ssl)
* idna (minimum 2.0, from cryptography)

However, pyopenssl depends on cryptography, which depends on idna, so while we
use all three directly here we end up having relatively few packages required.

You can install them with the following command:

    pip install pyopenssl cryptography idna

To activate certificate checking, call
:func:`~urllib3.contrib.pyopenssl.inject_into_urllib3` from your Python code
before you begin making HTTP requests. This can be done in a ``sitecustomize``
module, or at any other time before your application begins using ``urllib3``,
like this::

    try:
        import urllib3.contrib.pyopenssl
        urllib3.contrib.pyopenssl.inject_into_urllib3()
    except ImportError:
        pass

Now you can use :mod:`urllib3` as you normally would, and it will support SNI
when the required modules are installed.

Activating this module also has the positive side effect of disabling SSL/TLS
compression in Python 2 (see `CRIME attack`_).

If you want to configure the default list of supported cipher suites, you can
set the ``urllib3.contrib.pyopenssl.DEFAULT_SSL_CIPHER_LIST`` variable.

.. _sni: https://en.wikipedia.org/wiki/Server_Name_Indication
.. _crime attack: https://en.wikipedia.org/wiki/CRIME_(security_exploit)
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