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Based on kernel version 4.16.1. Page generated on 2018-04-09 11:53 EST.

1	This document describes how to use the kernel's L2TP drivers to
2	provide L2TP functionality. L2TP is a protocol that tunnels one or
3	more sessions over an IP tunnel. It is commonly used for VPNs
4	(L2TP/IPSec) and by ISPs to tunnel subscriber PPP sessions over an IP
5	network infrastructure. With L2TPv3, it is also useful as a Layer-2
6	tunneling infrastructure.
8	Features
9	========
11	L2TPv2 (PPP over L2TP (UDP tunnels)).
12	L2TPv3 ethernet pseudowires.
13	L2TPv3 PPP pseudowires.
14	L2TPv3 IP encapsulation.
15	Netlink sockets for L2TPv3 configuration management.
17	History
18	=======
20	The original pppol2tp driver was introduced in 2.6.23 and provided
21	L2TPv2 functionality (rfc2661). L2TPv2 is used to tunnel one or more PPP
22	sessions over a UDP tunnel.
24	L2TPv3 (rfc3931) changes the protocol to allow different frame types
25	to be passed over an L2TP tunnel by moving the PPP-specific parts of
26	the protocol out of the core L2TP packet headers. Each frame type is
27	known as a pseudowire type. Ethernet, PPP, HDLC, Frame Relay and ATM
28	pseudowires for L2TP are defined in separate RFC standards. Another
29	change for L2TPv3 is that it can be carried directly over IP with no
30	UDP header (UDP is optional). It is also possible to create static
31	unmanaged L2TPv3 tunnels manually without a control protocol
32	(userspace daemon) to manage them.
34	To support L2TPv3, the original pppol2tp driver was split up to
35	separate the L2TP and PPP functionality. Existing L2TPv2 userspace
36	apps should be unaffected as the original pppol2tp sockets API is
37	retained. L2TPv3, however, uses netlink to manage L2TPv3 tunnels and
38	sessions.
40	Design
41	======
43	The L2TP protocol separates control and data frames.  The L2TP kernel
44	drivers handle only L2TP data frames; control frames are always
45	handled by userspace. L2TP control frames carry messages between L2TP
46	clients/servers and are used to setup / teardown tunnels and
47	sessions. An L2TP client or server is implemented in userspace.
49	Each L2TP tunnel is implemented using a UDP or L2TPIP socket; L2TPIP
50	provides L2TPv3 IP encapsulation (no UDP) and is implemented using a
51	new l2tpip socket family. The tunnel socket is typically created by
52	userspace, though for unmanaged L2TPv3 tunnels, the socket can also be
53	created by the kernel. Each L2TP session (pseudowire) gets a network
54	interface instance. In the case of PPP, these interfaces are created
55	indirectly by pppd using a pppol2tp socket. In the case of ethernet,
56	the netdevice is created upon a netlink request to create an L2TPv3
57	ethernet pseudowire.
59	For PPP, the PPPoL2TP driver, net/l2tp/l2tp_ppp.c, provides a
60	mechanism by which PPP frames carried through an L2TP session are
61	passed through the kernel's PPP subsystem. The standard PPP daemon,
62	pppd, handles all PPP interaction with the peer. PPP network
63	interfaces are created for each local PPP endpoint. The kernel's PPP
64	subsystem arranges for PPP control frames to be delivered to pppd,
65	while data frames are forwarded as usual.
67	For ethernet, the L2TPETH driver, net/l2tp/l2tp_eth.c, implements a
68	netdevice driver, managing virtual ethernet devices, one per
69	pseudowire. These interfaces can be managed using standard Linux tools
70	such as "ip" and "ifconfig". If only IP frames are passed over the
71	tunnel, the interface can be given an IP addresses of itself and its
72	peer. If non-IP frames are to be passed over the tunnel, the interface
73	can be added to a bridge using brctl. All L2TP datapath protocol
74	functions are handled by the L2TP core driver.
76	Each tunnel and session within a tunnel is assigned a unique tunnel_id
77	and session_id. These ids are carried in the L2TP header of every
78	control and data packet. (Actually, in L2TPv3, the tunnel_id isn't
79	present in data frames - it is inferred from the IP connection on
80	which the packet was received.) The L2TP driver uses the ids to lookup
81	internal tunnel and/or session contexts to determine how to handle the
82	packet. Zero tunnel / session ids are treated specially - zero ids are
83	never assigned to tunnels or sessions in the network. In the driver,
84	the tunnel context keeps a reference to the tunnel UDP or L2TPIP
85	socket. The session context holds data that lets the driver interface
86	to the kernel's network frame type subsystems, i.e. PPP, ethernet.
88	Userspace Programming
89	=====================
91	For L2TPv2, there are a number of requirements on the userspace L2TP
92	daemon in order to use the pppol2tp driver.
94	1. Use a UDP socket per tunnel.
96	2. 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.
105	3. 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.
