troubleshooting: Add section for restoring config.xml from backup cache via serial console (#698)

Author: Monviech

source/troubleshooting.rst

@@ -72,6 +72,7 @@ be found in the list below.
    :glob:
 
    troubleshooting/password_reset
+   troubleshooting/config_reset
    troubleshooting/webgui
    troubleshooting/boot
    troubleshooting/hardening

source/troubleshooting/config_reset.rst

@@ -0,0 +1,93 @@
+====================================
+Restore Configuration via Console
+====================================
+
+Sometimes you may accidentally introduce a breaking change that interrupts network access, prevents user login or makes SSH and WebGUI inaccessible.
+Rolling back without a reinstall is possible if there is serial and console access.
+
+For serial access to the console, reference this guide: :doc:`Serial Console connectivity </hardware/serial_connectivity>`
+
+Console access is also possible directly when running a virtual machine or hardware with VGA capabilities.
+
+This guide describes step by step how to restore a previous configuration via console.
+
+.. Attention::
+
+    This requires automatic backups being retained. If you changed `Backup Count` settings in :menuselection:`System --> Configuration --> Backups`,
+    the history can be too short to restore a working configuration from the local backup cache. 
+    Mounting a USB flash drive with a previous configuration might be necessary.
+
+.. Tip::
+
+    Using :doc:`Snapshots </manual/snapshots>` with a ZFS filesystem can make rollbacks simpler.
+
+
+-----------------------------
+1. Power off the device
+-----------------------------
+
+Since you need to influence the boot process to reset the configuration, you should start with powering off the (virtual) device.
+
+.. Attention::
+
+    If you run a HA setup, ensure any cron jobs that synchronize the configuration are turned off. Otherwise they can overwrite the
+    configuration that you restored.
+
+-----------------------------
+2. Access console
+-----------------------------
+
+Make sure you are able to access the (virtual) console, in case it is a physical machine you might want to connect a
+monitor and keyboard, when it is an appliance with serial access, make sure to connect to the serial/usb port using an application
+like putty.
+
+.. Tip::
+
+    When using devices from the OPNsense shop (https://shop.opnsense.com), usually there's a mini-usb to serial cable included
+    in the box, the :doc:`Serial Console connectivity </hardware/serial_connectivity>` guide will help you with the setup.
+
+-----------------------------
+3. Power on and boot
+-----------------------------
+
+Switch the power and wait for the OPNsense splash screen to appear. Do not interact with the splash screen, wait for the actual boot to start.
+
+When the text scrolls fast, hold the **CTRL** button and press **C** rapidly to break out of the boot process into a shell. This does not need any
+authentication.
+
+-----------------------------
+4. Replace configuration
+-----------------------------
+
+Now that you are in the shell, you can do changes to the filesystem. In our example, we will restore a previous config.xml version.
+
+First we will evaluate which config.xml version should be restored:
+
+.. code-block:: sh
+
+    cd /conf/backup
+    ls -la
+
+Check out the timestamps of the backup configurations, copy the filename of one that was before you made the breaking change.
+We will backup our current config.xml and then overwrite it with a previous version.
+
+.. Attention::
+    
+    The below example must be adjusted to represent your config.xml timestamp.
+
+.. code-block:: sh
+
+    cp /conf/config.xml /conf/config.xml.backup
+    cp /conf/backup/config-YOURTIMESTAMP.xml /conf/config.xml
+
+Reboot and the replaced configuration will be loaded.
+
+.. code-block:: sh
+
+    reboot
+
+After the reboot, confirm that you can log in and that the breaking change has been rolled back. If not, repeat the above steps and go back 
+further with the backup configuration timestamp.
+
+If this cannot fix it, :doc:`reinstall your appliance </manual/install>` with the latest available image and restore a
+known-good configuration you have kept safe.