Dial-in is where you set up your PC so that others may dial in to your phone number and use your PC. The "point of view" is your PC. When you dial out from your PC you are also dialing in to another computer (but not dialing in to your own computer)
Dial-in works like this. Someone with a modem dials your telephone
number. Your modem answers the call and connects. Once the caller is
connected, your PC (via the getty program) starts the login
process for the caller. The original method was to send a login
prompt to the caller's screen (manual login). But a more modern
method (if you use mgetty) is to start PPP (pppd) and let PPP
automatically login the caller (no need to manually type in a name or
password). See the PPP-HOWTO (new revision expected soon) and docs
for mgetty for more details.
After the caller has logged in, the caller uses your PC. Using your
PC may mean that the caller has a shell account and can use your PC
just as if they logged in at the console (or from a text-terminal).
It could also mean that they get connected to the Internet thru your
PC (via PPP). The program that you use at your PC to handle dialin is
called getty or mgetty. See
About mgetty.
If you expect that people will be able to dial-in to you at 56k, it can't be done unless:
getty is the program you run for dialin. You don't need it
for dialout. In addition to presenting a login prompt, it also
answers the telephone. Originally getty was used for logging in to a
computer from a dumb terminal. It's currently used for logging in to a
Linux console). There are a few different getty programs with
slightly different names. Only certain ones work with modems for
dialin. This getty program is usually started at boot-time. It
must be called from the /etc/inittab file. You may find an example in
this file of a call to getty which you will likely need to edit a bit.
If you use a different getty program than the one shown in such an
example, then you will need to edit it quite a bit since the options
will have a different format.
There are four different getty programs to choose from that may be
used with modems for dialin: mgetty, uugetty, getty_em,
and agetty. A few details are given in the following
subsections. agetty is the simplest (and weakest) of the four
and some consider it mainly for use with directly connected
text-terminals. mgetty has support for fax and voice mail but
Uugetty doesn't. mgetty allegedly lacks a few of the features
of uugetty. getty_em is a simplified version of
uugetty. Thus mgetty is likely your best choice unless you
are already familiar with uugetty (or find it difficult to get
mgetty). The syntax for these getty programs differs, so be sure
to check that you are using the correct syntax in
/etc/inittab for whichever getty you use.
mgetty was written as a replacement for uugetty which was
in existence long before mgetty. Both are for use with
modems. Although mgetty may be also used for directly connected
terminals the documentation for this is hard to pinpoint and mgetty
will not (as of mid 1999) support software flow control (used on many
terminals) without recompiling. This defect is listed as a bug. In
addition to allowing dialup logins, mgetty also provides FAX
support and auto PPP detection. There is a supplemental program
called vgetty which handles voicemail for some modems.
mgetty documentation is good (except for voice mail), and does
not need supplementing . Please refer to it for installation
instructions. You can find the latest information on mgetty at
http://www.leo.org/~doering/mgetty/ and
http://alpha.greenie.net/mgetty
getty_ps contains two programs: getty is used for console
and terminal devices, and uugetty for modems. Greg Hankins
(former author of Serial-HOWTO) used uugetty so his writings
about it are included here. See
Uugetty.
The other gettys are well covered by the documentation that comes with
them.
This is a simplified version of ``uugetty''. It was written by Vern Hoxie after he became fully confused with complex support files needed for getty_ps and uugetty.
It is part of the collection of serial port utilities and information by Vern Hoxie available via ftp from scicom.alphacdc.com/pub/linux. The name of the collection is ``serial_suite.tgz''. When logging into ``scicom'' as "anonymous", you must use your full e-mail address as the password. For example: greg.hankins@cc.gatech.edu
agetty is a simple, completely functional implementation of
getty which is best suited for virtual consoles or terminals
rather than modems. But it works fine with modems under favorable
conditions (except you cannot dial out when agetty is running and
waiting for a call). agetty in the Debian distribution is just
named getty.
mingetty is a small getty that will work only for consoles
(monitors) so you can't use it with modems for dialin.
