[PATCH] PPPoL2TP: Add more code snippets

Samuel Thibault
2023-04-16T22:07:22Z

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 described, so that people were thinking
it was not supported, while it actually is.

Signed-off-by: Samuel Thibault <samuel.thibault@ens-lyon.org>

---
Documentation/networking/l2tp.rst | 59 ++++++++++++++++++++++++++++++++++++--
1 file changed, 56 insertions(+), 3 deletions(-)

--- a/Documentation/networking/l2tp.rst
+++ b/Documentation/networking/l2tp.rst
@@ -387,11 +387,12 @@ Sample userspace code:
- Create session PPPoX data socket::

struct sockaddr_pppol2tp sax;
- int fd;
+ int ret;

/* Note, the tunnel socket must be bound already, else it
* will not be ready
*/
+ int session_fd = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OL2TP);
sax.sa_family = AF_PPPOX;
sax.sa_protocol = PX_PROTO_OL2TP;
sax.pppol2tp.fd = tunnel_fd;
@@ -406,12 +407,64 @@ 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 ) {
return -errno;
}
return 0;

+ - Create PPP channel::
+
+ int chindx;
+ ret = ioctl(session_fd, PPPIOCGCHAN, &chindx);
+ if (ret < 0)
+ return -errno;
+
+ int ppp_chan_fd = open("/dev/ppp", O_RDWR);
+
+ ret = ioctl(ppp_chan_fd, PPPIOCATTCHAN, &chindx);
+ if (ret < 0)
+ return -errno;
+
+Non-data PPP frames will be available for read on `ppp_chan_fd`.
+
+ - Create PPP interface::
+
+ int ppp_if_fd = open("/dev/ppp", O_RDWR);
+
+ int ifunit;
+ ret = ioctl(ppp_if_fd, PPPIOCNEWUNIT, &ifunit);
+ if (ret < 0)
+ return -errno;
+
+ ret = ioctl(ppp_chan_fd, PPPIOCCONNECT, ifunit);
+ if (ret < 0)
+ return -errno;
+
+The ppp<ifunit> interface can then be configured as usual with SIOCSIFMTU,
+SIOCSIFADDR, SIOCSIFDSTADDR, SIOCSIFNETMASK, and activated by setting IFF_UP
+with SIOCSIFFLAGS
+
+ - Tunnel switching is supported by bridging channels::
+
+ int chindx;
+ ret = ioctl(session_fd, PPPIOCGCHAN, &chindx);
+ if (ret < 0)
+ return -errno;
+
+ int chindx2;
+ ret = ioctl(session_fd2, PPPIOCGCHAN, &chind2x);
+ if (ret < 0)
+ return -errno;
+
+ int ppp_chan_fd = open("/dev/ppp", O_RDWR);
+
+ ret = ioctl(ppp_chan_fd, PPPIOCBRIDGECHAN, &chindx2);
+ if (ret < 0)
+ return -errno;
+
+ close(ppp_chan_fd);
+
Old L2TPv2-only API
-------------------