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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] [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

       -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+send-
       fax/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

       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 pro-
       gram  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 sig-
       nalling), and if it doesn't work for you, please send me BOTH the mgetty and the callback log file, oth-
       erwise 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, <>.

greenie                            27 Oct 93                       callback(8)