[docs] Added "How to configure push updates" section #255

Closes #255

Author: nemesifier

Diff for: README.rst

@@ -813,6 +813,87 @@ The default values can then be overridden at
         "wlan0_password": "room_23pwd!321654"
     }
 
+How to configure push updates
+-----------------------------
+
+Follow the procedure described below to enable secure SSH access from OpenWISP to your
+devices, this is required to enable push updates (whenever the configuration is changed,
+OpenWISP will trigger the update in the background) and/or
+`firmware upgrades (via the additional module openwisp-firmware-upgrader)
+<https://github.com/openwisp/openwisp-firmware-upgrader>`_.
+
+1. Generate SSH key
+~~~~~~~~~~~~~~~~~~~
+
+First of all, we need to generate the SSH key which will be
+used by OpenWISP to access the devices, to do so, you can use the following command:
+
+.. code-block:: shell
+
+    echo './sshkey' | ssh-keygen -t rsa -b 4096 -C "openwisp"
+
+This will create two files in the current directory, one called ``sshkey`` (the private key) and one called
+``sshkey.pub`` (the public key).
+
+Store the content of these files in a secure location.
+
+2. Save SSH private key in OpenWISP (access credentials)
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. image:: https://raw.githubusercontent.com/openwisp/openwisp-controller/master/docs/add-ssh-credentials-private-key.png
+  :alt: add SSH private key as access credential in OpenWISP
+
+From the first page of OpenWISP click on "Access credentials", then click
+on the **"ADD ACCESS CREDENTIALS"** button in the upper right corner
+(alternatively, go to the following URL: ``/admin/connection/credentials/add/``).
+
+Select SSH as ``type``, enable the **Auto add** checkbox, then at the field
+"Credentials type" select "SSH (private key)", now type "root" in the ``username`` field,
+while in the ``key`` field you have to paste the contents of the private key just created.
+
+Now hit save.
+
+The credentials just created will be automatically enabled for all the devices in the system
+(both existing devices and devices which will be added in the future).
+
+3. Add the public key to your devices
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+.. image:: https://raw.githubusercontent.com/openwisp/openwisp-controller/master/docs/add-authorized-ssh-keys-template.png
+  :alt: Add authorized SSH public keys template to OpenWISP (OpenWRT)
+
+Now we need to instruct your devices to allow OpenWISP accessing via SSH,
+in order to do this we need to add the contents of the public key file created in step 1
+(``sshkey.pub``) in the file ``/etc/dropbear/authorized_keys`` on the devices, the
+recommended way to do this is to create a configuration template in OpenWISP:
+from the first page of OpenWISP, click on "Templates", then and click on the
+**"ADD TEMPLATE"** button in the upper right corner (alternatively, go to the following URL:
+``/admin/config/template/add/``).
+
+Check **enabled by default**, then scroll down the configuration section,
+click on "Configuration Menu", scroll down, click on "Files" then close the menu
+by clicking again on "Configuration Menu". Now type ``/etc/dropbear/authorized_keys``
+in the ``path`` field of the file, then paste the contents of ``sshkey.pub`` in ``contents``.
+
+Now hit save.
+
+**There's a catch**: you will need to assign the template to any existing device.
+
+4. Test it
+~~~~~~~~~~
+
+Once you have performed the 3 steps above, you can test it as follows:
+
+1. Ensure there's at least one device turned on and connected to OpenWISP, ensure
+   this device has the "SSH Authorized Keys" assigned to it.
+2. Ensure the celery worker of OpenWISP Controller is running (eg: ``ps aux | grep celery``)
+3. SSH into the device and wait (maximum 2 minutes) until ``/etc/dropbear/authorized_keys``
+   appears as specified in the template.
+4. While connected via SSH to the device run the following command in the console:
+   ``logread -f``, now try changing the device name in OpenWISP
+5. Shortly after you change the name in OpenWISP, you should see some output in the
+   SSH console indicating another SSH access and the configuration update being performed.
+
 Signals
 -------