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callback

Section: mgetty+callback manual (8)
Updated: 27 Oct 93
Index

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, askuser for one), and if a CONNECT is established, hand over control tomgetty(8) to present user with a login name prompt.

callbackis used for various purposes:

* security: make sure your users are who they pretend to be by calling awell-known phone number.

* cost savings: make your company call you back.

callbackcan be called directly from the command line (but you must be "root" to dothis, otherwise callback can't signal mgetty), or from mgetty's"login.config". See the login.config file shipped with mgetty for anexample.

OPTIONS


-x <debug level>: Use the given level of verbosity for logging - 0 means no logging, 5 isreally 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 withsendfax(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 commandresponses 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 canmake use of multiple modem lines, and with this options, you can force itto use just one modem, the one where a call comes in.

CONFIG FILE

callbackwill read all its configuration at run-time from a file, usually called/etc/mgetty+sendfax/callback.config. See the documentation in the mgetty.infomanual for details.

DIAGNOSTICS

In most cases,callbackcan't print any error messages to the console, because it must detachitself immediately from the terminal, in case someone wants to be calledback 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 (theextent of the data written is controlled by the "-x" option), especiallyincluding the reason why a call was not made, or what exactly failed.

Just two messages are printed on stdout, and those are self-explaining, acall 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 thenstart a "login shell", it won't work (this is for the same reasonmgetty(8)has to be started from /etc/inittab).

So, mgetty has to do the "asking for login name". But I do not want tohave all that dialout code in mgetty, bloating it even more.

The way it works is this:callbackdials out on a modem device. It will only take a modem device that has amgetty watching over it (!). When the connection is established (CONNECT),callbackwill send a signal SIGUSR1 to mgetty, which, in turn, will send the samesignal back to signal "I got your signal".callbackthen exits, andmgettytakes 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 BOTHthe mgetty and the callback log file, otherwise it's very hard to find thebugs.

BUGS

callbackis "alpha" code, not very stable right now.

callbackis fairly dumb concerning retries.

callbackmust be run as root.

AUTHOR

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