From 6d5c36565c169376e3c5fc54d01d7c6819381465 Mon Sep 17 00:00:00 2001 From: Samuel Thibault Date: Sat, 17 Feb 2024 22:14:25 +0100 Subject: [PATCH] PPPoL2TP: Add more code snippets 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 Acked-by: Tom Parkin Link: https://lore.kernel.org/r/20240217211425.qj576u3jmaa6yidf@begin Signed-off-by: Jakub Kicinski --- Documentation/networking/l2tp.rst | 137 ++++++++++++++++++++++++++++-- 1 file changed, 130 insertions(+), 7 deletions(-) diff --git a/Documentation/networking/l2tp.rst b/Documentation/networking/l2tp.rst index 7f383e99dbad..8496b467dea4 100644 --- a/Documentation/networking/l2tp.rst +++ b/Documentation/networking/l2tp.rst @@ -386,12 +386,19 @@ Sample userspace code: - Create session PPPoX data socket:: - struct sockaddr_pppol2tp sax; - int fd; - - /* Note, the tunnel socket must be bound already, else it - * will not be ready + /* Input: the L2TP tunnel UDP socket `tunnel_fd`, which needs to be + * bound already (both sockname and peername), otherwise it will not be + * ready. */ + + struct sockaddr_pppol2tp sax; + int session_fd; + int ret; + + session_fd = socket(AF_PPPOX, SOCK_DGRAM, PX_PROTO_OL2TP); + if (session_fd < 0) + return -errno; + sax.sa_family = AF_PPPOX; sax.sa_protocol = PX_PROTO_OL2TP; sax.pppol2tp.fd = tunnel_fd; @@ -406,12 +413,128 @@ 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 ppp_chan_fd; + int chindx; + int ret; + + 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 ifunit = -1; + int ppp_if_fd; + int ret; + + 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 ppp_if_fd; + +IPCP/IPv6CP PPP frames will be available for read on `ppp_if_fd`. + +The ppp 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 ppp_chan_fd; + int chindx1; + int chindx2; + int ret; + + 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; + return 0; +It can be noted that when bridging PPP channels, the PPP session is not locally +terminated, and no local PPP interface is created. PPP frames arriving on one +channel are directly passed to the other channel, and vice versa. + +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 -------------------