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callback(8) mgetty+callback manual callback(8)
NAME
callback - call a user back, presenting a login prompt
SYNOPSIS
callback [-x<debuglevel>] [-V] [-l<modemlines>] [-m<initstring>] [-s<speed>] [-d]
[-S] [phone-number]
DESCRIPTION
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.
OPTIONS
-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 debugging.
-l <modem lines>
Use the given modem lines. Multiple lines can be separated by ":", as
with sendfax(8). Example: callback -l tty1a:tty2a
-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 con-
nection. 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.
CONFIG FILE
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.
DIAGNOSTICS
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
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.
INTERNALS
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 /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 connec-
tion 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 connec-
tion, 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.
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".
SEE ALSO
mgetty(8), ct(1)
AUTHOR
callback is Copyright (C) 1993-1996 by Gert Doering, <gert AT greenie.de>.
greenie 27 Oct 93 callback(8)
Generated by $Id: phpMan.php,v 4.55 2007/09/05 04:42:51 chedong Exp $ Author: Che Dong
On Apache/1.3.41 (Unix) PHP/5.2.5 mod_perl/1.30 mod_gzip/1.3.26.1a
Under GNU General Public License
2009-01-08 07:57 @38.103.63.58 CrawledBy CCBot/1.0 (+http://www.commoncrawl.org/bot.html)