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SECURELEVEL(7)             OpenBSD Reference 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

      1 Secure mode
           -   default mode when system is multi-user
           -   securelevel may no longer be lowered except by init
           -   /dev/mem and /dev/kmem may not be written to
           -   raw disk devices of mounted file systems are read-only
           -   system immutable and append-only file flags may not be removed
           -   kernel modules may not be loaded or unloaded

      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 back-
               wards or close to overflow
           -   pfctl(8) may no longer alter filter or nat rules
           -   the ddb.console and ddb.panic sysctl(8) variables may not be

     Securelevel provides convenient means of ``locking down'' a system to a
     degree suited to its environment.  It is normally set at boot via the
     rc.securelevel(8) script, 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.  A kernel
     built with option INSECURE in the config file will default to permanently
     insecure mode.

     Highly secure mode may seem Draconian, but is intended as a last line of
     defence should the superuser account be compromised.  Its effects pre-
     clude 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 prohibit-
     ing 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 on highly se-
     cure systems.  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

     chflags(2), settimeofday(2), mem(4), options(4), init(8), rc(8),

     The securelevel manual page first appeared in OpenBSD 2.6.

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

OpenBSD 3.4                     January 4, 2000                              2


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