docs: add documentation for the WAN configuration

docs/dev/wan.rst

+WAN support
+===========
+
+As the WAN port of a node will be connected to a user's private network, it
+is essential that the node only uses the WAN when it is absolutely necessary.
+There are two cases in which the WAN port is used:
+
+* Mesh VPN (package ``gluon-mesh-vpn-fastd``
+* DNS to resolve the VPN servers' addresses (package ``gluon-wan-dnsmasq``)
+
+After the VPN connection has been established, the node should be able to reach
+the mesh's DNS servers and use these for all other name resolution.
+
+
+Routing tables
+~~~~~~~~~~~~~~
+As a node may get IPv6 default routes both over the WAN and the mesh, Gluon
+uses two routing tables for IPv6. As all normal traffic should go over the mesh,
+the mesh routes are added to the default table (table 0). All routes on the WAN interface
+are put into table 1 (see ``/lib/gluon/upgrade/110-network`` in ``gluon-core``).
+
+There is also an *ip -6 rule* which routes all IPv6 traffic with a packet mark with the
+bit 1 set though table 1.
+
+
+libpacketmark
+~~~~~~~~~~~~~
+*libpacketmark* is a library which can be loaded with ``LD_PRELOAD`` and will set the packet mark of all
+sockets created by a process in accordance with the ``LIBPACKETMARK_MARK`` environment variable. This allows setting
+the packet mark for processes which don't support this themselves. The process must run as root (or at least
+with ``CAP_NET_ADMIN``) for this to work.
+
+Unfortunately there's no nice way to set the packet mark via iptables for outgoing packets. The iptables will
+run after the packet has been created, to even when the packet mark is changed and the packet is re-routed, the
+source address won't be rewritten to the default source address of the newly chosen route. *libpacketmark* avoids
+this issue as the packet mark will already be set when the packet is created.
+
+gluon-wan-dnsmasq
+~~~~~~~~~~~~~~~~~
+To separate the DNS servers in the mesh from the ones on the WAN, the ``gluon-wan-dnsmasq`` package provides
+a secondary DNS daemon which runs on ``127.0.0.1:54``. It will automatically use all DNS servers explicitly
+configured in ``/etc/config/gluon-wan-dnsmasq`` or received via DNS/RA on the WAN port. It is important that
+no DNS servers for the WAN interface are configured in ``/etc/config/network`` and that ``peerdns`` is set to 0
+so the WAN DNS servers aren't leaked to the primary DNS daemon.
+
+*libpacketmark* is used to make the secondary DNS daemon send its requests over the WAN interface.
+
+The package ``gluon-mesh-vpn-fastd`` provides an iptables rule which will redirect all DNS requests from processes running
+with the primary group ``gluon-fastd`` to ``127.0.0.1:54``, thus making fastd use the secondary DNS daemon.

docs/index.rst

@@ -38,6 +38,7 @@ Developer Documentation
    dev/hardware
    dev/upgrade
    dev/configmode
+   dev/wan
 
 Supported Devices
 -----------------