1callback(8) mgetty+callback manual callback(8)
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6 callback - call a user back, presenting a login prompt
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9 callback [-x<debuglevel>] [-V] [-l<modemlines>] [-m<initstring>]
10 [-s<speed>] [-d] [-S] [phone-number]
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13 Call the given phone number (if none is given on the command line, ask
14 user for one), and if a CONNECT is established, hand over control to
15 mgetty(8) to present user with a login name prompt.
16 callback is used for various purposes:
18 * security: make sure your users are who they pretend to be by calling
20 a well-known phone number.
21 * cost savings: make your company call you back.
23 callback can be called directly from the command line (but you must be
25 "root" to do this, otherwise callback can't signal mgetty), or from
26 mgetty's "login.config". See the login.config file shipped with mgetty
27 for an example.
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31 -x <debug level>
32 Use the given level of verbosity for logging - 0 means no
33 logging, 5 is really noisy.
34 -V Print version number and quit.
36 -d Do not go into the background. This is helpful for debug‐
38 ging.
39 -l <modem lines>
41 Use the given modem lines. Multiple lines can be sepa‐
42 rated by ":", as with sendfax(8). Example: callback -l
43 tty1a:tty2a
44 -m <init sequence>
46 Set the modem initialization sequence (as usual: expect
47 send expect ...). This can do nearly everything, as long
48 as it leaves the modem command responses on (that is, no
49 ATQ1 here!) and switches the modem to data mode
50 (AT+FCLASS=0) if it is used in data/fax mode.
51 -s <speed>
53 This is the bit rate that should be used for the machine-
54 modem connection. Usually you'll set this via the "speed
55 <nnnn>" option in "callback.config".
56 -S Use the line where callback is started from for dialing
58 out. Callback can make use of multiple modem lines, and
59 with this options, you can force it to use just one
60 modem, the one where a call comes in.
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64 callback will read all its configuration at run-time from a
65 file, usually called /etc/mgetty+sendfax/callback.config. See
66 the documentation in the mgetty.info manual for details.
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70 In most cases, callback can't print any error messages to the
71 console, because it must detach itself immediately from the ter‐
72 minal, in case someone wants to be called back on the modem line
73 he called in. So, nothing to print messages to...
74 Because of this, all callback errors are logged to a protocol
76 file (the extent of the data written is controlled by the "-x"
77 option), especially including the reason why a call was not
78 made, or what exactly failed.
79 Just two messages are printed on stdout, and those are self-
81 explaining, a call from a non-root user, and an invalid option.
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85 How does it work?
86 This is a bit tricky, because of the way init(8) handles the
88 utmp(5) file. You can't just have any program ask the user for
89 a login name, and then start a "login shell", it won't work
90 (this is for the same reason mgetty(8) has to be started from
91 /etc/inittab).
92 So, mgetty has to do the "asking for login name". But I do not
94 want to have all that dialout code in mgetty, bloating it even
95 more.
96 The way it works is this: callback dials out on a modem device.
98 It will only take a modem device that has a mgetty watching over
99 it (!). When the connection is established (CONNECT), callback
100 will send a signal SIGUSR1 to mgetty, which, in turn, will send
101 the same signal back to signal "I got your signal". callback
102 then exits, and mgetty takes over the existing connection,
103 prompts the user for a login name, and forks off /bin/login.
104 Conclusion: this will not work with mgetty versions before Feb‐
106 ruary 04, 1996 (no support for this signalling), and if it
107 doesn't work for you, please send me BOTH the mgetty and the
108 callback log file, otherwise it's very hard to find the bugs.
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112 callback is "alpha" code, not very stable right now.
113 callback is fairly dumb concerning retries.
115 callback must be run as root.
117 Most of the documentation consists of "reading the source".
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122 mgetty(8), ct(1)
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125 callback is Copyright (C) 1993-1996 by Gert Doering, <gert@gree‐
126 nie.muc.de>.
127greenie 27 Oct 93 callback(8)