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James Chapman2f77a3f2010-04-02 06:19:46 +00001This document describes how to use the kernel's L2TP drivers to
2provide L2TP functionality. L2TP is a protocol that tunnels one or
3more sessions over an IP tunnel. It is commonly used for VPNs
James Chapman58e50a92007-06-27 15:53:49 -07004(L2TP/IPSec) and by ISPs to tunnel subscriber PPP sessions over an IP
James Chapman2f77a3f2010-04-02 06:19:46 +00005network infrastructure. With L2TPv3, it is also useful as a Layer-2
6tunneling infrastructure.
7
8Features
9========
10
11L2TPv2 (PPP over L2TP (UDP tunnels)).
12L2TPv3 ethernet pseudowires.
13L2TPv3 PPP pseudowires.
14L2TPv3 IP encapsulation.
15Netlink sockets for L2TPv3 configuration management.
16
17History
18=======
19
20The original pppol2tp driver was introduced in 2.6.23 and provided
21L2TPv2 functionality (rfc2661). L2TPv2 is used to tunnel one or more PPP
22sessions over a UDP tunnel.
23
24L2TPv3 (rfc3931) changes the protocol to allow different frame types
25to be passed over an L2TP tunnel by moving the PPP-specific parts of
26the protocol out of the core L2TP packet headers. Each frame type is
27known as a pseudowire type. Ethernet, PPP, HDLC, Frame Relay and ATM
28pseudowires for L2TP are defined in separate RFC standards. Another
29change for L2TPv3 is that it can be carried directly over IP with no
30UDP header (UDP is optional). It is also possible to create static
31unmanaged L2TPv3 tunnels manually without a control protocol
32(userspace daemon) to manage them.
33
34To support L2TPv3, the original pppol2tp driver was split up to
35separate the L2TP and PPP functionality. Existing L2TPv2 userspace
36apps should be unaffected as the original pppol2tp sockets API is
37retained. L2TPv3, however, uses netlink to manage L2TPv3 tunnels and
38sessions.
James Chapman58e50a92007-06-27 15:53:49 -070039
40Design
41======
42
James Chapman2f77a3f2010-04-02 06:19:46 +000043The L2TP protocol separates control and data frames. The L2TP kernel
44drivers handle only L2TP data frames; control frames are always
45handled by userspace. L2TP control frames carry messages between L2TP
46clients/servers and are used to setup / teardown tunnels and
47sessions. An L2TP client or server is implemented in userspace.
James Chapman58e50a92007-06-27 15:53:49 -070048
James Chapman2f77a3f2010-04-02 06:19:46 +000049Each L2TP tunnel is implemented using a UDP or L2TPIP socket; L2TPIP
50provides L2TPv3 IP encapsulation (no UDP) and is implemented using a
51new l2tpip socket family. The tunnel socket is typically created by
52userspace, though for unmanaged L2TPv3 tunnels, the socket can also be
53created by the kernel. Each L2TP session (pseudowire) gets a network
54interface instance. In the case of PPP, these interfaces are created
55indirectly by pppd using a pppol2tp socket. In the case of ethernet,
56the netdevice is created upon a netlink request to create an L2TPv3
57ethernet pseudowire.
58
59For PPP, the PPPoL2TP driver, net/l2tp/l2tp_ppp.c, provides a
60mechanism by which PPP frames carried through an L2TP session are
61passed through the kernel's PPP subsystem. The standard PPP daemon,
62pppd, handles all PPP interaction with the peer. PPP network
63interfaces are created for each local PPP endpoint. The kernel's PPP
James Chapman58e50a92007-06-27 15:53:49 -070064subsystem arranges for PPP control frames to be delivered to pppd,
65while data frames are forwarded as usual.
66
James Chapman2f77a3f2010-04-02 06:19:46 +000067For ethernet, the L2TPETH driver, net/l2tp/l2tp_eth.c, implements a
68netdevice driver, managing virtual ethernet devices, one per
69pseudowire. These interfaces can be managed using standard Linux tools
70such as "ip" and "ifconfig". If only IP frames are passed over the
71tunnel, the interface can be given an IP addresses of itself and its
72peer. If non-IP frames are to be passed over the tunnel, the interface
73can be added to a bridge using brctl. All L2TP datapath protocol
74functions are handled by the L2TP core driver.
