callback - call a user back, presenting a login prompt


SYNOPSIS

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


DESCRIPTION

       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.


OPTIONS

       -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 every­
              thing,  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".

              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.


CONFIG FILE

       callback  will read all its configuration at run-time from
       a file, usually  called  /etc/mgetty+sendfax/callback.con­
       fig.  See  the documentation in the mgetty.info manual for
       details.


DIAGNOSTICS

       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 mes­
       sages to...

       Because of this, all callback errors are logged to a  pro­
       tocol  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.


INTERNALS

       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

       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.


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".


SEE ALSO

       mgetty(8), ct(1)


AUTHOR

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


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