The caller runs some sort of communication program that dials your
telephone number and your telephone rings. There are two different
ways that your PC can answer the phone. One way is for the modem to
automatically answer the call. The other way is for getty to sense
the ringing and send a command to the modem to answer the call. Once
the call is answered, your modem sends tones to the other modem (and
conversely). The two modems negotiate how they will communicate and
when this is done your modem sends a "CONNECTed" message (or the like)
to getty. When getty gets this message, it sends a login
prompt out the serial port. Sometimes getty just calls on a
program named login to handle the logging in. getty usually
starts running at boot-time but it must wait until a connection is
made before sending out a "login" prompt.
Now for more details on the two methods of answering the call. By
setting the S0 register of the modem to 3, the modem will
automatically answer on the 3rd ring. If it's set to 0 then the modem
will only answer the call if getty sends it an "A" (= Answer) command
while the phone is ringing. Actually an "ATA" is sent since all modem
commands are prefixed by "AT". You might think it best to utilize the
ability of the modem to automatically answer the call, but it's
actually better if getty answers it. If the modem doesn't
automatically answer, it's called manual answer (even though
getty automatically handles it).
For the "manual" answer case, getty opens the port at boot-time and
listens. When the phone rings, a "RING" message is sent to the
listening getty. Then if getty wants to answer this ring, it
sends the modem an "ATA" command. The modem then makes a connection
and sends a "CONNECT ..." message to getty which then sends a login
prompt to the caller.
The automatic answer case uses the CD (Carrier Detect) wire from the
modem to the serial port to detect when a connection is made. It
works like this. At boot-time getty tries to open the serial
port but the attempt fails since there is normally no CD signal from
the modem. Then the getty program waits at the open statement in
the program until a CD signal appears. When a CD signal arrives
(perhaps hours later) then the port is opened and getty sends the
login prompt. While getty is waiting (sleeping) at the open
statement, other processes can run since Linux is a multiprocessing
operating system. What actually wakes getty up is an interrupt
which is issued when the CD line from the modem changes state to on.
You may wonder how getty is able to open the serial port in the manual-answer case since there is no CD signal. Well, there's a way to write a program to force the port to open even if there is no CD signal present.
The difference between the two ways of answering will show itself
when the computer happens to be down but the modem is still working.
For the manual case, the "RING" message is sent to getty but since the
computer is down, getty isn't there and the phone never gets answered.
There are no telephone charges when there is no answer. For the
automatic answer case, the phone is answered but no login message is
ever sent since the computer is down. The phone bill runs up as the
waiting continues. If the phone call is toll-free, it doesn't make
much difference, although it may be frustrating waiting for a login
prompt that never arrives. mgetty uses manual answer. Uugetty
can do this too using a configuration script.
Callback is where someone first dials in to your modem. Then, you get a little info from the caller and then call it right back. Why would you want to do this? One reason is to save on telephone bills if you can call the caller cheaper than the caller can call you. Another is to make sure that the caller really is who it claims to be. If a caller calls you and claims to be calling from its usual phone number, then one way to verify this is to actually place a new call to that number.
There's a program for Linux called "callback" that works with mgetty. It's at ftp://ftp.icce.rug.nl/pub/unix/. Step-by-step instructions on how someone installed it (and PPP) is at http://www.stokely.com/unix.serial.port.resources/callback.html
Voice mail is like an answering machine run by a computer. To do this you must have a modem that supports "voice" and supporting software. Instead of storing the messages on tape, they are stored in digital format on a disk. When a person phones you, they hear a "greeting" message and can then leave a message for you. More advanced systems would have caller-selectable mail boxes and caller-selectable messages to listen to. Free software is available in Linux for simple answering, but doesn't seem to be available yet for the more advanced stuff.
I know of two different voicemail packages for Linux. One is a very
minimal package (see
Voicemail Software).
The other, more advanced, but currently poorly documented, is
vgetty. It's an optional addition to the well documented and
widely distributed mgetty program. It supports ZyXEL-like voice
modem commands. In the Debian distribution, you must get the
mgetty-voice package in addition to the mgetty package and mgetty-doc
package. Obsolete documentation has been removed from mgetty but
replacement documentation is lacking (except if you use the -h (help)
option when running certain programs, etc.). But one sees postings
about using it on the mgetty newsgroup. See
About mgetty. It seems that vgetty is currently not very
stable but it's successfully being used and development of it
continues. If this is the latest version of this HOWTO can someone
who is familiar with vgetty please let me know its current status.