SECURELEVEL(7)         Miscellaneous Information Manual         SECURELEVEL(7)


     securelevel - securelevel and its effects


     The OpenBSD kernel provides four levels of system security:

     -1 Permanently insecure mode
           -   init(8) will not attempt to raise the securelevel
           -   may only be set with sysctl(8) while the system is insecure
           -   otherwise identical to securelevel 0

      0 Insecure mode
           -   used during bootstrapping and while the system is single-user
           -   all devices may be read or written subject to their permissions
           -   system file flags may be cleared with chflags(2)

      1 Secure mode
           -   default mode when system is multi-user
           -   securelevel may no longer be lowered except by init
           -   /dev/mem and /dev/kmem cannot be opened
           -   raw disk devices of mounted file systems are read-only
           -   system immutable and append-only file flags may not be removed
           -   the fs.posix.setuid, hw.allowpowerdown, kern.allowkmem,
               net.inet.ip.sourceroute, and machdep.kbdreset sysctl(8)
               variables may not be changed
           -   the ddb.console, ddb.panic, and machdep.allowaperture sysctl(8)
               variables may not be raised
           -   gpioctl(8) may only access GPIO pins configured at system

      2 Highly secure mode
           -   all effects of securelevel 1
           -   raw disk devices are always read-only whether mounted or not
           -   settimeofday(2) and clock_settime(2) may not set the time
               backwards or close to overflow
           -   pf(4) filter and NAT rules may not be altered

     Securelevel provides convenient means of "locking down" a system to a
     degree suited to its environment.  It is normally set at boot by rc(8),
     or the superuser may raise securelevel at any time by modifying the
     kern.securelevel sysctl(8) variable.  However, only init(8) may lower it
     once the system has entered secure mode.

     Highly secure mode may seem Draconian, but is intended as a last line of
     defence should the superuser account be compromised.  Its effects
     preclude circumvention of file flags by direct modification of a raw disk
     device, or erasure of a file system by means of newfs(8).  Further, it
     can limit the potential damage of a compromised "firewall" by prohibiting
     the modification of packet filter rules.  Preventing the system clock
     from being set backwards aids in post-mortem analysis and helps ensure
     the integrity of logs.  Precision timekeeping is not affected because the
     clock may still be slowed.

     Because securelevel can be modified with the in-kernel debugger ddb(4), a
     convenient means of locking it off (if present) is provided at
     securelevels 1 and 2.  This is accomplished by setting ddb.console and
     ddb.panic to 0 with the sysctl(8) utility.


     /etc/rc.securelevel  commands that run before the security level changes


     init(8), rc(8), sysctl(8)


     The securelevel manual page first appeared in OpenBSD 2.6.


     The list of securelevel's effects may not be comprehensive.

OpenBSD 6.4                   September 12, 2017                   OpenBSD 6.4

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