.\"     $OpenBSD: securelevel.7,v 1.31 2019/08/21 20:44:09 cheloha Exp $
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.\" Copyright (c) 2000 Hugh Graham
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.Dd $Mdocdate: August 21 2019 $
.Dt SECURELEVEL 7
.Os
.Sh NAME
.Nm securelevel
.Nd securelevel and its effects
.Sh DESCRIPTION
The
.Ox
kernel provides four levels of system security:
.Bl -tag -width flag
.It \&-1 Em Permanently insecure mode
.Bl -hyphen -compact
.It
.Xr init 8
will not attempt to raise the securelevel
.It
may only be set with
.Xr sysctl 8
while the system is insecure
.It
otherwise identical to securelevel 0
.El
.It \ 0 Em Insecure mode
.Bl -hyphen -compact
.It
used during bootstrapping and while the system is single-user
.It
all devices may be read or written subject to their permissions
.It
system file flags may be cleared with
.Xr chflags 2
.El
.It \ 1 Em Secure mode
.Bl -hyphen -compact
.It
default mode when system is multi-user
.It
securelevel may no longer be lowered except by init
.It
.Pa /dev/mem
and
.Pa /dev/kmem
cannot be opened
.It
raw disk devices of mounted file systems are read-only
.It
system immutable and append-only file flags may not be removed
.It
the
.Va fs.posix.setuid ,
.Va hw.allowpowerdown ,
.Va kern.allowkmem ,
.Va kern.utc_offset ,
.Va net.inet.ip.sourceroute ,
and
.Va machdep.kbdreset
.Xr sysctl 8
variables may not be changed
.It
the
.Va ddb.console ,
.Va ddb.panic ,
and
.Va machdep.allowaperture
.Xr sysctl 8
variables may not be raised
.It
.Xr gpioctl 8
may only access GPIO pins configured at system startup
.El
.It \ 2 Em Highly secure mode
.Bl -hyphen -compact
.It
all effects of securelevel 1
.It
raw disk devices are always read-only whether mounted or not
.It
.Xr settimeofday 2
and
.Xr clock_settime 2
may not set the time backwards or close to overflow
.It
.Xr pf 4
filter and NAT rules may not be altered
.El
.El
.Pp
Securelevel provides convenient means of
.Dq locking down
a system to a degree suited to its environment.
It is normally set at boot by
.Xr rc 8 ,
or the superuser may raise securelevel at any time by modifying the
.Va kern.securelevel
.Xr sysctl 8
variable.
However, only
.Xr init 8
may lower it once the system has entered secure mode.
.Pp
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
.Xr newfs 8 .
Further, it can limit the potential damage of a compromised
.Dq 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.
.Pp
Because securelevel can be modified with the in-kernel debugger
.Xr ddb 4 ,
a convenient means of locking it off (if present) is provided
at securelevels 1 and 2.
This is accomplished by setting
.Va ddb.console
and
.Va ddb.panic
to 0 with the
.Xr sysctl 8
utility.
.Sh FILES
.Bl -tag -width /etc/rc.securelevel -compact
.It Pa /etc/rc.securelevel
commands that run before the security level changes
.El
.Sh SEE ALSO
.Xr init 8 ,
.Xr rc 8 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
manual page first appeared in
.Ox 2.6 .
.Sh BUGS
The list of securelevel's effects may not be comprehensive.