The exit status of pppd is set to indicate whether any error was
detected, or the reason for the link being terminated. The
values used are:
0 Pppd has detached, or otherwise the connection was
successfully established and terminated at the peer's
request.
1 An immediately fatal error of some kind occurred, such as
an essential system call failing, or running out of
virtual memory.
2 An error was detected in processing the options given,
such as two mutually exclusive options being used.
3 Pppd is not setuid-root and the invoking user is not root.
4 The kernel does not support PPP, for example, the PPP
kernel driver is not included or cannot be loaded.
5 Pppd terminated because it was sent a SIGINT, SIGTERM or
SIGHUP signal.
6 The serial port could not be locked.
7 The serial port could not be opened.
8 The connect script failed (returned a non-zero exit
status).
9 The command specified as the argument to the pty option
could not be run.
10 The PPP negotiation failed, that is, it didn't reach the
point where at least one network protocol (e.g. IP) was
running.
11 The peer system failed (or refused) to authenticate
itself.
12 The link was established successfully and terminated
because it was idle.
13 The link was established successfully and terminated
because the connect time limit was reached.
14 Callback was negotiated and an incoming call should arrive
shortly.
15 The link was terminated because the peer is not responding
to echo requests.
16 The link was terminated by the modem hanging up.
17 The PPP negotiation failed because serial loopback was
detected.
18 The init script failed (returned a non-zero exit status).
19 We failed to authenticate ourselves to the peer.
