callback man page on Oracle

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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 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 debug‐

       -l <modem lines>
	      Use the given modem lines. Multiple lines	 can  be  sepa‐
	      rated  by	 ":",  as with sendfax(8). Example: callback -l

       -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+sendfax/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 ter‐
       minal, 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  Feb‐
       ruary  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, <gert@gree‐>.

greenie				   27 Oct 93			   callback(8)

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