Kernel support with an old kernel

[feba-code]

Describe the bug
I'm trying to get xl2tpd working on an embedded system. It is a pure L2TP, without IPSec.
Having kernel support switched off, it works just fine, but I'd really like to have it working with kernel support.

When there is a connection to be established, older versions of xl2tpd (1.2.0-1.3.1) report the following:

xl2tpd[20336]: start_pppd: Unable to connect PPPoL2TP socket.
xl2tpd[20336]: Call established with 192.168.0.10, Local: 60620, Remote: 5760, Serial: 2
xl2tpd[20336]: write_packet: tty is not open yet.
xl2tpd[20336]: write_packet: tty is not open yet...

When I use xl2tpd v1.3.6 this issue disappears, but connection is not established anyway:

xl2tpd[4538]: Connection established to 192.168.0.10, 1701. Local: 28339, Remote: 27256 (ref=0/0). LNS session is '2'
xl2tpd[4538]: control_finish: Warning: Peer did not specify transmit speed
xl2tpd[4538]: start_pppd: I'm running:
xl2tpd[4538]: "/usr/sbin/pppd"
xl2tpd[4538]: "passive"
xl2tpd[4538]: "nodetach"
xl2tpd[4538]: "10.2.2.22:10.2.2.2"
xl2tpd[4538]: "file"
xl2tpd[4538]: "/data/manager/config/options.lns.2"
xl2tpd[4538]: "plugin"
xl2tpd[4538]: "pppol2tp.so"
xl2tpd[4538]: "pppol2tp"
xl2tpd[4538]: "7"
xl2tpd[4538]: "pppol2tp_lns_mode"
xl2tpd[4538]: "pppol2tp_tunnel_id"
xl2tpd[4538]: "28339"
xl2tpd[4538]: "pppol2tp_session_id"
xl2tpd[4538]: "35470"
xl2tpd[4538]: Call established with 192.168.0.10, Local: 35470, Remote: 938, Serial: 3
xl2tpd[4538]: child_handler : pppd exited for call 938 with code 1
xl2tpd[4538]: call_close: Call 35470 to 192.168.0.10 disconnected
xl2tpd[4538]: Terminating pppd: sending KILL signal to pid 4859

In this case, there is a message from the pppol2tp driver: 'No PPPoL2TP FD specified'.

What should I do? Please help me. Thanks in advance!

To Reproduce
Try to connect a client to a server with kernel module pppol2tp being active.

Expected behavior
Connection is established.

l2tp detail:

• kernel version: [2.6.31]
• libc6 version: [2.6]
• x2ltpd version: [1.2.0 - 1.3.6]
• ppp version: [2.4.5]

xl2tpd.conf
Relevant kernel modules:

• CONFIG_PPP=y
• CONFIG_PPP_ASYNC=y
• CONFIG_PPP_MPPE=y
• CONFIG_PPP_SYNC_TTY=y
• CONFIG_PPP_DEFLATE=y
• CONFIG_PPPOE=y
• CONFIG_PPPOL2TP=y