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ACUCNTRL(8)        BSD System Manager's Manual        ACUCNTRL(8)


NAME
       acucntrl - turn around tty line between dialin and dialout

SYNOPSIS
       /usr/lib/uucp/acucntrl keyword ttyline

DESCRIPTION
       Acucntrl turns around the 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 /etc/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 /etc/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 /etc/utmp to check that the line is not in
              use by another user




4.3 Berkeley Distribution  June 1, 1994                         1








ACUCNTRL(8)        BSD System Manager's Manual        ACUCNTRL(8)


       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 /etc/utmp to let world
              know device has been grabbed

       9)     make sure that DTR is on

       Steps taken when enabling (i.e.  setup for dialing in)

       1)     check input arguments

       2)     look  in /etc/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 /etc/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 subroutine stolen from a program written
       by Tsutomu Shimomura {astrovax,escher}!tsutomu

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




4.3 Berkeley Distribution  June 1, 1994                         2








ACUCNTRL(8)        BSD System Manager's Manual        ACUCNTRL(8)


FILES
       /dev/kmem, /vmunix, /etc/ttys, /etc/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  dis-
       abling  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 deliber-
       ately no error  message  given  when  the  TIOCMGET  ioctl
       fails.

       Previously  there  were  similar  synchronization problems
       with the init process.  When dialins are disabled the cap-
       italized  name  of  the  process  cannot  be  posted  into
       /etc/utmp until  init  has  finished  clearing  /etc/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 /etc/utmp, and 4) posting the name again.

























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