75
James Chapman58e50a92007-06-27 15:53:49 -070076Each tunnel and session within a tunnel is assigned a unique tunnel_id
77and session_id. These ids are carried in the L2TP header of every
James Chapman2f77a3f2010-04-02 06:19:46 +000078control and data packet. (Actually, in L2TPv3, the tunnel_id isn't
79present in data frames - it is inferred from the IP connection on
80which the packet was received.) The L2TP driver uses the ids to lookup
81internal tunnel and/or session contexts to determine how to handle the
82packet. Zero tunnel / session ids are treated specially - zero ids are
83never assigned to tunnels or sessions in the network. In the driver,
84the tunnel context keeps a reference to the tunnel UDP or L2TPIP
85socket. The session context holds data that lets the driver interface
86to the kernel's network frame type subsystems, i.e. PPP, ethernet.
James Chapman58e50a92007-06-27 15:53:49 -070087
James Chapman2f77a3f2010-04-02 06:19:46 +000088Userspace Programming
89=====================
James Chapman58e50a92007-06-27 15:53:49 -070090
James Chapman2f77a3f2010-04-02 06:19:46 +000091For L2TPv2, there are a number of requirements on the userspace L2TP
92daemon in order to use the pppol2tp driver.
James Chapman58e50a92007-06-27 15:53:49 -070093
941. Use a UDP socket per tunnel.
95
962. Create a single PPPoL2TP socket per tunnel bound to a special null
97 session id. This is used only for communicating with the driver but
98 must remain open while the tunnel is active. Opening this tunnel
99 management socket causes the driver to mark the tunnel socket as an
100 L2TP UDP encapsulation socket and flags it for use by the
101 referenced tunnel id. This hooks up the UDP receive path via
102 udp_encap_rcv() in net/ipv4/udp.c. PPP data frames are never passed
103 in this special PPPoX socket.
104
1053. Create a PPPoL2TP socket per L2TP session. This is typically done
106 by starting pppd with the pppol2tp plugin and appropriate
107 arguments. A PPPoL2TP tunnel management socket (Step 2) must be
108 created before the first PPPoL2TP session socket is created.
109
110When creating PPPoL2TP sockets, the application provides information
111to the driver about the socket in a socket connect() call. Source and
112destination tunnel and session ids are provided, as well as the file
113descriptor of a UDP socket. See struct pppol2tp_addr in
Paul Mackerras4b32da2b2012-03-04 12:56:55 +0000114include/linux/if_pppol2tp.h. Note that zero tunnel / session ids are
James Chapman58e50a92007-06-27 15:53:49 -0700115treated specially. When creating the per-tunnel PPPoL2TP management
116socket in Step 2 above, zero source and destination session ids are
117specified, which tells the driver to prepare the supplied UDP file
118descriptor for use as an L2TP tunnel socket.
119
120Userspace may control behavior of the tunnel or session using
121setsockopt and ioctl on the PPPoX socket. The following socket
122options are supported:-
123
124DEBUG - bitmask of debug message categories. See below.
125SENDSEQ - 0 => don't send packets with sequence numbers
126 1 => send packets with sequence numbers
127RECVSEQ - 0 => receive packet sequence numbers are optional
128 1 => drop receive packets without sequence numbers
129LNSMODE - 0 => act as LAC.
130 1 => act as LNS.
131REORDERTO - reorder timeout (in millisecs). If 0, don't try to reorder.
132
133Only the DEBUG option is supported by the special tunnel management
134PPPoX socket.
135
136In addition to the standard PPP ioctls, a PPPIOCGL2TPSTATS is provided
137to retrieve tunnel and session statistics from the kernel using the
138PPPoX socket of the appropriate tunnel or session.
139
James Chapman2f77a3f2010-04-02 06:19:46 +0000140For L2TPv3, userspace must use the netlink API defined in
141include/linux/l2tp.h to manage tunnel and session contexts. The
142general procedure to create a new L2TP tunnel with one session is:-
143
1441. Open a GENL socket using L2TP_GENL_NAME for configuring the kernel
145 using netlink.
146
1472. Create a UDP or L2TPIP socket for the tunnel.
148
1493. Create a new L2TP tunnel using a L2TP_CMD_TUNNEL_CREATE
150 request. Set attributes according to desired tunnel parameters,
151 referencing the UDP or L2TPIP socket created in the previous step.
152
1534. Create a new L2TP session in the tunnel using a
154 L2TP_CMD_SESSION_CREATE request.
155
156The tunnel and all of its sessions are closed when the tunnel socket
157is closed. The netlink API may also be used to delete sessions and
158tunnels. Configuration and status info may be set or read using netlink.
