Oof. Shame on me -- I have to read man more attentively.
/usr/local/sbinl2tpd -f -d 255
The "-f" tells it not to fork into the background, and to remain attached to the terminal.
OK. With "magic key" -f I receive in console "Error: Unable to set line discipline to N_HDLC" and with google help find your message in archive about this error. I need n_hdlc module loaded or builded into kernel, don't I.
I'll play with it and believe it helps me.
Many thanks for advise.
Regards.