VPN: split different providers into individual files

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diff --git a/docs/vpn/l2tp_ipsec.rst b/docs/vpn/l2tp_ipsec.rst
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+.. _l2tp_ipsec:
+
+L2TP over IPsec
+---------------
+
+Example for configuring a simple L2TP over IPsec VPN for remote access (works
+with native Windows and Mac VPN clients):
+
+.. code-block:: sh
+
+  set vpn ipsec ipsec-interfaces interface eth0
+  set vpn ipsec nat-traversal enable
+  set vpn ipsec nat-networks allowed-network 0.0.0.0/0
+
+  set vpn l2tp remote-access outside-address 203.0.113.2
+  set vpn l2tp remote-access client-ip-pool start 192.168.255.1
+  set vpn l2tp remote-access client-ip-pool stop 192.168.255.254
+  set vpn l2tp remote-access ipsec-settings authentication mode pre-shared-secret
+  set vpn l2tp remote-access ipsec-settings authentication pre-shared-secret <secret>
+  set vpn l2tp remote-access authentication mode local
+  set vpn l2tp remote-access authentication local-users username <username> password <password>
+
+In the example above an external IP of 203.0.113.2 is assumed.
+
+If a local firewall policy is in place on your external interface you will need
+to open:
+
+* UDP port 500 (IKE)
+* IP protocol number 50 (ESP)
+* UDP port 1701 for IPsec
+
+In addition when NAT is detected by the VPN client ESP is encapsulated in UDP
+for NAT-traversal:
+
+* UDP port 4500 (NAT-T)
+
+Example:
+
+.. code-block:: sh
+
+  set firewall name OUTSIDE-LOCAL rule 40 action 'accept'
+  set firewall name OUTSIDE-LOCAL rule 40 destination port '50'
+  set firewall name OUTSIDE-LOCAL rule 40 protocol 'esp'
+  set firewall name OUTSIDE-LOCAL rule 41 action 'accept'
+  set firewall name OUTSIDE-LOCAL rule 41 destination port '500'
+  set firewall name OUTSIDE-LOCAL rule 41 protocol 'udp'
+  set firewall name OUTSIDE-LOCAL rule 42 action 'accept'
+  set firewall name OUTSIDE-LOCAL rule 42 destination port '4500'
+  set firewall name OUTSIDE-LOCAL rule 42 protocol 'udp'
+  set firewall name OUTSIDE-LOCAL rule 43 action 'accept'
+  set firewall name OUTSIDE-LOCAL rule 43 destination port '1701'
+  set firewall name OUTSIDE-LOCAL rule 43 ipsec 'match-ipsec'
+  set firewall name OUTSIDE-LOCAL rule 43 protocol 'udp'
+
+Also note that if you wish to allow the VPN to be used for external access you
+will need to add the appropriate source NAT rules to your configuration.
+
+.. code-block:: sh
+
+  set nat source rule 110 outbound-interface 'eth0'
+  set nat source rule 110 source address '192.168.255.0/24'
+  set nat source rule 110 translation address masquerade
+
+To be able to resolve when connected to the VPN, the following DNS rules are
+needed as well.
+
+.. code-block:: sh
+
+  set vpn l2tp remote-access dns-servers server-1 '8.8.8.8'
+  set vpn l2tp remote-access dns-servers server-2 '8.8.4.4'
+
+.. note:: Those are the `Google public DNS`_ servers. You can also use the
+   public available servers from Quad9_ (9.9.9.9) or Cloudflare_ (1.1.1.1).
+
+Established sessions can be viewed using the **show vpn remote-access**
+operational command.
+
+.. code-block:: sh
+
+  vyos@vyos:~$ show vpn remote-access
+  Active remote access VPN sessions:
+  User            Proto Iface     Tunnel IP       TX byte RX byte  Time
+  ----            ----- -----     ---------       ------- -------  ----
+  vyos            L2TP  l2tp0     192.168.255.1      3.2K    8.0K  00h06m13s
+
+RADIUS authentication
+^^^^^^^^^^^^^^^^^^^^^
+
+The above configuration made use of local accounts on the VyOS router for
+authenticating L2TP/IPSec clients. In bigger environments usually something
+like RADIUS_ (FreeRADIUS_ or Microsoft `Network Policy Server`_, NPS) is used.
+
+VyOS supports either `local` or `radius` user authentication:
+
+.. code-block:: sh
+
+  set vpn l2tp remote-access authentication mode <local|radius>
+
+In addition one or more RADIUS_ servers can be configured to server for user
+authentication. This is done using the `radius server` and `radius server key`
+nodes:
+
+.. code-block:: sh
+
+  set vpn l2tp remote-access authentication radius server 1.1.1.1 key 'foo'
+  set vpn l2tp remote-access authentication radius server 2.2.2.2 key 'foo'
+
+.. note:: Some RADIUS_ severs make use of an access control list who is allowed
+   to query the server. Please configure your VyOS router in the allowed client
+   list.
+
+RADIUS source address
+*********************
+
+If you are using e.g. OSPF as IGP always the nearest interface facing the RADIUS
+server is used. With VyOS 1.2 you can bind all outgoing RADIUS requests to a
+single source IP e.g. the loopback interface.
+
+.. code-block:: sh
+
+  set vpn l2tp remote-access authentication radius source-address 3.3.3.3
+
+Above command will use `3.3.3.3` as source IPv4 address for all RADIUS queries
+on this NAS.