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+.. _l2tp:
+
+L2TP
+----
+
+VyOS utilizes accel-ppp_ to provide L2TP server functionality. It can be used
+with local authentication or a connected RADIUS server.
+
+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:: none
+
+  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 192.0.2.2
+  set vpn l2tp remote-access client-ip-pool start 192.168.255.2
+  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 test password 'test'
+
+In the example above an external IP of 192.0.2.2 is assumed.
+
+If a local firewall policy is in place on your external interface you will need
+to allow the ports below:
+
+* UDP port 500 (IKE)
+* IP protocol number 50 (ESP)
+* UDP port 1701 for IPsec
+
+As well as the below to allow NAT-traversal (when NAT is detected by the
+VPN client, ESP is encapsulated in UDP for NAT-traversal):
+
+* UDP port 4500 (NAT-T)
+
+Example:
+
+.. code-block:: none
+
+  set firewall name OUTSIDE-LOCAL rule 40 action 'accept'
+  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'
+
+To allow VPN-clients access via your external address, a NAT rule is required:
+
+
+.. code-block:: none
+
+  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
+
+
+VPN-clients will request configuration parameters, optionally you can DNS
+parameter to the client.
+
+.. code-block:: none
+
+  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, but you can choose
+   any public available servers, like Quad9_ (9.9.9.9), Cloudflare_ (1.1.1.1)
+   or OpenNIC_.
+
+Established sessions can be viewed using the **show vpn remote-access**
+operational command, or **show l2tp-server sessions**
+
+.. code-block:: none
+
+  vyos@vyos:~$ show vpn remote-access
+   ifname | username | calling-sid  |      ip       | rate-limit | type | comp | state  |  uptime
+  --------+----------+--------------+---------------+------------+------+------+--------+----------
+   ppp0   | vyos     | 192.168.0.36 | 192.168.255.1 |            | l2tp |      | active | 00:06:13
+
+
+LNS (L2TP Network Server)
+=========================
+
+LNS are often used to connect to a LAC (L2TP Access Concentrator).
+
+Below is an example to configure a LNS:
+
+.. code-block:: none
+
+  set vpn l2tp remote-access outside-address 192.0.2.2
+  set vpn l2tp remote-access client-ip-pool start 192.168.255.2
+  set vpn l2tp remote-access client-ip-pool stop 192.168.255.254
+  set vpn l2tp remote-access lns shared-secret 'secret'
+  set vpn l2tp remote-access ccp-disable
+  set vpn l2tp remote-access authentication mode local
+  set vpn l2tp remote-access authentication local-users username test password 'test'
+
+The example above uses 192.0.2.2 as external IP address. A LAC normally
+requires an authentication password, which is set in the example configuration
+to ``lns shared-secret 'secret'``. This setup requires the Compression Control
+Protocol (CCP) being disabled, the command ``set vpn l2tp remote-access ccp-disable``
+accomplishes that.
+
+
+Bandwidth Shaping
+=================
+
+Bandwidth rate limits can be set for local users or via RADIUS based attributes.
+
+Bandwidth Shaping for local users
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+The rate-limit is set in kbit/sec.
+
+.. code-block:: none
+
+  set vpn l2tp remote-access outside-address 192.0.2.2
+  set vpn l2tp remote-access client-ip-pool start 192.168.255.2
+  set vpn l2tp remote-access client-ip-pool stop 192.168.255.254
+  set vpn l2tp remote-access authentication mode local
+  set vpn l2tp remote-access authentication local-users username test password test
+  set vpn l2tp remote-access authentication local-users username test rate-limit download 20480
+  set vpn l2tp remote-access authentication local-users username test rate-limit upload 10240
+
+  vyos@vyos:~$ show vpn remote-access
+  ifname | username | calling-sid  |      ip       | rate-limit  | type | comp | state  |  uptime
+  -------+----------+--------------+---------------+-------------+------+------+--------+-----------
+  ppp0   | test     | 192.168.0.36 | 192.168.255.2 | 20480/10240 | l2tp |      | active | 00:06:30
+
+RADIUS authentication
+======================
+
+To enable RADIUS based authentication, the authentication mode needs to be
+changed within the configuration. Previous settings like the local users, still
+exists within the configuration, however they are not used if the mode has been
+changed from local to radius. Once changed back to local, it will use all local
+accounts again.
+
+.. code-block:: none
+
+  set vpn l2tp remote-access authentication mode <local|radius>
+
+Since the RADIUS server would be a single point of failure, multiple RADIUS
+servers can be setup and will be used subsequentially.
+
+.. code-block:: none
+
+  set vpn l2tp remote-access authentication radius server 10.0.0.1 key 'foo'
+  set vpn l2tp remote-access authentication radius server 10.0.0.2 key 'foo'
+
+.. note:: Some RADIUS_ severs use an access control list which allows or denies
+   queries, make sure to add your VyOS router to the allowed client list.
+
+RADIUS source address
+^^^^^^^^^^^^^^^^^^^^^
+
+If you are using OSPF as IGP always the closets interface connected to 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:: none
+
+  set vpn l2tp remote-access authentication radius source-address 10.0.0.3
+
+Above command will use `10.0.0.3` as source IPv4 address for all RADIUS queries
+on this NAS.
+
+.. note:: The ``source-address`` must be configured on one of VyOS interface.
+   Best proctice would be a loopback or dummy interface.
+
+RADIUS bandwidth shaping attribute
+^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+
+To enable bandwidth shaping via RADIUS, the option rate-limit needs to be enabled.
+
+.. code-block:: none
+
+  set vpn l2tp remote-access authentication radius rate-limit enable
+
+The default RADIUS attribute for rate limiting is ``Filter-Id``, but you may also
+redefine it.
+
+.. code-block:: none
+
+  set vpn l2tp remote-access authentication radius rate-limit attribute Download-Speed
+
+.. note:: If you set a custom RADIUS attribute you must define it on both
+   dictionaries at RADIUS server and client, which is the vyos router in our
+   example.
+
+The RADIUS dictionaries in VyOS are located at ``/usr/share/accel-ppp/radius/``
+
+RADIUS advanced features
+^^^^^^^^^^^^^^^^^^^^^^^^
+
+Received RADIUS attributes have a higher priority than parameters defined within
+the CLI configuration, refer to the explanation below.
+
+Allocation clients ip addresses by RADIUS
+*****************************************
+
+If the RADIUS server sends the attribute ``Framed-IP-Address`` then this IP
+address will be allocated to the client and the option ip-pool within the CLI
+config is being ignored.
+
+Renaming clients interfaces by RADIUS
+*************************************
+
+If the RADIUS server uses the attribute ``NAS-Port-Id``, ppp tunnels will be
+renamed.
+
+.. note:: The value of the attribute ``NAS-Port-Id`` must be less than 16
+   characters, otherwise the interface won't be renamed.
+
+
+.. _`Google Public DNS`: https://developers.google.com/speed/public-dns
+.. _Quad9: https://quad9.net
+.. _CloudFlare: https://blog.cloudflare.com/announcing-1111
+.. _OpenNIC: https://www.opennic.org/
+.. _RADIUS: https://en.wikipedia.org/wiki/RADIUS
+.. _FreeRADIUS: https://freeradius.org
+.. _`Network Policy Server`: https://en.wikipedia.org/wiki/Network_Policy_Server
+.. _accel-ppp: https://accel-ppp.org/