346 lines
14 KiB
Plaintext
346 lines
14 KiB
Plaintext
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This document describes how to use the kernel's L2TP drivers to
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provide L2TP functionality. L2TP is a protocol that tunnels one or
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more sessions over an IP tunnel. It is commonly used for VPNs
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(L2TP/IPSec) and by ISPs to tunnel subscriber PPP sessions over an IP
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network infrastructure. With L2TPv3, it is also useful as a Layer-2
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tunneling infrastructure.
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Features
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========
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L2TPv2 (PPP over L2TP (UDP tunnels)).
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L2TPv3 ethernet pseudowires.
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L2TPv3 PPP pseudowires.
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L2TPv3 IP encapsulation.
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Netlink sockets for L2TPv3 configuration management.
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History
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=======
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The original pppol2tp driver was introduced in 2.6.23 and provided
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L2TPv2 functionality (rfc2661). L2TPv2 is used to tunnel one or more PPP
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sessions over a UDP tunnel.
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L2TPv3 (rfc3931) changes the protocol to allow different frame types
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to be passed over an L2TP tunnel by moving the PPP-specific parts of
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the protocol out of the core L2TP packet headers. Each frame type is
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known as a pseudowire type. Ethernet, PPP, HDLC, Frame Relay and ATM
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pseudowires for L2TP are defined in separate RFC standards. Another
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change for L2TPv3 is that it can be carried directly over IP with no
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UDP header (UDP is optional). It is also possible to create static
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unmanaged L2TPv3 tunnels manually without a control protocol
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(userspace daemon) to manage them.
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To support L2TPv3, the original pppol2tp driver was split up to
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separate the L2TP and PPP functionality. Existing L2TPv2 userspace
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apps should be unaffected as the original pppol2tp sockets API is
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retained. L2TPv3, however, uses netlink to manage L2TPv3 tunnels and
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sessions.
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Design
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======
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The L2TP protocol separates control and data frames. The L2TP kernel
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drivers handle only L2TP data frames; control frames are always
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handled by userspace. L2TP control frames carry messages between L2TP
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clients/servers and are used to setup / teardown tunnels and
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sessions. An L2TP client or server is implemented in userspace.
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Each L2TP tunnel is implemented using a UDP or L2TPIP socket; L2TPIP
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provides L2TPv3 IP encapsulation (no UDP) and is implemented using a
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new l2tpip socket family. The tunnel socket is typically created by
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userspace, though for unmanaged L2TPv3 tunnels, the socket can also be
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created by the kernel. Each L2TP session (pseudowire) gets a network
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interface instance. In the case of PPP, these interfaces are created
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indirectly by pppd using a pppol2tp socket. In the case of ethernet,
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the netdevice is created upon a netlink request to create an L2TPv3
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ethernet pseudowire.
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For PPP, the PPPoL2TP driver, net/l2tp/l2tp_ppp.c, provides a
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mechanism by which PPP frames carried through an L2TP session are
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passed through the kernel's PPP subsystem. The standard PPP daemon,
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pppd, handles all PPP interaction with the peer. PPP network
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interfaces are created for each local PPP endpoint. The kernel's PPP
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subsystem arranges for PPP control frames to be delivered to pppd,
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while data frames are forwarded as usual.
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For ethernet, the L2TPETH driver, net/l2tp/l2tp_eth.c, implements a
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netdevice driver, managing virtual ethernet devices, one per
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pseudowire. These interfaces can be managed using standard Linux tools
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such as "ip" and "ifconfig". If only IP frames are passed over the
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tunnel, the interface can be given an IP addresses of itself and its
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peer. If non-IP frames are to be passed over the tunnel, the interface
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can be added to a bridge using brctl. All L2TP datapath protocol
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functions are handled by the L2TP core driver.
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Each tunnel and session within a tunnel is assigned a unique tunnel_id
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and session_id. These ids are carried in the L2TP header of every
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control and data packet. (Actually, in L2TPv3, the tunnel_id isn't
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present in data frames - it is inferred from the IP connection on
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which the packet was received.) The L2TP driver uses the ids to lookup
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internal tunnel and/or session contexts to determine how to handle the
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packet. Zero tunnel / session ids are treated specially - zero ids are
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never assigned to tunnels or sessions in the network. In the driver,
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the tunnel context keeps a reference to the tunnel UDP or L2TPIP
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socket. The session context holds data that lets the driver interface
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to the kernel's network frame type subsystems, i.e. PPP, ethernet.
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Userspace Programming
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=====================
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For L2TPv2, there are a number of requirements on the userspace L2TP
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daemon in order to use the pppol2tp driver.
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1. Use a UDP socket per tunnel.
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2. Create a single PPPoL2TP socket per tunnel bound to a special null
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session id. This is used only for communicating with the driver but
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must remain open while the tunnel is active. Opening this tunnel
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management socket causes the driver to mark the tunnel socket as an
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L2TP UDP encapsulation socket and flags it for use by the
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referenced tunnel id. This hooks up the UDP receive path via
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udp_encap_rcv() in net/ipv4/udp.c. PPP data frames are never passed
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in this special PPPoX socket.
