Add section on re-enabling Indirect GLX.

Closes #13.

content/guides/running_applications/how_to_setup_x11_forwarding.md

@@ -48,7 +48,45 @@ weight = "35"
 4.  Close the xeyes application by "Ctrl + c" from the terminal or click
     the close button on the up-right corner of the graphical window.
 
- 
+#### Special note about Indirect GLX
+
+If you are using OS X or Linux, you may need to re-enable indirect GLX (IGLX) in order for
+the GUI application to run properly. IGLX was disabled for security reasons starting with
+X.org version 1.17 (Linux) and XQuartz version 2.7.9 (OS X).  More information about this
+change is available at [this link](https://www.phoronix.com/scan.php?page=news_item&px=Xorg-IGLX-Potential-Bye-Bye).
+
+To re-enable IGLX, use the follwing instructions:
+
+##### OS X
+
+Open a terminal on your Mac, and run the following command:
+
+{{% panel theme="info" header="Enable ILGX on OS X" %}}
+{{< highlight bash >}}
+$ defaults write org.macosforge.xquartz.X11 enable_iglx -bool true
+{{< /highlight >}}
+{{% /panel %}}
+
+##### Linux
+
+You will need to create a small Xorg configuration file.  This requires `root` access, so
+run commands with care.
+
+{{% notice warning %}}
+Be extremely cautious running commands as `root`.  You can permanently make your system unusable.
+{{% /notice %}}
+
+Create a file at `/etc/X11/xorg.conf.d/iglx.conf` with the following contents:
+
+{{% panel theme="info" header="/etc/X11/xorg.conf.d/iglx.conf" %}}
+{{< highlight bash >}}
+Section "ServerFlags"
+	Option "IndirectGLX" "on"
+EndSection
+{{< /highlight >}}
+{{% /panel %}}
+
+Restart X11 by logging out, or by rebooting your machine.
 
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