tailscale debug ts2021 now starts with an advisory TLS check against
the control plane over port 443: it compares a TLS 1.3 handshake using
Go's default key exchange groups (which include ML-KEM hybrid
post-quantum groups and produce a large ClientHello) with one
restricted to classic groups (a small ClientHello). If the large
ClientHello fails while the small one succeeds, the network path does
not support large (post-quantum) ClientHellos, likely due to a
middlebox dropping ClientHellos that span multiple TCP segments.
The check honors --connect-ip (dialing that IP while keeping the host
for SNI), can be skipped with --skip-tls-check, and never aborts the
surrounding command.
Updates #21708
Signed-off-by: James Tucker <james@tailscale.com>
James Tucker
55642c2154