title: Running mitmproxy on macOS Sierra: traffic not passing through
body: <p>I have installed mitmproxy with homebrew. </p>
  
  <pre><code>mitmproxy --version
  Mitmproxy version: 2.0.0 (release version)
  Python version: 3.6.0
  Platform: Darwin-16.3.0-x86_64-i386-64bit
  SSL version: OpenSSL 1.1.0e  16 Feb 2017
  Mac version: 10.12.2 ('', '', '') x86_64
  
  mitmproxy -T --host
  </code></pre>
  
  <p>And then point a browser at mitm.it and I see the message "If you can see this, traffic is not passing through mitmproxy."  Is there a known problem or fix? If not where should I investigate? </p>