TLSv1-only SMTP servers not supported under Django 1.9.7 / Python 3.4.4 (win32) / OpenSSL 1.0.2d

[Rudolf Cardinal]

Connecting to a Microsoft (E)SMTP mail server with the Django default e-mail backend (django.core.mail.backends.smtp.EmailBackend) and TLS can fail with the following error:

ssl.SSLEOFError: EOF occurred in violation of protocol (_ssl.c:600)

This appears to occur with some (more recent) versions of Python's smtplib, ssl (and underlying OpenSSL) with servers that only support TLSv1. There is no option in Django to force the TLS protocol version that I can see.

The simplest illustration of failure is with:

import smtplib
s = smtplib.SMTP(host, port) # port typically 587
s.ehlo()
s.starttls() # fails with ssl.SSLEOFError as above

which succeeds (with such servers) with a small modification to force TLSv1:

import smtplib
import ssl
s = smtplib.SMTP(host, port)
c = ssl.SSLContext(ssl.PROTOCOL_TLSv1)
s.ehlo()
s.starttls(context=c) # works

There is a suggestion of a dependence on exact OpenSSL version in the docs at ​https://docs.python.org/3.4/library/ssl.html ("Which connections succeed will vary depending on the version of OpenSSL...").

Therefore, a proposed framework alternative backend is one that enforces TLSv1 when required, e.g. as attached. This can be selected with Django's EMAIL_BACKEND setting, so can be added by users, but I'd have thought that continuing to support such servers is important for the Django core project as well.

Test software: Django 1.9.7; Python 3.4.4 (win32) using OpenSSL 1.0.2d (from ssl.OPENSSL_VERSION). Test server: Microsoft SMTP Server id 8.3.83.0 (from resulting e-mail headers).

[Rudolf Cardinal]

New e-mail backend forcing TLSv1

[Tim Graham]

This is the first I've heard of the problem, so I can't say myself that supporting old protocols is critically important for Django core. Feel free to raise it on the DevelopersMailingList. If there's consensus there to make a change in Django, we can reopen the ticket.