4.4BSD/usr/share/man/cat8/acucntrl.0
ACUCNTRL(8) BSD System Manager's Manual ACUCNTRL(8)
NNAAMMEE
acucntrl - turn around tty line between dialin and dialout
SSYYNNOOPPSSIISS
//uussrr//lliibb//uuuuccpp//aaccuuccnnttrrll keyword ttyline
DDEESSCCRRIIPPTTIIOONN
_A_c_u_c_n_t_r_l 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.
_k_e_y_w_o_r_d is chosen from the list: _d_i_s_a_b_l_e or _d_i_a_l_o_u_t, to
condition a line for dialout; and _e_n_a_b_l_e or _d_i_a_l_i_n, 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. _A_c_u_c_n_t_r_l 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.
_T_t_y_l_i_n_e 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
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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 dialin)
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
HHIISSTTOORRYY
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)
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ACUCNTRL(8) BSD System Manager's Manual ACUCNTRL(8)
FFIILLEESS
/dev/kmem, /vmunix, /etc/ttys, /etc/utmp, /dev/tty*
BBUUGGSS
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 was similar synchronization problem with
the init process. When dialins are disabled the capital-
ized 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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