I just tried to use X11 Forwarding from a RedHat 6 box to my Mac failing miserably.

You have to make sure to have the xorg-x11-xauth.x86_64 package installed on the RedHat box in order for MacOS to start the tunnel for X11 forwarding – otherwise it won’t work …

See the logs:

{% highlight bash %}
debug1: Requesting X11 forwarding with authentication spoofing.
…
debug1: Remote: No xauth program; cannot forward with spoofing.
{% endhighlight %}

After that X11 forwarding works like a charm 🙂