome
- Go to
chrome://flags, search "QUIC", set "Experimental QUIC protocol" to Disabled, relaunch
Linux
- Test UDP reachability:
curl --http3 -I https://example.com(needs curl built with HTTP/3) or compare with--http2
Nginx
- To remove the Alt-Svc advertisement while debugging, drop
add_header Alt-Svc 'h3=":443"; ma=86400';
How to diagnose
- Switch — Does disabling QUIC fix it?
- Firewall — Is UDP 443 allowed?
- Server — Is Alt-Svc advertised?
🔧 Still not fixed?
Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:
Similar errors
Most viewed in Browser
Other ways to find it
Where this error sits
Origin: On your device. Errors that are often confused with or caused by this one (click to open):
🧠 Still stuck? Analyze your error
Paste the full message, response headers or stack trace — we'll detect the platform and point to the most likely cause.
💬 What worked for others
No tips yet — did something else fix this for you?
Share what worked for you
Tips are checked by an editor before they appear. Links are removed.
Report a correction or suggest an improvement
Last updated 15 Oct 2026