110	When creating PPPoL2TP sockets, the application provides information
111	to the driver about the socket in a socket connect() call. Source and
112	destination tunnel and session ids are provided, as well as the file
113	descriptor of a UDP socket. See struct pppol2tp_addr in
114	include/linux/if_pppol2tp.h. Note that zero tunnel / session ids are
115	treated specially. When creating the per-tunnel PPPoL2TP management
116	socket in Step 2 above, zero source and destination session ids are
117	specified, which tells the driver to prepare the supplied UDP file
118	descriptor for use as an L2TP tunnel socket.
120	Userspace may control behavior of the tunnel or session using
121	setsockopt and ioctl on the PPPoX socket. The following socket
122	options are supported:-
124	DEBUG     - bitmask of debug message categories. See below.
125	SENDSEQ   - 0 => don't send packets with sequence numbers
126	            1 => send packets with sequence numbers
127	RECVSEQ   - 0 => receive packet sequence numbers are optional
128	            1 => drop receive packets without sequence numbers
129	LNSMODE   - 0 => act as LAC.
130	            1 => act as LNS.
131	REORDERTO - reorder timeout (in millisecs). If 0, don't try to reorder.
133	Only the DEBUG option is supported by the special tunnel management
134	PPPoX socket.
136	In addition to the standard PPP ioctls, a PPPIOCGL2TPSTATS is provided
137	to retrieve tunnel and session statistics from the kernel using the
138	PPPoX socket of the appropriate tunnel or session.
140	For L2TPv3, userspace must use the netlink API defined in
141	include/linux/l2tp.h to manage tunnel and session contexts. The
142	general procedure to create a new L2TP tunnel with one session is:-
144	1. Open a GENL socket using L2TP_GENL_NAME for configuring the kernel
145	   using netlink.
147	2. Create a UDP or L2TPIP socket for the tunnel.
149	3. 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.
153	4. Create a new L2TP session in the tunnel using a
154	   L2TP_CMD_SESSION_CREATE request.
156	The tunnel and all of its sessions are closed when the tunnel socket
157	is closed. The netlink API may also be used to delete sessions and
158	tunnels. Configuration and status info may be set or read using netlink.
160	The L2TP driver also supports static (unmanaged) L2TPv3 tunnels. These
161	are where there is no L2TP control message exchange with the peer to
162	setup the tunnel; the tunnel is configured manually at each end of the
163	tunnel. There is no need for an L2TP userspace application in this
164	case -- the tunnel socket is created by the kernel and configured
165	using parameters sent in the L2TP_CMD_TUNNEL_CREATE netlink
166	request. The "ip" utility of iproute2 has commands for managing static
167	L2TPv3 tunnels; do "ip l2tp help" for more information.
169	Debugging
170	=========
172	The driver supports a flexible debug scheme where kernel trace
173	messages may be optionally enabled per tunnel and per session. Care is
174	needed when debugging a live system since the messages are not
175	rate-limited and a busy system could be swamped. Userspace uses
176	setsockopt on the PPPoX socket to set a debug mask.
178	The following debug mask bits are available:
180	L2TP_MSG_DEBUG    verbose debug (if compiled in)
181	L2TP_MSG_CONTROL  userspace - kernel interface
182	L2TP_MSG_SEQ      sequence numbers handling
183	L2TP_MSG_DATA     data packets
185	If enabled, files under a l2tp debugfs directory can be used to dump
186	kernel state about L2TP tunnels and sessions. To access it, the
187	debugfs filesystem must first be mounted.
189	# mount -t debugfs debugfs /debug
191	Files under the l2tp directory can then be accessed.
193	# cat /debug/l2tp/tunnels
195	The debugfs files should not be used by applications to obtain L2TP
196	state information because the file format is subject to change. It is
197	implemented to provide extra debug information to help diagnose
198	problems.) Users should use the netlink API.
200	/proc/net/pppol2tp is also provided for backwards compatibility with
201	the original pppol2tp driver. It lists information about L2TPv2
202	tunnels and sessions only. Its use is discouraged.
204	Unmanaged L2TPv3 Tunnels
205	========================
207	Some commercial L2TP products support unmanaged L2TPv3 ethernet
208	tunnels, where there is no L2TP control protocol; tunnels are
209	configured at each side manually. New commands are available in
210	iproute2's ip utility to support this.
212	To create an L2TPv3 ethernet pseudowire between local host
213	and peer, using IP addresses and for the
214	tunnel endpoints:-
216	# ip l2tp add tunnel tunnel_id 1 peer_tunnel_id 1 udp_sport 5000 \
217	  udp_dport 5000 encap udp local remote
218	# ip l2tp add session tunnel_id 1 session_id 1 peer_session_id 1
219	# ip -s -d show dev l2tpeth0
220	# ip addr add peer dev l2tpeth0
221	# ip li set dev l2tpeth0 up
223	Choose IP addresses to be the address of a local IP interface and that
224	of the remote system. The IP addresses of the l2tpeth0 interface can be
225	anything suitable.
227	Repeat the above at the peer, with ports, tunnel/session ids and IP
228	addresses reversed.  The tunnel and session IDs can be any non-zero
229	32-bit number, but the values must be reversed at the peer.
