Add a patch from the isdn mailinglist, Juha writes:

ppp in 4.x apparently does a close(2) after opening the tun device;
 i4brunppp starts up with only file descriptors 0 and 1 open (to the rbch
 device) -> tun gets opened as 2 -> tun gets closed -> later use results
 in EBADF. A quick fix to i4brunppp.c makes the thing work (I know, this
 is ugly, but I needed it up quick...):

Submitted by:	Juha-Matti Liukkonen <jml@cubical.fi>

share/examples/isdn/i4brunppp/i4brunppp.c

@@ -164,6 +164,7 @@ main(int argc, char **argv)
 	i = getdtablesize();
 
 	for(;i >= 0; i--)
+           if (i != 2)
 		close(i);
 
 	/* fiddle a terminating zero after the rbch unit number */