5.6. L2TP device type commands

This chapter describes commands that are specific to L2TP type links. These commands apply to the currently active link, and are only valid if the currently active link has type l2tp.

The L2TP protocol utilizes UDP datagrams on port 1701 (and this is the default for port in the commands below) to create and maintain virtual tunnel between IP peers. One or more independent PPP connections (sessions) can be carried inside this tunnel.

Complete L2TP network topology looks like:

client <- some link type -> LAC <- L2TP tunnel -> LNS

LAC is physical level repeater, which receives PPP connection of some type and forwards it to LNS using L2TP protocol. LNS is ppp endpoint, which receives ppp frames via L2TP tunnel and processes them.

In simple case, when physical conversion is not required, topology can be simplified to:

client (LAC emulator) <- L2TP tunnel -> LNS

Mpd is able to operate in both LAC and LNS modes. As LAC mpd supports both simple case LAC emulator and complete LAC topologies. Complete LAC can be configured by joining two physical devices using mpd's repeater functionality.

Windows L2TP client uses IPSec encryption for the additional tunnel security. So, to let it connect you must configure IPSec on your MPD router or disable IPSec on Windows by setting registry DWORD value ProhibitIpSec at the key HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\RasMan\Parameters\ into "1".

set l2tp self ipaddr [ port ]

Sets the local IP address and port for the L2TP connection. There is known implementation bug if this option is not set while accepting incoming connections on the router with multiple IPs and clients are connecting not to the nearest address of this router.

set l2tp peer ipaddr [ port ]

Sets the peer IP address and port for the L2TP connection. This command applies to both incoming and outgoing connections. For outgoing connections, this command is required in order to specify where to connect to. For incoming connections, this command is optional; if not given, mpd accepts incoming connections from any host. Otherwise, only connections from the stated IP address (and, optionally, port) are allowed.

set l2tp hostname name

Sets the L2TP tunnel local hostname. For server side, only one unique hostname supported for every pair of listening IP (set l2tp self ...) and peer ip (set l2tp peer ...). If several hostnames defined, only the first matching will be used for all incoming connections.

set l2tp secret secret

Sets the L2TP tunnel secret. Used to authenticate tunnel connection and encrypt important control packets avpairs. For server side, only one unique secret supported for every pair of listening IP (set l2tp self ...) and peer ip (set l2tp peer ...). If several secrets defined, only the first matching will be used for all incoming connections.

NOTE: This options is not related with usual PPP authentication. Windows client does not support tunnel authentication.

set l2tp callingnum number

set l2tp callednum number

Sets the calling and called telephone number to use when initiating a L2TP connection. For most VPN applications this is ignored, but in certain cases an actual phone number is required. The default is the empty string.

set l2tp enable option ...

set l2tp disable option ...

Enable and disable L2TP device type options for the link.

The following options are supported:

originate

This option enables the initiation of L2TP connections to the peer. If this option is disabled, mpd will not initiate any L2TP connections. You must configure the peer's IP address with the set l2tp peer command.

incoming

This option enables the acceptance of incoming L2TP connections. If this option is disabled, mpd will not accept any incoming connections.

outcall

Inside L2TP tunnel, each individual PPP connection (there may be several), is initiated as either an incoming or an outgoing call. This is to allow, for example, making an outgoing telephone call (by LNS) via a remote access server (LAC), as well as the more common use of forwarding of an incoming telephone call from an access server (LAC) to a remote L2TP server (LNS).

When this option is enabled, mpd will initiate outgoing calls (LNS); otherwise mpd will initiate incoming calls (LAC).

hidden

When L2TP tunnel secret is configured it is possible hide (encrypt) some control data for additional protection.

Default is disabled.

length

By default L2TP uses header Length field for control packets, but not for data packets. This option enables Length field for data packets. This feature enabled may be useful on links where packets padding may happend. Disabling it reduces overhead by 2 bytes per packet.

Default is disabled.

dataseq

By default L2TP requires header sequence fields for control packets, but not require them for data packets. This option enables sequence fields for data packets. This feature enabled may be useful on links where packets reordering may happend but it is intolerable. Disabling it reduces overhead by 4 bytes per packet.

Default is enabled.