title: use ssh private key from host in vagrant guest
body: <p>I want to clone a bunch of private git repositories while provisioning a vagrant box. According to <a href="https://coderwall.com/p/q8z1rg">this article</a> this should be possible using <code>config.ssh.forward_agent = true</code>. However, when trying to connect to github via something like <code>ssh -T git@github.com -o StrictHostKeyChecking=no</code> it fails with the following error:</p>
  
  <blockquote>
    <p>Warning: Permanently added 'github.com,192.30.252.130' (RSA) to the list of known hosts.
    Permission denied (publickey).</p>
  </blockquote>
  
  <p>I cut my configuration down to the simplest possible configuration. You can find it here: <a href="https://gist.github.com/TomTasche/31f7c45fcffc2997d43a">https://gist.github.com/TomTasche/31f7c45fcffc2997d43a</a></p>
  
  <p>When I do "vagrant ssh" and try the same again, a similar error occurs:</p>
  
  <blockquote>
    <p>Cloning into 'private-repositories'...
    Warning: Permanently added the RSA host key for IP address '192.30.252.130' to the list of known hosts.
    Permission denied (publickey).
    fatal: The remote end hung up unexpectedly</p>
  </blockquote>
  
  <p><strong>Edit:</strong> the configuration linked above does work on a host running Ubuntu, but does neither work on a Mac host, nor on a Windows host. My goal is to have a configuration that works on all these three hosts.</p>