smtplib.SMTP starttls fails with tlsv1 alert decode error

Question

I encountered the following perculiar behaviour today.

The following code works on Python 3.3:

smtp = smtplib.SMTP()
smtp.connect(host="smtp.gmail.com", port=587)
smtp.ehlo()
smtp.starttls()

In Python 3.4 the above code doesn't work, instead the following error is encountered:

   File "smtp_test.py", line 10, in <module>
    smtp.starttls()
   File "/usr/lib/python3.4/smtplib.py", line 676, in starttls
    server_hostname=server_hostname)
   File "/usr/lib/python3.4/ssl.py", line 344, in wrap_socket
    _context=self)
   File "/usr/lib/python3.4/ssl.py", line 540, in __init__
    self.do_handshake()
   File "/usr/lib/python3.4/ssl.py", line 767, in do_handshake
    self._sslobj.do_handshake()
   ssl.SSLError: [SSL: TLSV1_ALERT_DECODE_ERROR] tlsv1 alert decode error (_ssl.c:598)

If the above code is modified to specify the host and port in the constructor and not use the connect method, as in the code below, then it works.

smtp = smtplib.SMTP(host="smtp.gmail.com", port=587)
smtp.ehlo()
smtp.starttls()

The above behaviour occurs with OpenSSL version 1.0.1f and OpenSSL 1.0.1g

Could someone explain this behaviour to me ?

Answer 1

According to a tcpdump the code in 3.4 sends in SNI extension with an empty target name. SNI (Server Name Indication) is used when having different certificates behind the same IP address. I consider this a bug: if it does not have a name it should not send the SNI extension instead of sending an extension with a zero-length name in it.