callback(8) mgetty+callback manual callback(8)
callback - call a user back, presenting a login prompt
callback [-x<debuglevel>] [-V] [-l<modemlines>] [-m<initstring>]
[-s<speed>] [-d] [-S] [phone-number]
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.
callback 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.
callback 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.
-V Print version number and quit.
-d Do not go into the background. This is helpful for debug-
-l <modem lines>
Use the given modem lines. Multiple lines can be sepa-
rated 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.
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".
-S 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.
callback will read all its configuration at run-time from a
file, usually called /etc/mgetty+sendfax/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 ter-
minal, in case someone wants to be called back on the modem line
he called in. So, nothing to print messages to...
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 mgetty(8) has to be started from
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
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 BOTH the mgetty and the
callback log file, otherwise it’s very hard to find the bugs.
callback is "alpha" code, not very stable right now.
callback is fairly dumb concerning retries.
callback must be run as root.
Most of the documentation consists of "reading the source".
callback is Copyright (C) 1993-1996 by Gert Doering, <gert@gree-
greenie 27 Oct 93 callback(8)
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