159
160The L2TP driver also supports static (unmanaged) L2TPv3 tunnels. These
161are where there is no L2TP control message exchange with the peer to
162setup the tunnel; the tunnel is configured manually at each end of the
163tunnel. There is no need for an L2TP userspace application in this
164case -- the tunnel socket is created by the kernel and configured
165using parameters sent in the L2TP_CMD_TUNNEL_CREATE netlink
166request. The "ip" utility of iproute2 has commands for managing static
167L2TPv3 tunnels; do "ip l2tp help" for more information.
168
James Chapman58e50a92007-06-27 15:53:49 -0700169Debugging
170=========
171
172The driver supports a flexible debug scheme where kernel trace
173messages may be optionally enabled per tunnel and per session. Care is
174needed when debugging a live system since the messages are not
175rate-limited and a busy system could be swamped. Userspace uses
176setsockopt on the PPPoX socket to set a debug mask.
177
178The following debug mask bits are available:
179
180PPPOL2TP_MSG_DEBUG verbose debug (if compiled in)
181PPPOL2TP_MSG_CONTROL userspace - kernel interface
182PPPOL2TP_MSG_SEQ sequence numbers handling
183PPPOL2TP_MSG_DATA data packets
184
James Chapman2f77a3f2010-04-02 06:19:46 +0000185If enabled, files under a l2tp debugfs directory can be used to dump
186kernel state about L2TP tunnels and sessions. To access it, the
187debugfs filesystem must first be mounted.
188
189# mount -t debugfs debugfs /debug
190
191Files under the l2tp directory can then be accessed.
192
193# cat /debug/l2tp/tunnels
194
195The debugfs files should not be used by applications to obtain L2TP
196state information because the file format is subject to change. It is
197implemented to provide extra debug information to help diagnose
198problems.) Users should use the netlink API.
199
200/proc/net/pppol2tp is also provided for backwards compaibility with
201the original pppol2tp driver. It lists information about L2TPv2
202tunnels and sessions only. Its use is discouraged.
203
204Unmanaged L2TPv3 Tunnels
205========================
206
207Some commercial L2TP products support unmanaged L2TPv3 ethernet
208tunnels, where there is no L2TP control protocol; tunnels are
209configured at each side manually. New commands are available in
210iproute2's ip utility to support this.
211
212To create an L2TPv3 ethernet pseudowire between local host 192.168.1.1
213and peer 192.168.1.2, using IP addresses 10.5.1.1 and 10.5.1.2 for the
214tunnel endpoints:-
215
216# modprobe l2tp_eth
217# modprobe l2tp_netlink
218
219# ip l2tp add tunnel tunnel_id 1 peer_tunnel_id 1 udp_sport 5000 \
220 udp_dport 5000 encap udp local 192.168.1.1 remote 192.168.1.2
221# ip l2tp add session tunnel_id 1 session_id 1 peer_session_id 1
222# ifconfig -a
223# ip addr add 10.5.1.2/32 peer 10.5.1.1/32 dev l2tpeth0
224# ifconfig l2tpeth0 up
225
226Choose IP addresses to be the address of a local IP interface and that
227of the remote system. The IP addresses of the l2tpeth0 interface can be
228anything suitable.
229
230Repeat the above at the peer, with ports, tunnel/session ids and IP
231addresses reversed. The tunnel and session IDs can be any non-zero
23232-bit number, but the values must be reversed at the peer.
233
234Host 1 Host2
235udp_sport=5000 udp_sport=5001
236udp_dport=5001 udp_dport=5000
237tunnel_id=42 tunnel_id=45
238peer_tunnel_id=45 peer_tunnel_id=42
239session_id=128 session_id=5196755
240peer_session_id=5196755 peer_session_id=128
241
242When done at both ends of the tunnel, it should be possible to send
243data over the network. e.g.
