# Chrome: This site can’t be reached — ERR_QUIC_PROTOCOL_ERROR (HTTP/3 over UDP blocked)

**Code:** `ERR_QUIC_PROTOCOL_ERROR` · **Category:** Browser · **Seen on:** Chrome, Edge

Chrome tried HTTP/3 (QUIC over UDP) and the connection failed — usually a firewall, VPN or proxy that blocks or mangles UDP port 443.

## 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- Outdated Chrome with a QUIC bug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

1. Test with QUIC disabled: `chrome://flags/#enable-quic` → Disabled
2. Allow outbound UDP 443 on the firewall
3. Update Chrome

## Detailed fix by platform

### Chrome

1. Go to `chrome://flags`, search "QUIC", set "Experimental QUIC protocol" to Disabled, relaunch

### Linux

1. Test UDP reachability: `curl --http3 -I https://example.com` (needs curl built with HTTP/3) or compare with `--http2`

### Nginx

1. To remove the Alt-Svc advertisement while debugging, drop `add_header Alt-Svc 'h3=":443"; ma=86400';`

## How to diagnose

1. **Switch** — Does disabling QUIC fix it?
2. **Firewall** — Is UDP 443 allowed?
3. **Server** — Is Alt-Svc advertised?

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Source: [WhatIsMyError](https://whatismyerror.com/errors/browser/err-quic-protocol-error)