Usenet 1994 January/usenetsourcesnewsgroupsinfomagicjanuary1994.iso/sources/misc/volume10/callback_hg/part01/README

Text File | 1990-01-06 | 10.5 KB | 283 lines

$Header: README,v 1.2 89/12/29 10:17:06 howard Exp $

      DESCRIPTION

The callback system implements callback for dialup login
sessions.  A user dials in and enters a symbolic name for a
stored telephone number.  The line is dropped, and a new call is
placed to the corresponding number.  On connection a normal
login session is established.  Unlike callback firmware in some
modems, the list of phone numbers may be arbitrarily long.
Different lines may use different lists, or any number of lines
may share the same list.

The system has two purposes: to charge (almost all of) the cost
of the telephone calls to the host, and to improve security.  I
do no guarantee that callback is secure, but at least you can
look at the source code; it's not firmware burned into a PROM.

Callback has been tested on an ALM port on a Sun 3/280 running
SunOS 4.0.3, and on a CPU serial port on a Sun 3/60 runing first
SunOS 4.0.1, then SunOS 4.0.3.  Callback may work on other UNIX
versions that use termio and /dev/cua* special files for dialing
out.  It will not work on SunOS 3.x.  It has been tested with
Octocom OSI8596 modems and Alfanet ANC 8x224TT modems, both of
which are Hayes-compatible.  Callback is designed to be general
enough to operate with most autodial modems.  Extensive logging
shows all traffic to and from the modem, with timestamps.

Callback works with the normal getty and login functionality,
including line speed selection.  It allows different modem
commands for each line speed, if necessary.

As distributed, callback needs verbose text responses from
modems, e.g. "CONNECT 2400" instead of "10."  This makes it
incompatible with some UUCPs, since they require numeric
response codes.  It should be easy to modify callback to handle
numeric response codes, however.


      LICENSE

Callback is free software under the GNU general public license.


      HOW IT WORKS (KLUDGEOLOGISTS TAKE NOTE)

The /etc/ttytab file is modified for the dialup line.  Instead
of running the usual getty, callback is run.  Callback
immediately execs a special getty, named getty-cb.  Getty-cb
works exactly like the ordinary getty, except that after it has
the tty set up correctly and has read the login name, it does
not exec login but instead execs callback0.  Callback0 does the
actual callback.  It prompts the user for the symbolic name for
the telephone number, hangs up, sends the dialing command to the
modem, waits for answer, turns off the hangup-on-close (HUPCL)
bit, and then simply exits.  Since the getty process has died,
init starts up another one, so another callback is started.  But
this time, callback does not exec getty-cb, it execs login, so
the user can finish logging in.  Callback execs either getty-cb
or login, depending on a state file that callback0 modifies.
And getty-cb is really just the vendor-supplied getty
executable, with "/bin/login" changed to a path to callback0.


      PREREQUISITES

You must have my library of C functions.  It was posted to
comp.sources.misc on 1 October 1989 (volume 8, issues 80-87) and
28 October 1989 (volume 8, issue 96).  The archive name was
libhoward.

You also need some way to change a string in an executable, or
you need source for a getty that will work on your system.  GNU
Emacs is an excellent tool for editing executables; adb will
also work.

The libhoward package itself has a number of prerequisites.
ANSI-C float.h and limits.h include files must be on the C
include file search path.  If you don't have them, you can use
Steven Pemberton's config program to make them.  Config was
posted to comp.sources.misc on 30 April 1989 as volume 6 issue
96, archive name config2.

The makefiles use features that may only be in GNU make, version
3.54 or later.  (There are a lot of different make commands out
there, and I haven't tested them all, so I don't know for sure
which will work and which won't.  GNU make 3.54 will definitely
work.  The make supplied with SunOS 4.0.3 will not work.)

Chris Tweed's sets command must be on the command search path.
It was posted to comp.sources.unix on 24 February 1988 as volume
13, issue 68, archive name sets.

Some of the documentation is in LaTeX.  You do not *have* to
read it, but it may be helpful.  If you want to read it, you
will need LaTeX to format it.


      INSTALLATION

It will be easier to understand the installation if you first
read my report "Sharing software in a network of heterogeneous
UNIX hosts."  This comes with libhoward.

Install the dialup lines in the normal way, including the
/dev/cua* files.  For Suns, see section 11.4, "Adding a Modem to
Your System," in the System & Network Administration Manual.
Make sure the dialup lines are working in the normal manner,
without callback, before installing callback.

Figure out where you want to install various files.  Here I'll
assume a separate directory for everything, but it's easy to
make a different choice.  I discuss an alternative example
below.

I'll call the master source directory $DD.

Create a directory for compiling on this machine type.  I'll
call it /usr/local/free/callback.

Find out the version.  Look at the Header line (typically the
first line) of the FREEZE file.  The version is the first
number, and ends at the period.  The first externally released
version was version 2.

Create a subdirectory for this version. e.g.
/usr/local/free/callback/2.

Create the following subdirectories for results:
   bin       Installed executables.
   man/man1  Manual entries.
   man/cat1  On-line formatted manual entries.