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3. Create a PPPoL2TP socket per L2TP session. This is typically done
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by starting pppd with the pppol2tp plugin and appropriate
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arguments. A PPPoL2TP tunnel management socket (Step 2) must be
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created before the first PPPoL2TP session socket is created.
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When creating PPPoL2TP sockets, the application provides information
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to the driver about the socket in a socket connect() call. Source and
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destination tunnel and session ids are provided, as well as the file
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descriptor of a UDP socket. See struct pppol2tp_addr in
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include/linux/if_pppol2tp.h. Note that zero tunnel / session ids are
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treated specially. When creating the per-tunnel PPPoL2TP management
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socket in Step 2 above, zero source and destination session ids are
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specified, which tells the driver to prepare the supplied UDP file
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descriptor for use as an L2TP tunnel socket.
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Userspace may control behavior of the tunnel or session using
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setsockopt and ioctl on the PPPoX socket. The following socket
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options are supported:-
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DEBUG - bitmask of debug message categories. See below.
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SENDSEQ - 0 => don't send packets with sequence numbers
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1 => send packets with sequence numbers
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RECVSEQ - 0 => receive packet sequence numbers are optional
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1 => drop receive packets without sequence numbers
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LNSMODE - 0 => act as LAC.
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1 => act as LNS.
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REORDERTO - reorder timeout (in millisecs). If 0, don't try to reorder.
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Only the DEBUG option is supported by the special tunnel management
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PPPoX socket.
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In addition to the standard PPP ioctls, a PPPIOCGL2TPSTATS is provided
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to retrieve tunnel and session statistics from the kernel using the
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PPPoX socket of the appropriate tunnel or session.
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For L2TPv3, userspace must use the netlink API defined in
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include/linux/l2tp.h to manage tunnel and session contexts. The
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general procedure to create a new L2TP tunnel with one session is:-
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1. Open a GENL socket using L2TP_GENL_NAME for configuring the kernel
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using netlink.
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2. Create a UDP or L2TPIP socket for the tunnel.
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3. Create a new L2TP tunnel using a L2TP_CMD_TUNNEL_CREATE
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request. Set attributes according to desired tunnel parameters,
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referencing the UDP or L2TPIP socket created in the previous step.
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4. Create a new L2TP session in the tunnel using a
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L2TP_CMD_SESSION_CREATE request.
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The tunnel and all of its sessions are closed when the tunnel socket
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is closed. The netlink API may also be used to delete sessions and
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tunnels. Configuration and status info may be set or read using netlink.
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The L2TP driver also supports static (unmanaged) L2TPv3 tunnels. These
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are where there is no L2TP control message exchange with the peer to
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setup the tunnel; the tunnel is configured manually at each end of the
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tunnel. There is no need for an L2TP userspace application in this
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case -- the tunnel socket is created by the kernel and configured
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using parameters sent in the L2TP_CMD_TUNNEL_CREATE netlink
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request. The "ip" utility of iproute2 has commands for managing static
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L2TPv3 tunnels; do "ip l2tp help" for more information.
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Debugging
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=========
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The driver supports a flexible debug scheme where kernel trace
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messages may be optionally enabled per tunnel and per session. Care is
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needed when debugging a live system since the messages are not
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rate-limited and a busy system could be swamped. Userspace uses
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setsockopt on the PPPoX socket to set a debug mask.
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The following debug mask bits are available:
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L2TP_MSG_DEBUG verbose debug (if compiled in)
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L2TP_MSG_CONTROL userspace - kernel interface
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L2TP_MSG_SEQ sequence numbers handling
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L2TP_MSG_DATA data packets
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If enabled, files under a l2tp debugfs directory can be used to dump
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kernel state about L2TP tunnels and sessions. To access it, the
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debugfs filesystem must first be mounted.
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# mount -t debugfs debugfs /debug
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Files under the l2tp directory can then be accessed.
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# cat /debug/l2tp/tunnels
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The debugfs files should not be used by applications to obtain L2TP
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state information because the file format is subject to change. It is
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implemented to provide extra debug information to help diagnose
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problems.) Users should use the netlink API.
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/proc/net/pppol2tp is also provided for backwards compatibility with
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the original pppol2tp driver. It lists information about L2TPv2
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tunnels and sessions only. Its use is discouraged.
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Unmanaged L2TPv3 Tunnels
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========================
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Some commercial L2TP products support unmanaged L2TPv3 ethernet
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tunnels, where there is no L2TP control protocol; tunnels are
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configured at each side manually. New commands are available in
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iproute2's ip utility to support this.
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To create an L2TPv3 ethernet pseudowire between local host 192.168.1.1
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and peer 192.168.1.2, using IP addresses 10.5.1.1 and 10.5.1.2 for the
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tunnel endpoints:-
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# ip l2tp add tunnel tunnel_id 1 peer_tunnel_id 1 udp_sport 5000 \
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udp_dport 5000 encap udp local 192.168.1.1 remote 192.168.1.2
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# ip l2tp add session tunnel_id 1 session_id 1 peer_session_id 1
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# ip -s -d show dev l2tpeth0
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# ip addr add 10.5.1.2/32 peer 10.5.1.1/32 dev l2tpeth0
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# ip li set dev l2tpeth0 up
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Choose IP addresses to be the address of a local IP interface and that
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of the remote system. The IP addresses of the l2tpeth0 interface can be
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anything suitable.
