170 lines
7.3 KiB
Text
170 lines
7.3 KiB
Text
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This brief document describes how to use the kernel's PPPoL2TP driver
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to provide L2TP functionality. L2TP is a protocol that tunnels one or
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more PPP sessions over a UDP 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.
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Design
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======
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The PPPoL2TP driver, drivers/net/pppol2tp.c, provides a mechanism by
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which PPP frames carried through an L2TP session are passed through
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the kernel's PPP subsystem. The standard PPP daemon, pppd, handles all
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PPP interaction with the peer. PPP network interfaces are created for
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each local PPP endpoint.
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The L2TP protocol http://www.faqs.org/rfcs/rfc2661.html defines L2TP
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control and data frames. L2TP control frames carry messages between
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L2TP clients/servers and are used to setup / teardown tunnels and
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sessions. An L2TP client or server is implemented in userspace and
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will use a regular UDP socket per tunnel. L2TP data frames carry PPP
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frames, which may be PPP control or PPP data. 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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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. The pppol2tp driver uses them to lookup
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internal tunnel and/or session contexts. Zero tunnel / session ids are
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treated specially - zero ids are never assigned to tunnels or sessions
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in the network. In the driver, the tunnel context keeps a pointer to
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the tunnel UDP socket. The session context keeps a pointer to the
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PPPoL2TP socket, as well as other data that lets the driver interface
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to the kernel PPP subsystem.
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Note that the pppol2tp kernel driver handles only L2TP data frames;
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L2TP control frames are simply passed up to userspace in the UDP
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tunnel socket. The kernel handles all datapath aspects of the
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protocol, including data packet resequencing (if enabled).
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There are a number of requirements on the userspace L2TP daemon in
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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_ppp.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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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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PPPOL2TP_MSG_DEBUG verbose debug (if compiled in)
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PPPOL2TP_MSG_CONTROL userspace - kernel interface
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PPPOL2TP_MSG_SEQ sequence numbers handling
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PPPOL2TP_MSG_DATA data packets
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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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Miscellanous
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============
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The PPPoL2TP driver was 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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which allows pppd to use the kernel driver. Details can be found at
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http://openl2tp.sourceforge.net.
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