For example:
   % cd /usr/local/free/callback/2
   % mkdir bin man man/{cat,man}1

Create a subdirectory for doing the actual compilations.  I'll
call it sun34:
   % mkdir sun34
   % cd sun34

Create an mk shell file.  Make it executable.  Here's a starting
point for SunOS 4.x:

DD=/usr/local/free-dist/callback/2/dist
SRCS=`cd $DD; echo *`
RM='rm -f'
export DD SRCS RM
make -f uMakefile
exec make \
   CATMAN=catman \
   FMTMAN=mkManPS \
   $*

If you don't have a catman command, drop that line.  FMTMAN
should be the name of a command to turn manual entries into
printable form.  Add any local changes you need here.  If your C
compiler doesn't understand different optimization levels, add
CFLAGS=-O as an argument to the last make.

Here is an alternate mk file for a system using GCC, and with
results installed in more conventional places.

DD=/usr/local/free-dist/callback/2/dist
SRCS=`cd $DD; echo *`
CC=gcc
CFLAGS='-g -O -traditional'
INCLUDES=-I/usr/local/include
RM='rm -f'
export DD SRCS CC CFLAGS INCLUDES RM
make -f uMakefile
exec make \
   CATMAN=catman \
   CID=/usr/local/bin \
   LIBPATH='-L/usr/local/lib' \
   MID=/usr/local/man \
   $*

Get MakeCommon and uMakefile from the master source directory,
e.g.
   % mkDistI MakeCommon $DD
   % mkDistI uMakefile  $DD

Make any changes you need to make to callback.h.  Near the top
are some absolute path names that you may want to change.

If your telephone line has double clearing, and if you're
concerned about security, you should increase the value of the
DROPSEC macro defined near the top of callback.h.  Double
clearing is a telephony feature.  A calls B and B answers.  With
double clearing, B can hang up for a short time and then pick up
the call again.  This is convenient, for example, to walk to a
phone in a different room.  However, double clearing also makes
spoofing possible.  The bad guy calls up and initiates the
callback, but does not hang up.  Instead, the bad guy supplies
dial tone.  The host modem hangs up the line for a while, and
when it goes off hook again it detects the dial tone supplied by
the bad guy.  It dials the number and then waits for answer.
The bad guy removes the dial tone and answers.

To avoid spoofing, DROPSEC must be longer than the double
clearing timeout.  This is typically about 90 seconds, but you
can test for yourself by having a partner call you, hanging up,
and timing how long you need to stay on hook before you can go
off hook and get dial tone instead of your partner.
Unfortunately, DROPSEC is the time between when the callback
request is accepted and when it is processed, so making it
longer than a few seconds will annoy your users and lead them to
suspect that the callbacks they request have failed.  It's much
better to get a line with single clearing, if possible.  Most
PABX lines have single clearing.

Callback as distributed assumes verbose responses from the
modem.  With Hayes-compatible modems, the command "atv1" puts
them in verbose mode.  If you prefer numeric response codes, you
will need to modify the mrctab[] table in callback.h to handle
them.  You will probably also need to modify the code in main()
in callback0.c to do exact matching on the result codes instead
of prefix matching.

When you have the modem programmed with the configuration you
wish, save the configuration in non-volatile memory.  With
Hayes-compatible modems, the command "at&w" does this.

Run mk.

Run "mk install".

Make a getty that execs callback0 instead of login.  If you have
source for getty, you can use that.  If you don't, make a copy
of your vendor-supplied getty executable, and edit it.  I call
the result getty-cb.  It should go where callback and callback0
go.  I used GNU Emacs to edit a copy of /usr/etc/getty.  I used
incremental search to find /bin/login, then I changed it to
/bin/.cb0 followed by an extra NUL (Ctl-Q 000) so the new string
takes up exactly the same space as the old.  I then made
/bin/.cb0 a symbolic link to where callback0 really is:
   # mount -o remount,rw /usr  (If /usr is mounted read-only)
   # ln -s /usr/local/free/callback/0/bin/callback0 /bin/.cb0

Set up control, log, and state files.
I put these in /etc/local/callback.  In this example there's
only one callback line, ttyd1:
   # cd /etc
   # mkdir local
   # chmod g+s local
   # cd local
   # mkdir callback
   # chmod o= callback
   # cd callback
   # mkdir control log state
   # chmod o= *
   # touch {control,log}/ttyd1
   # chmod o= control/* log/*

Edit the control files.  See callback(1) for an example.

The log files grow without bound, so you should include them in
your periodic log file trimming scripts.

Edit /etc/ttytab.  The lines that use callback should have
/usr/etc/getty replaced by a full path to callback, e.g.:

   ttyd1   "/usr/local/free/callback/0/bin/callback 2400-baud" dialup          on

Make the newly installed version of callback version 0:
   # cd /usr/local/free/callback
   # /bin/rm -f 0; ls -s 2 0

Have init read the changed /etc/ttytab:
   # kill -1 1
--
Howard Gayle
TN/ETX/TT/HL
Ericsson Telecom AB
S-126 25 Stockholm
Sweden
[e-mail]
uunet!ericsson.se!howard
Phone: [phone]
FAX  : [phone]