Problem Description

Firefox fails to access cloudflare-quic.com over HTTP3 while using mitmproxy in local mode. Other websites such as http3.is load up normally using HTTP3
refs #7025

Steps to reproduce the behavior:

1. Start local mode: mitmdump --mode local:firefox --set experimental_transparent_http3=true
2. Start firefox and access http3.is to ensure HTTP3 works
3. Accessing cloudflare-quic.com does not work over HTTP3

Error raised: TLS Error: [('SSL routines', '', '')]
Server QUIC handshake failed. unknown error (0x128)
Client QUIC handshake failed. unknown error (0x179)

System Information

Mitmproxy: 11.0.0.dev (+18, commit 6bb536e)
Python: 3.11.3
OpenSSL: OpenSSL 3.2.2 4 Jun 2024
Platform: Windows-10-10.0.22631-SP0