callback - call a user back, presenting a login 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.
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.
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.
callback is fairly dumb concerning retries.
callback must be run as root.
Most of the documentation consists of "reading the source".
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Created 1996-2021 by Maxim Chirkov
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