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DESCRIPTION

If the console entry in the ttys(5) file is marked ``insecure'', then init will require that the superuser password be entered before the system will start a single-user shell. The password check is skipped if the console is marked as ``secure''.

The kernel runs with four different levels of security. Any superuser process can raise the security level, but only init can lower it. Security levels are defined as follows:

-1

Permanently insecure mode - always run system in level 0 mode.

0

Insecure mode - immutable and append-only flags may be turned off. All devices may be read or written subject to their permissions.

1

Secure mode - immutable and append-only flags may not be changed; disks for mounted filesystems, /dev/mem, and /dev/kmem are read-only. The settimeofday(2) system call can only advance the time.

2

Highly secure mode - same as secure mode, plus disks are always read-only whether mounted or not. This level precludes tampering with filesystems by unmounting them, but also inhibits running newfs(8) while the system is multi-user.

Normally, the system runs in level 0 mode while single user and in level 1 mode while multiuser. If the level 2 mode is desired while running multiuser, it can be set in the startup script /etc/rc using sysctl(8). If it is desired to run the system in level 0 mode while multiuser, the administrator must build a kernel with the variable securelevel defined in the file /sys/compile/MACHINE/param.c and initialize it to -1.

In multi-user operation, init maintains processes for the terminal ports found in the file ttys(5). Init reads this file, and executes the command found in the second field. This command is usually getty(8); getty opens and initializes the tty line and executes the login program. The login program, when a valid user logs in, executes a shell for that user. When this shell dies, either because the user logged out or an abnormal termination occurred (a signal), the init program wakes up, deletes the user from the utmp(5) file of current users and records the logout in the wtmp file. The cycle is then restarted by init executing a new getty for the line.

Line status (on, off, secure, getty, or window information) may be changed in the ttys file without a reboot by sending the signal SIGHUP to init with the command “kill -s HUP 1”. On receipt of this signal, init re-reads the ttys file. When a line is turned off in ttys, init will send a SIGHUP signal to the controlling process for the session associated with the line. For any lines that were previously turned off in the ttys file and are now on, init executes a new getty to enable a new login. If the getty or window field for a line is changed, the change takes effect at the end of the current login session (e.g., the next time init starts a process on the line). If a line is commented out or deleted from ttys, init will not do anything at all to that line. However, it will complain that the relationship between lines in the ttys file and records in the utmp file is out of sync, so this practice is not recommended.

Init will terminate multi-user operations and resume single-user mode if sent a terminate (TERM) signal, for example, “kill -s TERM 1”. If there are processes outstanding that are deadlocked (because of hardware or software failure), init will not wait for them all to die (which might take forever), but will time out after 30 seconds and print a warning message.

Init will cease creating new getty's and allow the system to slowly die away, if it is sent a terminal stop (TSTP) signal, i.e. “kill -s TSTP 1”. A later hangup will resume full multi-user operations, or a terminate will start a single user shell. This hook is used by reboot(8) and halt(8).

The role of init is so critical that if it dies, the system will reboot itself automatically. If, at bootstrap time, the init process cannot be located, the system will panic with the message ``panic: "init died (signal %d, exit %d)''.