Permanently insecure mode - always run the system in level 0 mode. This is the default initial value.
Insecure mode - immutable and append-only flags may be turned off. All devices may be read or written subject to their permissions.
Secure mode - the system immutable and system append-only flags may not be turned off; disks for mounted file systems, /dev/mem, /dev/kmem and /dev/io (if your platform has it) may not be opened for writing; kernel modules (see kld(4)) may not be loaded or unloaded.
Highly secure mode - same as secure mode, plus disks may not be opened for writing (except by mount(2)) whether mounted or not. This level precludes tampering with file systems by unmounting them, but also inhibits running newfs(8) while the system is multi-user.
In addition, kernel time changes are restricted to less than or equal to one second. Attempts to change the time by more than this will log the message "Time adjustment clamped to +1 second".
Network secure mode - same as highly secure mode, plus IP packet filter rules (see ipfw(8), ipfirewall(4) and pfctl(8)) cannot be changed and dummynet(4) or pf(4) configuration cannot be adjusted.
If the security level is initially nonzero, then init leaves it unchanged. Otherwise, init raises the level to 1 before going multi-user for the first time. Since the level cannot be reduced, it will be at least 1 for subsequent operation, even on return to single-user. If a level higher than 1 is desired while running multi-user, it can be set before going multi-user, e.g., by the startup script rc(8), using sysctl(8) to set the kern.securelevel variable to the required security level.
If init is run in a jail, the security level of the "host system" will not be effected. Part of the information set up in the kernel to support a jail is a per-jail "securelevel" setting. This allows running a higher security level inside of a jail than that of the host system. See jail(8) for more information about jails.
In multi-user operation, init maintains processes for the terminal ports found in the file ttys(5). The init utility reads this file and executes the command found in the second field, unless the first field refers to a device in /dev which is not configured. The first field is supplied as the final argument to the command. This command is usually getty(8); getty opens and initializes the tty line and executes the login(1) 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 utility wakes up, deletes the user from the utmp(5) file of current users and records the logout in the wtmp(5) file. The cycle is then restarted by init executing a new getty for the line.
The init utility can also be used to keep arbitrary daemons running, automatically restarting them if they die. In this case, the first field in the ttys(5) file must not reference the path to a configured device node and will be passed to the daemon as the final argument on its command line. This is similar to the facility offered in the AT&T V /etc/inittab.
Line status (on, off, secure, getty, or window information) may be changed in the ttys(5) file without a reboot by sending the signal SIGHUP to init with the command ""kill -HUP 1"". On receipt of this signal, init re-reads the ttys(5) file. When a line is turned off in ttys(5), 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(5) file and are now on, init executes the command specified in the second field. If the command 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(5), init will not do anything at all to that line. However, it will complain that the relationship between lines in the ttys(5) file and records in the utmp(5) file is out of sync, so this practice is not recommended.
The init utility will terminate multi-user operations and resume single-user mode if sent a terminate (TERM) signal, for example, ""kill -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.
The init utility will cease creating new processes and allow the system to slowly die away, if it is sent a terminal stop (TSTP) signal, i.e. ""kill -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 init utility will terminate all possible processes (again, it will not wait for deadlocked processes) and reboot the machine if sent the interrupt (INT) signal, i.e. ""kill -INT 1"". This is useful for shutting the machine down cleanly from inside the kernel or from X when the machine appears to be hung.
The init utility will do the same, except it will halt the machine if sent the user defined signal 1 (USR1), or will halt and turn the power off (if hardware permits) if sent the user defined signal 2 (USR2).
When shutting down the machine, init will try to run the /etc/rc.shutdown script. This script can be used to cleanly terminate specific programs such as innd (the InterNetNews server). If this script does not terminate within 120 seconds, init will terminate it. The timeout can be configured via the sysctl(8) variable kern.init_shutdown_timeout.
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)"".
If run as a user process as shown in the second synopsis line, init will emulate AT&T V behavior, i.e., super-user can specify the desired run-level on a command line, and init will signal the original (PID 1) init as follows:
"Run-level Signal Action 0 SIGUSR2 "Halt and turn the power off" 1 SIGTERM "Go to single-user mode" 6 SIGINT "Reboot the machine" c SIGTSTP "Block further logins" q SIGHUP Rescan the
"getty repeating too quickly on port %s, sleeping."
A process being started to service a line is exiting quickly each time it is started. This is often caused by a ringing or noisy terminal line. .Bf -emphasis Init will sleep for 30 seconds, then continue trying to start the process. .Ef
"some processes would not die; ps axl advised."
A process is hung and could not be killed when the system was shutting down. This condition is usually caused by a process that is stuck in a device driver because of a persistent device error condition.