acucntrl(8)						      System Manager's Manual						       acucntrl(8)

NAME

       acucntrl - turn around tty line between dialin and dialout

SYNOPSIS

       /usr/libexec/acucntrl keyword ttyline

DESCRIPTION

       Acucntrl  turns	around	terminal  line,  enabling it to be used for both dialin and dialout.  On dialin a terminal line is assumed to have
       modem control enabled and a getty process in existence waiting for logins.  On dialout modem control is disabled  and  there  is  no  getty
       process.

       This program must be run setuid to root.

       keyword is chosen from the list: disable or dialout, to condition a line for dialout; and enable or dialin, to condition a line for dialin.

       When  the  line is conditioned for dialing out, the login name of the real uid of the process is placed in /var/run/utmp in capitals.  This
       declares that the line is in use and acts as an additional locking mechanism.  Acucntrl will refuse to act if the /var/run/utmp	entry  for
       the  line is not null, is not the the user's login name (capitalized or not), and if the process is not running as the superuser.  The last
       condition is to allow the superuser to clear the state of the line.

       Turning modem control on or off is handled by poking into /dev/kmem.  It is currently implemented for dz, dh, and dmf lines.

       Under 4.2 BSD the program will also refuse to disable a line if carrier is sensed on it.  This is to avoid the dead  period  where  someone
       has just dialed in and made the connection but has not yet logged in.

       Ttyline	can  be either of the form tty* or /dev/tty*.  Enabling/disabling a line whose name does not begin with ttyd? is prohibited unless
       the real uid of the process is 0 or if the login name corresponding to the real uid is uucp.  This is a security precaution.

       Steps taken when disabling (i.e.  setup for dialing out)

       1)     check input arguments

       2)     look in /var/run/utmp to check that the line is not in use by another user

       3)     disable modem control on line

       4)     check for carrier on device

       5)     change owner of device to real uid

       6)     edit /etc/ttys,  changing the first character of the appropriate line to 0

       7)     send a hangup to process 1 to poke init to disable getty

       8)     post uid name in capitals in /var/run/utmp to let world know device has been grabbed

       9)     make sure that DTR is on

       Steps taken when enabling (i.e.	setup for dialin)

       1)     check input arguments

       2)     look in /var/run/utmp to check that the line is not in use by another user

       3)     make sure modem control on line is disabled

       4)     turn off DTR to make sure line is hung up

       5)     condition line: clear exclusive use and set hangup on close modes

       6)     turn on modem control

       7)     edit /etc/ttys,  changing the first character of the appropriate line to 1

       8)     send a hangup to process 1 to poke init to enable getty

       9)     clear uid name for /var/run/utmp

HISTORY

       First written by Allan Wilkes (fisher!allan)

       Modified June 8,1983 by W.Sebok (astrovax!wls) to poke the kernel rather than use a kernel hack to turn on/off modem control, using a  sub-
       routine stolen from a program written by Tsutomu Shimomura {astrovax,escher}!tsutomu

       Worked over many times by W.Sebok (i.e.	hacked to death)

FILES

       /dev/kmem, /vmunix, /etc/ttys, /var/run/utmp, /dev/tty*

BUGS

       Sensing	carrier  requires  the	4.2 BSD TIOCMGET ioctl call.  Unfortunately this ioctl is not implemented in the vanilla 4.2 BSD dh driver
       even though the dz and dmf drivers use an emulation of the DH11's modem control bits. This has been fixed here.

       Some time (currently 2 seconds) is required between disabling modem control and opening the device.  This is probably  because  of  a  race
       with  getty  whose  open  is  finally being allowed to complete.  This time interval may not be enough on a loaded system.  Because of this
       problem and the above problem with the dh driver there is deliberately no error message given when the TIOCMGET ioctl fails.

       Previously there was similar synchronization problem with the init process.  When dialins are disabled the capitalized name of the  process
       cannot  be  posted  into  /var/run/utmp until init has finished clearing /var/run/utmp.	However one does not know how long that will take,
       and, on a loaded system,  it can take quite a while.  This was solved by the strategy of 1) posting the name, 2) poking init, 3) going into
       a  loop	where  the  process repeatedly waits a second and checks whether the entry has been cleared from /var/run/utmp, and 4) posting the
       name again.

4.3 Berkeley Distribution					 November 27, 1996						       acucntrl(8)