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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]

       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

       * 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 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  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  man-
       ual 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 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 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 estab-
       lished (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".

       mgetty(8), ct(1)

       callback is Copyright (C) 1993-1996 by Gert Doering, <gert@greenie.muc.de>.

greenie 				    27 Oct 93				      callback(8)
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