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       callback  [-x<debuglevel>] [-V] [-l<modemlines>] [-m<init­
       string>] [-s<speed>] [-d] [-S] [phone-number]


       Call the given phone number (if none is given on the  com­
       mand  line,  ask user for one), and if a CONNECT is estab­
       lished, 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 help­
              ful 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

       -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".


       In  most cases, callback can't print any error mes­
       sages 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 includ­
       ing 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 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  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 sig­
       nalling), 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
       callback is Copyright (C) 1993-1996 by  Gert  Doer­
       ing, <gert@greenie.muc.de>.

greenie                     27 Oct 93                 callback(8)

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