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Repeat the above at the peer, with ports, tunnel/session ids and IP
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addresses reversed. The tunnel and session IDs can be any non-zero
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32-bit number, but the values must be reversed at the peer.
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Host 1 Host2
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udp_sport=5000 udp_sport=5001
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udp_dport=5001 udp_dport=5000
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tunnel_id=42 tunnel_id=45
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peer_tunnel_id=45 peer_tunnel_id=42
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session_id=128 session_id=5196755
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peer_session_id=5196755 peer_session_id=128
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When done at both ends of the tunnel, it should be possible to send
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data over the network. e.g.
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# ping 10.5.1.1
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Sample Userspace Code
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=====================
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1. Create tunnel management PPPoX socket
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kernel_fd = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OL2TP);
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if (kernel_fd >= 0) {
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struct sockaddr_pppol2tp sax;
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struct sockaddr_in const *peer_addr;
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peer_addr = l2tp_tunnel_get_peer_addr(tunnel);
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memset(&sax, 0, sizeof(sax));
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sax.sa_family = AF_PPPOX;
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sax.sa_protocol = PX_PROTO_OL2TP;
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sax.pppol2tp.fd = udp_fd; /* fd of tunnel UDP socket */
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sax.pppol2tp.addr.sin_addr.s_addr = peer_addr->sin_addr.s_addr;
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sax.pppol2tp.addr.sin_port = peer_addr->sin_port;
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sax.pppol2tp.addr.sin_family = AF_INET;
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sax.pppol2tp.s_tunnel = tunnel_id;
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sax.pppol2tp.s_session = 0; /* special case: mgmt socket */
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sax.pppol2tp.d_tunnel = 0;
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sax.pppol2tp.d_session = 0; /* special case: mgmt socket */
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if(connect(kernel_fd, (struct sockaddr *)&sax, sizeof(sax) ) < 0 ) {
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perror("connect failed");
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result = -errno;
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goto err;
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}
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}
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2. Create session PPPoX data socket
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struct sockaddr_pppol2tp sax;
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int fd;
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/* Note, the target socket must be bound already, else it will not be ready */
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sax.sa_family = AF_PPPOX;
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sax.sa_protocol = PX_PROTO_OL2TP;
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sax.pppol2tp.fd = tunnel_fd;
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sax.pppol2tp.addr.sin_addr.s_addr = addr->sin_addr.s_addr;
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sax.pppol2tp.addr.sin_port = addr->sin_port;
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sax.pppol2tp.addr.sin_family = AF_INET;
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sax.pppol2tp.s_tunnel = tunnel_id;
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sax.pppol2tp.s_session = session_id;
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sax.pppol2tp.d_tunnel = peer_tunnel_id;
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sax.pppol2tp.d_session = peer_session_id;
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/* session_fd is the fd of the session's PPPoL2TP socket.
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* tunnel_fd is the fd of the tunnel UDP socket.
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*/
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fd = connect(session_fd, (struct sockaddr *)&sax, sizeof(sax));
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if (fd < 0 ) {
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return -errno;
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}
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return 0;
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Internal Implementation
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=======================
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The driver keeps a struct l2tp_tunnel context per L2TP tunnel and a
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struct l2tp_session context for each session. The l2tp_tunnel is
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always associated with a UDP or L2TP/IP socket and keeps a list of
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sessions in the tunnel. The l2tp_session context keeps kernel state
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about the session. It has private data which is used for data specific
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to the session type. With L2TPv2, the session always carried PPP
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traffic. With L2TPv3, the session can also carry ethernet frames
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(ethernet pseudowire) or other data types such as ATM, HDLC or Frame
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Relay.
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When a tunnel is first opened, the reference count on the socket is
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increased using sock_hold(). This ensures that the kernel socket
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cannot be removed while L2TP's data structures reference it.
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Some L2TP sessions also have a socket (PPP pseudowires) while others
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do not (ethernet pseudowires). We can't use the socket reference count
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as the reference count for session contexts. The L2TP implementation
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therefore has its own internal reference counts on the session
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contexts.
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To Do
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=====
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Add L2TP tunnel switching support. This would route tunneled traffic
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from one L2TP tunnel into another. Specified in
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http://tools.ietf.org/html/draft-ietf-l2tpext-tunnel-switching-08
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Add L2TPv3 VLAN pseudowire support.
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Add L2TPv3 IP pseudowire support.
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Add L2TPv3 ATM pseudowire support.
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Miscellaneous
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=============
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The L2TP drivers were developed as part of the OpenL2TP project by
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Katalix Systems Ltd. OpenL2TP is a full-featured L2TP client / server,
|
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designed from the ground up to have the L2TP datapath in the
|
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kernel. The project also implemented the pppol2tp plugin for pppd
|
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|
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which allows pppd to use the kernel driver. Details can be found at
|
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http://www.openl2tp.org.
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