1 This brief document describes how to use the kernel's PPPoL2TP driver
2 to provide L2TP functionality. L2TP is a protocol that tunnels one or
3 more PPP sessions over a UDP tunnel. It is commonly used for VPNs
4 (L2TP/IPSec) and by ISPs to tunnel subscriber PPP sessions over an IP
5 network infrastructure.
10 The PPPoL2TP driver, drivers/net/pppol2tp.c, provides a mechanism by
11 which PPP frames carried through an L2TP session are passed through
12 the kernel's PPP subsystem. The standard PPP daemon, pppd, handles all
13 PPP interaction with the peer. PPP network interfaces are created for
14 each local PPP endpoint.
16 The L2TP protocol http://www.faqs.org/rfcs/rfc2661.html defines L2TP
17 control and data frames. L2TP control frames carry messages between
18 L2TP clients/servers and are used to setup / teardown tunnels and
19 sessions. An L2TP client or server is implemented in userspace and
20 will use a regular UDP socket per tunnel. L2TP data frames carry PPP
21 frames, which may be PPP control or PPP data. The kernel's PPP
22 subsystem arranges for PPP control frames to be delivered to pppd,
23 while data frames are forwarded as usual.
25 Each tunnel and session within a tunnel is assigned a unique tunnel_id
26 and session_id. These ids are carried in the L2TP header of every
27 control and data packet. The pppol2tp driver uses them to lookup
28 internal tunnel and/or session contexts. Zero tunnel / session ids are
29 treated specially - zero ids are never assigned to tunnels or sessions
30 in the network. In the driver, the tunnel context keeps a pointer to
31 the tunnel UDP socket. The session context keeps a pointer to the
32 PPPoL2TP socket, as well as other data that lets the driver interface
33 to the kernel PPP subsystem.
35 Note that the pppol2tp kernel driver handles only L2TP data frames;
36 L2TP control frames are simply passed up to userspace in the UDP
37 tunnel socket. The kernel handles all datapath aspects of the
38 protocol, including data packet resequencing (if enabled).
40 There are a number of requirements on the userspace L2TP daemon in
41 order to use the pppol2tp driver.
43 1. Use a UDP socket per tunnel.
45 2. Create a single PPPoL2TP socket per tunnel bound to a special null
46 session id. This is used only for communicating with the driver but
47 must remain open while the tunnel is active. Opening this tunnel
48 management socket causes the driver to mark the tunnel socket as an
49 L2TP UDP encapsulation socket and flags it for use by the
50 referenced tunnel id. This hooks up the UDP receive path via
51 udp_encap_rcv() in net/ipv4/udp.c. PPP data frames are never passed
52 in this special PPPoX socket.
54 3. Create a PPPoL2TP socket per L2TP session. This is typically done
55 by starting pppd with the pppol2tp plugin and appropriate
56 arguments. A PPPoL2TP tunnel management socket (Step 2) must be
57 created before the first PPPoL2TP session socket is created.
59 When creating PPPoL2TP sockets, the application provides information
60 to the driver about the socket in a socket connect() call. Source and
61 destination tunnel and session ids are provided, as well as the file
62 descriptor of a UDP socket. See struct pppol2tp_addr in
63 include/linux/if_ppp.h. Note that zero tunnel / session ids are
64 treated specially. When creating the per-tunnel PPPoL2TP management
65 socket in Step 2 above, zero source and destination session ids are
66 specified, which tells the driver to prepare the supplied UDP file
67 descriptor for use as an L2TP tunnel socket.
69 Userspace may control behavior of the tunnel or session using
70 setsockopt and ioctl on the PPPoX socket. The following socket
71 options are supported:-
73 DEBUG - bitmask of debug message categories. See below.
74 SENDSEQ - 0 => don't send packets with sequence numbers
75 1 => send packets with sequence numbers
76 RECVSEQ - 0 => receive packet sequence numbers are optional
77 1 => drop receive packets without sequence numbers
78 LNSMODE - 0 => act as LAC.
80 REORDERTO - reorder timeout (in millisecs). If 0, don't try to reorder.
82 Only the DEBUG option is supported by the special tunnel management
85 In addition to the standard PPP ioctls, a PPPIOCGL2TPSTATS is provided
86 to retrieve tunnel and session statistics from the kernel using the
87 PPPoX socket of the appropriate tunnel or session.
92 The driver supports a flexible debug scheme where kernel trace
93 messages may be optionally enabled per tunnel and per session. Care is
94 needed when debugging a live system since the messages are not
95 rate-limited and a busy system could be swamped. Userspace uses
96 setsockopt on the PPPoX socket to set a debug mask.
98 The following debug mask bits are available:
100 PPPOL2TP_MSG_DEBUG verbose debug (if compiled in)
101 PPPOL2TP_MSG_CONTROL userspace - kernel interface
102 PPPOL2TP_MSG_SEQ sequence numbers handling
103 PPPOL2TP_MSG_DATA data packets
105 Sample Userspace Code
106 =====================
108 1. Create tunnel management PPPoX socket
110 kernel_fd = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OL2TP);
111 if (kernel_fd >= 0) {
112 struct sockaddr_pppol2tp sax;
113 struct sockaddr_in const *peer_addr;
115 peer_addr = l2tp_tunnel_get_peer_addr(tunnel);
116 memset(&sax, 0, sizeof(sax));
117 sax.sa_family = AF_PPPOX;
118 sax.sa_protocol = PX_PROTO_OL2TP;
119 sax.pppol2tp.fd = udp_fd; /* fd of tunnel UDP socket */
120 sax.pppol2tp.addr.sin_addr.s_addr = peer_addr->sin_addr.s_addr;
121 sax.pppol2tp.addr.sin_port = peer_addr->sin_port;
122 sax.pppol2tp.addr.sin_family = AF_INET;
123 sax.pppol2tp.s_tunnel = tunnel_id;
124 sax.pppol2tp.s_session = 0; /* special case: mgmt socket */
125 sax.pppol2tp.d_tunnel = 0;
126 sax.pppol2tp.d_session = 0; /* special case: mgmt socket */
128 if(connect(kernel_fd, (struct sockaddr *)&sax, sizeof(sax) ) < 0 ) {
129 perror("connect failed");
135 2. Create session PPPoX data socket
137 struct sockaddr_pppol2tp sax;
140 /* Note, the target socket must be bound already, else it will not be ready */
141 sax.sa_family = AF_PPPOX;
142 sax.sa_protocol = PX_PROTO_OL2TP;
143 sax.pppol2tp.fd = tunnel_fd;
144 sax.pppol2tp.addr.sin_addr.s_addr = addr->sin_addr.s_addr;
145 sax.pppol2tp.addr.sin_port = addr->sin_port;
146 sax.pppol2tp.addr.sin_family = AF_INET;
147 sax.pppol2tp.s_tunnel = tunnel_id;
148 sax.pppol2tp.s_session = session_id;
149 sax.pppol2tp.d_tunnel = peer_tunnel_id;
150 sax.pppol2tp.d_session = peer_session_id;
152 /* session_fd is the fd of the session's PPPoL2TP socket.
153 * tunnel_fd is the fd of the tunnel UDP socket.
155 fd = connect(session_fd, (struct sockaddr *)&sax, sizeof(sax));
164 The PPPoL2TP driver was developed as part of the OpenL2TP project by
165 Katalix Systems Ltd. OpenL2TP is a full-featured L2TP client / server,
166 designed from the ground up to have the L2TP datapath in the
167 kernel. The project also implemented the pppol2tp plugin for pppd
168 which allows pppd to use the kernel driver. Details can be found at
169 http://openl2tp.sourceforge.net.