The existing documentation was not telling that one has to create a PPP
channel and a PPP interface to get PPPoL2TP data offloading working.
Also, tunnel switching was not mentioned, so that people were thinking
it was not supported, while it actually is.
Signed-off-by: Samuel Thibault <samuel.thibault@xxxxxxxxxxxx>
---
Difference from v1:
- follow kernel coding style
- check for failures
- also mention netlink and ip for configuring the link
- fix bridging channels
Difference from v2:
- fix text alignment
Difference from v3:
- fix some variables references
- explicit inputs of the code snippets
- explicit that bridging is supported for l2tp with PPP pseudowire type.
- explicit that after bridging only the pppox sockets need to be kept
- explicit that bridging can also be done with other types of ppp
channels
--- a/Documentation/networking/l2tp.rst
+++ b/Documentation/networking/l2tp.rst
@@ -386,12 +386,17 @@ Sample userspace code:
- Create session PPPoX data socket::
+ /* Input: the L2TP tunnel UDP socket `tunnel_fd`, which needs to be
+ * bound already, otherwise it will not be ready. */
+
struct sockaddr_pppol2tp sax;
- int fd;
+ int session_fd;
+ int ret;
+
+ session_fd = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OL2TP);
+ if (session_fd < 0)
+ return -errno;
- /* Note, the tunnel socket must be bound already, else it
- * will not be ready
- */
sax.sa_family = AF_PPPOX;
sax.sa_protocol = PX_PROTO_OL2TP;
sax.pppol2tp.fd = tunnel_fd;
@@ -406,11 +411,117 @@ Sample userspace code:
/* session_fd is the fd of the session's PPPoL2TP socket.
* tunnel_fd is the fd of the tunnel UDP / L2TPIP socket.
*/
- fd = connect(session_fd, (struct sockaddr *)&sax, sizeof(sax));
- if (fd < 0 ) {
+ ret = connect(session_fd, (struct sockaddr *)&sax, sizeof(sax));
+ if (ret < 0 ) {
+ close(session_fd);
+ return -errno;
+ }
+
+ return session_fd;
+
+L2TP control packets will still be available for read on `tunnel_fd`.
+
+ - Create PPP channel::
+
+ /* Input: the session PPPoX data socket session_fd which was created as
+ * described above. */
+
+ int chindx;
+ int ppp_chan_fd;
+
+ ret = ioctl(session_fd, PPPIOCGCHAN, &chindx);
+ if (ret < 0)
+ return -errno;
+
+ ppp_chan_fd = open("/dev/ppp", O_RDWR);
+ if (ppp_chan_fd < 0)
+ return -errno;
+
+ ret = ioctl(ppp_chan_fd, PPPIOCATTCHAN, &chindx);
+ if (ret < 0) {
+ close(ppp_chan_fd);
+ return -errno;
+ }
+
+ return ppp_chan_fd;
+
+LCP PPP frames will be available for read on `ppp_chan_fd`.
+
+ - Create PPP interface::
+
+ /* Input: the PPP channel ppp_chan_fd which was created as described
+ * above */
+
+ int ppp_if_fd;
+ int ifunit = -1;
+
+ ppp_if_fd = open("/dev/ppp", O_RDWR);
+ if (ppp_if_fd < 0)
+ return -errno;
+
+ ret = ioctl(ppp_if_fd, PPPIOCNEWUNIT, &ifunit);
+ if (ret < 0) {
+ close(ppp_if_fd);
+ return -errno;
+ }
+
+ ret = ioctl(ppp_chan_fd, PPPIOCCONNECT, ifunit);
+ if (ret < 0) {
+ close(ppp_if_fd);
return -errno;
}
- return 0;
+
+ return ppp_if_fd;
+
+IPCP/IPv6CP PPP frames will be available for read on `ppp_if_fd`.
+
+The ppp<ifunit> interface can then be configured as usual with netlink's
+RTM_NEWLINK, RTM_NEWADDR, RTM_NEWROUTE, or ioctl's SIOCSIFMTU, SIOCSIFADDR,
+SIOCSIFDSTADDR, SIOCSIFNETMASK, SIOCSIFFLAGS, or with the `ip` command.
+
+ - Bridging L2TP sessions which have PPP pseudowire types (this is also called
+ L2TP tunnel switching or L2TP multihop) is supported by bridging the ppp
+ channels of the two L2TP sessions to be bridged::
+
+ /* Input: the session PPPoX data sockets session_fd1 and session_fd2
+ * which were created as described further above. */
+
+ int chindx1;
+ int chindx2;
+ int ppp_chan_fd;
+
+ ret = ioctl(session_fd1, PPPIOCGCHAN, &chindx1);
+ if (ret < 0)
+ return -errno;
+
+ ret = ioctl(session_fd2, PPPIOCGCHAN, &chindx2);
+ if (ret < 0)
+ return -errno;
+
+ ppp_chan_fd = open("/dev/ppp", O_RDWR);
+ if (ppp_chan_fd < 0) {
+ return -errno;
+ }
+
+ ret = ioctl(ppp_chan_fd, PPPIOCATTCHAN, &chindx1);
+ if (ret < 0) {
+ close(ppp_chan_fd);
+ return -errno;
+ }
+
+ ret = ioctl(ppp_chan_fd, PPPIOCBRIDGECHAN, &chindx2);
+ close(ppp_chan_fd);
+ if (ret < 0)
+ return -errno;
+
+It can be noted that in this case no PPP interface is needed, and the PPP
+channel does not need to be kept open. Only the session PPPoX data sockets need
+to be kept open.
+
+More generally, it is also possible in the same way to e.g. bridge a PPPol2tp
+ppp channel with other types of ppp channels, such as PPPoE.
+
+See more details for the PPP side in ppp_generic.rst.
Old L2TPv2-only API
-------------------
