NAME

       callback - call a user back, presenting a login prompt

SYNOPSIS

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

DESCRIPTION

       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.

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
              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.

CONFIG FILE

       callback    will   read   all   its   configuration   at   run-time   from   a   file,   usually   called
       /etc/mgetty/callback.config. 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 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.

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 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.

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>.

greenie                                             27 Oct 93                                        callback(8)