244
245# ping 10.5.1.1
246
247
James Chapman58e50a92007-06-27 15:53:49 -0700248Sample Userspace Code
249=====================
250
2511. Create tunnel management PPPoX socket
252
253 kernel_fd = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OL2TP);
254 if (kernel_fd >= 0) {
255 struct sockaddr_pppol2tp sax;
256 struct sockaddr_in const *peer_addr;
257
258 peer_addr = l2tp_tunnel_get_peer_addr(tunnel);
259 memset(&sax, 0, sizeof(sax));
260 sax.sa_family = AF_PPPOX;
261 sax.sa_protocol = PX_PROTO_OL2TP;
262 sax.pppol2tp.fd = udp_fd; /* fd of tunnel UDP socket */
263 sax.pppol2tp.addr.sin_addr.s_addr = peer_addr->sin_addr.s_addr;
264 sax.pppol2tp.addr.sin_port = peer_addr->sin_port;
265 sax.pppol2tp.addr.sin_family = AF_INET;
266 sax.pppol2tp.s_tunnel = tunnel_id;
267 sax.pppol2tp.s_session = 0; /* special case: mgmt socket */
268 sax.pppol2tp.d_tunnel = 0;
269 sax.pppol2tp.d_session = 0; /* special case: mgmt socket */
270
271 if(connect(kernel_fd, (struct sockaddr *)&sax, sizeof(sax) ) < 0 ) {
272 perror("connect failed");
273 result = -errno;
274 goto err;
275 }
276 }
277
2782. Create session PPPoX data socket
279
280 struct sockaddr_pppol2tp sax;
281 int fd;
282
283 /* Note, the target socket must be bound already, else it will not be ready */
284 sax.sa_family = AF_PPPOX;
285 sax.sa_protocol = PX_PROTO_OL2TP;
286 sax.pppol2tp.fd = tunnel_fd;
287 sax.pppol2tp.addr.sin_addr.s_addr = addr->sin_addr.s_addr;
288 sax.pppol2tp.addr.sin_port = addr->sin_port;
289 sax.pppol2tp.addr.sin_family = AF_INET;
290 sax.pppol2tp.s_tunnel = tunnel_id;
291 sax.pppol2tp.s_session = session_id;
292 sax.pppol2tp.d_tunnel = peer_tunnel_id;
293 sax.pppol2tp.d_session = peer_session_id;
294
295 /* session_fd is the fd of the session's PPPoL2TP socket.
296 * tunnel_fd is the fd of the tunnel UDP socket.
297 */
298 fd = connect(session_fd, (struct sockaddr *)&sax, sizeof(sax));
299 if (fd < 0 ) {
300 return -errno;
301 }
302 return 0;
303
James Chapman2f77a3f2010-04-02 06:19:46 +0000304Internal Implementation
305=======================
James Chapman58e50a92007-06-27 15:53:49 -0700306
James Chapman2f77a3f2010-04-02 06:19:46 +0000307The driver keeps a struct l2tp_tunnel context per L2TP tunnel and a
308struct l2tp_session context for each session. The l2tp_tunnel is
309always associated with a UDP or L2TP/IP socket and keeps a list of
310sessions in the tunnel. The l2tp_session context keeps kernel state
311about the session. It has private data which is used for data specific
312to the session type. With L2TPv2, the session always carried PPP
313traffic. With L2TPv3, the session can also carry ethernet frames
314(ethernet pseudowire) or other data types such as ATM, HDLC or Frame
315Relay.
316
317When a tunnel is first opened, the reference count on the socket is
318increased using sock_hold(). This ensures that the kernel socket
319cannot be removed while L2TP's data structures reference it.
320
321Some L2TP sessions also have a socket (PPP pseudowires) while others
322do not (ethernet pseudowires). We can't use the socket reference count
323as the reference count for session contexts. The L2TP implementation
324therefore has its own internal reference counts on the session
325contexts.
326
327To Do
328=====
329
330Add L2TP tunnel switching support. This would route tunneled traffic
331from one L2TP tunnel into another. Specified in
332http://tools.ietf.org/html/draft-ietf-l2tpext-tunnel-switching-08
333
334Add L2TPv3 VLAN pseudowire support.
335
336Add L2TPv3 IP pseudowire support.
337
338Add L2TPv3 ATM pseudowire support.
339
340Miscellaneous
341=============
342
343The L2TP drivers were developed as part of the OpenL2TP project by
James Chapman58e50a92007-06-27 15:53:49 -0700344Katalix Systems Ltd. OpenL2TP is a full-featured L2TP client / server,
345designed from the ground up to have the L2TP datapath in the
346kernel. The project also implemented the pppol2tp plugin for pppd
347which allows pppd to use the kernel driver. Details can be found at
James Chapman2f77a3f2010-04-02 06:19:46 +0000348http://www.openl2tp.org.