callback - call a user back, presenting a login prompt
Call the given phone number (if none is given on the command line, ask user for
one), and if a CONNECT is established, hand over control to mgetty(8) to
present user with a login name prompt.
is used for various purposes:
* security: make sure your users are who they pretend to be by calling a
well-known phone number.
* cost savings: make your company call you back.
can be called directly from the command line (but you must be
"root" to do this, otherwise callback can't signal mgetty), or from
mgetty's "login.config". See the login.config file shipped with
mgetty for an example.
- -x <debug level>
- Use the given level of verbosity for logging - 0 means no
logging, 5 is really noisy.
- Print version number and quit.
- Do not go into the background. This is helpful for
- -l <modem lines>
- Use the given modem lines. Multiple lines can be separated
by ":", as with sendfax(8). Example: callback -l
- -m <init sequence>
- Set the modem initialization sequence (as usual: expect
send expect ...). This can do nearly everything, as long as it leaves the
modem command responses on (that is, no ATQ1 here!) and switches the modem
to data mode (AT+FCLASS=0) if it is used in data/fax mode.
- -s <speed>
- This is the bit rate that should be used for the
machine-modem connection. Usually you'll set this via the "speed
<nnnn>" option in "callback.config".
- Use the line where callback is started from for dialing
out. Callback can make use of multiple modem lines, and with this options,
you can force it to use just one modem, the one where a call comes in.
will read all its configuration at run-time from a file, usually
called /etc/mgetty/callback.config. See the documentation in the mgetty.info
manual for details.
In most cases, callback
can't print any error messages to the console,
because it must detach itself immediately from the terminal, in case someone
wants to be called back on the modem line he called in. So, nothing to print
Because of this, all callback errors are logged to a protocol file (the extent
of the data written is controlled by the "-x" option), especially
including the reason why a call was not made, or what exactly failed.
Just two messages are printed on stdout, and those are self-explaining, a call
from a non-root user, and an invalid option.
How does it work?
This is a bit tricky, because of the way init(8) handles the utmp(5) file. You
can't just have any program ask the user for a login name, and then start a
"login shell", it won't work (this is for the same reason
has to be started from /etc/inittab).
So, mgetty has to do the "asking for login name". But I do not want to
have all that dialout code in mgetty, bloating it even more.
The way it works is this: callback
dials out on a modem device. It will
only take a modem device that has a mgetty watching over it (!). When the
connection is established (CONNECT), callback
will send a signal
SIGUSR1 to mgetty, which, in turn, will send the same signal back to signal
"I got your signal". callback
then exits, and mgetty
takes over the existing connection, prompts the user for a login name, and
forks off /bin/login.
Conclusion: this will not work with mgetty versions before February 04, 1996 (no
support for this signalling), and if it doesn't work for you, please send me
the mgetty and the callback log file, otherwise it's very hard to
find the bugs.
is "alpha" code, not very stable right now.
is fairly dumb concerning retries.
must be run as root.
Most of the documentation consists of "reading the source".
is Copyright (C) 1993-1996 by Gert Doering,