Provided by: mgetty_1.2.1-1_amd64 bug


       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

       -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/callback.config. See the documentation in the 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 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 /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 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, <>.