231	Host 1                         Host2
232	udp_sport=5000                 udp_sport=5001
233	udp_dport=5001                 udp_dport=5000
234	tunnel_id=42                   tunnel_id=45
235	peer_tunnel_id=45              peer_tunnel_id=42
236	session_id=128                 session_id=5196755
237	peer_session_id=5196755        peer_session_id=128
239	When done at both ends of the tunnel, it should be possible to send
240	data over the network. e.g.
242	# ping
245	Sample Userspace Code
246	=====================
248	1. Create tunnel management PPPoX socket
250	        kernel_fd = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OL2TP);
251	        if (kernel_fd >= 0) {
252	                struct sockaddr_pppol2tp sax;
253	                struct sockaddr_in const *peer_addr;
255	                peer_addr = l2tp_tunnel_get_peer_addr(tunnel);
256	                memset(&sax, 0, sizeof(sax));
257	                sax.sa_family = AF_PPPOX;
258	                sax.sa_protocol = PX_PROTO_OL2TP;
259	                sax.pppol2tp.fd = udp_fd;       /* fd of tunnel UDP socket */
260	                sax.pppol2tp.addr.sin_addr.s_addr = peer_addr->sin_addr.s_addr;
261	                sax.pppol2tp.addr.sin_port = peer_addr->sin_port;
262	                sax.pppol2tp.addr.sin_family = AF_INET;
263	                sax.pppol2tp.s_tunnel = tunnel_id;
264	                sax.pppol2tp.s_session = 0;     /* special case: mgmt socket */
265	                sax.pppol2tp.d_tunnel = 0;
266	                sax.pppol2tp.d_session = 0;     /* special case: mgmt socket */
268	                if(connect(kernel_fd, (struct sockaddr *)&sax, sizeof(sax) ) < 0 ) {
269	                        perror("connect failed");
270	                        result = -errno;
271	                        goto err;
272	                }
273	        }
275	2. Create session PPPoX data socket
277	        struct sockaddr_pppol2tp sax;
278	        int fd;
280	        /* Note, the target socket must be bound already, else it will not be ready */
281	        sax.sa_family = AF_PPPOX;
282	        sax.sa_protocol = PX_PROTO_OL2TP;
283	        sax.pppol2tp.fd = tunnel_fd;
284	        sax.pppol2tp.addr.sin_addr.s_addr = addr->sin_addr.s_addr;
285	        sax.pppol2tp.addr.sin_port = addr->sin_port;
286	        sax.pppol2tp.addr.sin_family = AF_INET;
287	        sax.pppol2tp.s_tunnel  = tunnel_id;
288	        sax.pppol2tp.s_session = session_id;
289	        sax.pppol2tp.d_tunnel  = peer_tunnel_id;
290	        sax.pppol2tp.d_session = peer_session_id;
292	        /* session_fd is the fd of the session's PPPoL2TP socket.
293	         * tunnel_fd is the fd of the tunnel UDP socket.
294	         */
295	        fd = connect(session_fd, (struct sockaddr *)&sax, sizeof(sax));
296	        if (fd < 0 )    {
297	                return -errno;
298	        }
299	        return 0;
301	Internal Implementation
302	=======================
304	The driver keeps a struct l2tp_tunnel context per L2TP tunnel and a
305	struct l2tp_session context for each session. The l2tp_tunnel is
306	always associated with a UDP or L2TP/IP socket and keeps a list of
307	sessions in the tunnel. The l2tp_session context keeps kernel state
308	about the session. It has private data which is used for data specific
309	to the session type. With L2TPv2, the session always carried PPP
310	traffic. With L2TPv3, the session can also carry ethernet frames
311	(ethernet pseudowire) or other data types such as ATM, HDLC or Frame
312	Relay.
314	When a tunnel is first opened, the reference count on the socket is
315	increased using sock_hold(). This ensures that the kernel socket
316	cannot be removed while L2TP's data structures reference it.
318	Some L2TP sessions also have a socket (PPP pseudowires) while others
319	do not (ethernet pseudowires). We can't use the socket reference count
320	as the reference count for session contexts. The L2TP implementation
321	therefore has its own internal reference counts on the session
322	contexts.
324	To Do
325	=====
327	Add L2TP tunnel switching support. This would route tunneled traffic
328	from one L2TP tunnel into another. Specified in
329	http://tools.ietf.org/html/draft-ietf-l2tpext-tunnel-switching-08
331	Add L2TPv3 VLAN pseudowire support.
333	Add L2TPv3 IP pseudowire support.
335	Add L2TPv3 ATM pseudowire support.
337	Miscellaneous
338	=============
340	The L2TP drivers were developed as part of the OpenL2TP project by
341	Katalix Systems Ltd. OpenL2TP is a full-featured L2TP client / server,
342	designed from the ground up to have the L2TP datapath in the
343	kernel. The project also implemented the pppol2tp plugin for pppd
344	which allows pppd to use the kernel driver. Details can be found at
345	http://www.openl2tp.org.
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