.\"	$OpenBSD: sysctl.8,v 1.213 2018/01/12 04:36:44 deraadt Exp $
.\"	$NetBSD: sysctl.8,v 1.4 1995/09/30 07:12:49 thorpej Exp $
.\"
.\" Copyright (c) 1993
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.\"	@(#)sysctl.8	8.2 (Berkeley) 5/9/95
.\"
.Dd $Mdocdate: January 12 2018 $
.Dt SYSCTL 8
.Os
.Sh NAME
.Nm sysctl
.Nd get or set kernel state
.Sh SYNOPSIS
.Nm sysctl
.Op Fl Aan
.Nm sysctl
.Op Fl n
.Ar name ...
.Nm sysctl
.Op Fl nq
.Ar name Ns = Ns Ar value ...
.Sh DESCRIPTION
The
.Nm
utility retrieves kernel state and allows processes with
appropriate privilege to set kernel state.
The state to be retrieved or set is described using a
.Dq Management Information Base
.Pq MIB
style name, described as a dotted set of components.
.Pp
When retrieving a variable,
a subset of the MIB name may be specified to retrieve a list of
variables in that subset.
For example, to list all the machdep variables:
.Pp
.Dl $ sysctl machdep
.Pp
When setting a variable,
the MIB name should be followed by an equal sign and the new value.
.Pp
The options are as follows:
.Bl -tag -width xxx
.It Fl A
List all the known MIB names including tables.
Those with string or integer values will be printed as with the
.Fl a
flag; for the table values, the name of the utility to retrieve them is given.
.It Fl a
List all the currently available string or integer values.
This is the default, if no parameters are given to
.Nm .
.It Fl n
Suppress printing of the field name, only output the field value.
Useful for setting shell variables.
For example, to set the psize shell variable to the pagesize of the hardware:
.Pp
.Dl # set psize=`sysctl -n hw.pagesize`
.It Fl q
Suppress all output when setting a variable.
This option overrides the behaviour of
.Fl n .
.It Ar name Ns = Ns Ar value
Attempt to set the specified variable
.Ar name
to
.Ar value .
.El
.Pp
The information available from
.Nm
consists of integers, strings, and tables.
For a detailed description of the variables, see
.Xr sysctl 2 .
Tables can only be retrieved by special purpose programs such as
.Xr ps 1 ,
.Xr systat 1 ,
and
.Xr netstat 1 .
.Pp
.Nm
can extract information about the filesystems that have been compiled
into the running system.
This information can be obtained by using the command:
.Pp
.Dl $ sysctl vfs.mounts
.Pp
By default, only filesystems that are actively being used are listed.
Use of the
.Fl A
flag lists all the filesystems compiled into the running kernel.
.Sh FILES
.Bl -tag -width "/etc/sysctl.confXX" -compact
.It Pa /etc/sysctl.conf
sysctl variables to set at system startup
.El
.Sh EXAMPLES
To retrieve the maximum number of processes allowed
in the system:
.Pp
.Dl $ sysctl kern.maxproc
.Pp
To set the maximum number of processes allowed
in the system to 1000:
.Pp
.Dl # sysctl kern.maxproc=1000
.Pp
To retrieve information about the system clock rate:
.Pp
.Dl $ sysctl kern.clockrate
.Pp
To retrieve information about the load average history:
.Pp
.Dl $ sysctl vm.loadavg
.Pp
To make the
.Xr chown 2
system call use traditional
.Bx
semantics (don't clear setuid/setgid bits):
.Pp
.Dl # sysctl fs.posix.setuid=0
.Pp
To set the list of reserved TCP ports that should not be allocated
by the kernel dynamically:
.Pp
.Dl # sysctl net.inet.tcp.baddynamic=749,750,751,760,761,871
.Dl # sysctl net.inet.udp.baddynamic=749,750,751,760,761,871,1024-2048
.Pp
This can be used to keep daemons
from stealing a specific port that another program needs to function.
List elements may be separated by commas and/or whitespace;
a hyphen may be used to specify a range of ports.
.Pp
It is also possible to add or remove ports from the current list:
.Bd -literal -offset indent
# sysctl net.inet.tcp.baddynamic=+748,+6000-6999
# sysctl net.inet.tcp.baddynamic=-871
.Ed
.Pp
To set the amount of shared memory available in the system and
the maximum number of shared memory segments:
.Bd -literal -offset indent
# sysctl kern.shminfo.shmmax=33554432
# sysctl kern.shminfo.shmseg=32
.Ed
.Pp
To place core dumps from
.Xr issetugid 2
programs (in this example
.Xr bgpd 8 )
into a safe place for debugging purposes:
.Bd -literal -offset indent
# mkdir -m 700 /var/crash/bgpd
# sysctl kern.nosuidcoredump=3
.Ed
.Sh SEE ALSO
.Xr sysctl 2 ,
.Xr options 4 ,
.Xr sysctl.conf 5
.Sh HISTORY
.Nm
first appeared in
.Bx 4.4 .