docs: add general documentation about vpn protocols

docs/features/vpn.rst

@@ -0,0 +1,45 @@
+VPN
+===
+
+Gluon supports different options to establish vpn tunnels,
+which connect mesh clouds and provide internet access.
+Currently the available vpn protocols options are:
+
+- fastd
+- L2TP (tunneldigger)
+
+Fastd is a lightweight vpn daemon in userspace, which is
+especially designed for embedded hardware. It supports
+encryption and authentication.
+
+L2TP is implemented inside the linux kernel and has
+therefore performance advantages over fastd. The
+disadvantage of L2TP is, that it does not support any
+encryption. So everything is sent in plain.
+
+Optional Encryption (fastd only):
+---------------------------------
+
+When using fastd, the firmware can allow the user to
+decide by itself, whether he want's to use encryption
+or not. If the firmware builder doesn't like this, he
+is also able to hide (or even forbid) the encryptionless
+option to the user.
+
+If you want to allow users to decide by themselves:
+
+- Be sure, the package ``gluon-web-mesh-vpn-fastd`` is enabled in ``site.mk``
+- Set the option ``mesh_vpn.fastd.configurable = true`` in ``site.conf``
+- On the server side, be sure that ``null`` cipher is allowed and preferred over ``salsa2012+umac``. You can ensure this by inserting the ``method "null";`` entry before the ``method "salsa2012+umac";`` in your site.conf.
+
+Users now should have the choice in expert mode to decide
+by themselves, which looks like this:
+
+.. image:: fastd_mode.gif
+
+If you want to ensure, that the correct chipher is chosen,
+you can use the following command on a router. You maybe
+have to install socat before.
+
+       socat - UNIX-CONNECT:/var/run/fastd.mesh_vpn.socket
+