The ssl.match_hostname() function from Python 3.5

The Secure Sockets Layer is only actually secure if you check the hostname in the certificate returned by the server to which you are connecting, and verify that it matches to hostname that you are trying to reach.

But the matching logic, defined in RFC2818, can be a bit tricky to implement on your own. So the ssl package in the Standard Library of Python 3.2 and greater now includes a match_hostname() function for performing this check instead of requiring every application to implement the check separately.

This backport brings match_hostname() to users of earlier versions of Python. Simply make this distribution a dependency of your package, and then use it like this:

from backports.ssl_match_hostname import match_hostname, CertificateError
sslsock = ssl.wrap_socket(sock, ssl_version=ssl.PROTOCOL_SSLv23,
                          cert_reqs=ssl.CERT_REQUIRED, ca_certs=...)
    match_hostname(sslsock.getpeercert(), hostname)
except CertificateError, ce:

Brandon Craig Rhodes is merely the packager of this distribution; the actual code inside comes from Python 3.5 with small changes for portability.


  • If you want to verify hosts match with certificates via ServerAltname IPAddress fields, you need to install the `ipaddress module`_. backports.ssl_match_hostname will continue to work without ipaddress but will only be able to handle ServerAltName DNSName fields, not IPAddress. System packagers (Linux distributions, et al) are encouraged to add this as a hard dependency in their packages.
  • If you need to use this on Python versions earlier than 2.6 you will need to install the `ssl module`_. From Python 2.6 upwards ssl is included in the Python Standard Library so you do not need to install it separately.


  • This function was introduced in python-3.2
  • It was updated for python-3.4a1 for a CVE (backports-ssl_match_hostname-
  • It was updated from RFC2818 to RFC 6125 compliance in order to fix another security flaw for python-3.3.3 and python-3.4a5 (backports-ssl_match_hostname-
  • It was updated in python-3.5 to handle IPAddresses in ServerAltName fields (something that backports.ssl_match_hostname will do if you also install the ipaddress library from pypi).