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       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

       -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

       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".


       mgetty(8), ct(1)


       callback  is  Copyright   (C)   1993-1996   by   Gert   Doering,