connect() times out on Python 3.13 with AWSIoTPythonSDK 1.6.1 and 1.6.2

[Anna-Ku]

Summary
MQTT connection to AWS IoT Core fails with connectTimeoutException after 20 seconds in SDK versions 1.6.1 and 1.6.2.
Version 1.6.0 works with the same endpoint, certificates, and private key.
TLS socket wrapping succeeds with check_hostname=True and verify_mode=2. _send_connect() returns 0, but the socket returns b'' before receiving MQTT CONNACK.

Relevant logs:
_ERROR Connect timed out
DEBUG hostname: 'MYHOST'
DEBUG verify_mode: 2
DEBUG check_hostname: True
DEBUG: _send_connect returned: 0
DEBUG: command received = b''
DEBUG: EMPTY COMMAND — connection closed
DEBUG: packet_read returned: 1

[Anna-Ku]

@xiazhvera @sfod could you please take a look at this issue? Your help would be greatly appreciated. Thanks!

[xiazhvera]

Hi @Anna-Ku, starting in 1.6.1 the SDK sets server_hostname when wrapping the TLS socket. This enables client-side hostname verification, and it also sends SNI (Server Name Indication) to AWS IoT Core.

Some suggestions for further investigation:

1. As a quick check, use 1.6.0 to connect to the same endpoint on port 443 with ALPN (x-amzn-mqtt-ca). That path already sent SNI in 1.6.0.
2. Check the AWS IoT Core logs for a failed attempt, if logging is enabled. The Connect failure entry includes a reason (for example, an authorization failure) that shows why the connection was closed.