sbin/sysctl/sysctl.8

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.\"	@(#)sysctl.8	8.1 (Berkeley) 6/6/93
.\"
.Dd December 4, 2006
.Dt SYSCTL 8
.Os
.Sh NAME
.Nm sysctl
.Nd get or set kernel state
.Sh SYNOPSIS
.Nm sysctl
.Op Fl AdeMn
.Oo
.Fl r |
.Fl x
.Oc
.Op Ar name ...
.Nm sysctl
.Op Fl nq
.Oo
.Fl r |
.Fl x
.Oc
.Fl w
.Ar name Ns Li = Ns Ar value ...
.Nm sysctl
.Op Fl en
.Oo
.Fl r |
.Fl x
.Oc
.Fl a
.Nm sysctl
.Op Fl nq
.Oo
.Fl r |
.Fl x
.Oc
.Fl f
.Ar file
.Sh DESCRIPTION
The
.Nm sysctl
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
``Management Information Base'' (``MIB'') style name,
described as a dotted set of components.
The
.Sq /
character may also be used as a separator and a leading separator
character is accepted.
If
.Ar name
specifies a non-leaf node in the MIB, all the nodes underneath
.Ar name
will be printed.
.Pp
The following options are available:
.Bl -tag -width indent
.It Fl A
List all the known MIB names including tables, unless any MIB
arguments or
.Fl f Ar file
are given.
Those with string or integer values will be printed as with the
.Fl a
flag; for table or structure values that
.Nm
is not able to print,
the name of the utility to retrieve them is given.
Errors in retrieving or setting values will be directed to stdout
instead of stderr.
.It Fl a
List all the currently available string or integer values.
The use of a solitary separator character (either
.Sq \&.
or
.Sq / )
by
itself has the same effect.
Any given
.Ar name
arguments are ignored if this option is specified.
.It Fl d
Descriptions of each of the nodes selected will be printed instead of
their values.
.It Fl e
Separate the name and the value of the variable(s) with
.Ql = .
This is useful for producing output which can be fed back to the
.Nm
utility.
This option is ignored if
.Fl n
is specified or a variable is being set.
.It Fl f
Specifies the name of a file to read and process.
Blank lines and comments (beginning with
.Ql # )
are ignored.
Line continuations with
.Ql \e
are permitted.
Remaining lines are processed similarly to
command line arguments of the form
.Ar name
or
.Ar name Ns Li = Ns Ar value .
The
.Fl w
flag is implied by
.Fl f .
Any
.Ar name
arguments are ignored.
.It Fl M
Makes
.Nm
print the MIB instead of any of the actual values contained in the
MIB.
This causes the entire MIB to be printed unless specific MIB arguments
or
.Fl f Ar file
are also given.
.It Fl n
Specifies that the printing of the field name should be
suppressed and that only its value should be output.
This flag is useful for setting shell variables.
For example, to save the pagesize in variable psize, use:
.Bd -literal -offset indent -compact
set psize=`sysctl -n hw.pagesize`
.Ed
.It Fl q
Used to indicate that nothing should be printed for writes unless an
error is detected.
.It Fl r
Raw output form.
Values printed are in their raw binary forms as retrieved directly
from the kernel.
Some additional nodes that
.Nm
cannot print directly can be retrieved with this flag.
This option conflicts with the
.Fl x
option.
.It Fl w
Sets the MIB style name given to the value given.
The MIB style name and value must be separated by
.Ql =
with no whitespace.
Only integral and string values can be set via this method.
.It Fl x
Makes
.Nm
print the requested value in a hexadecimal representation instead of
its regular form.
If specified more than once, the output for each value resembles that of
.Xr hexdump 1
when given the
.Fl C
flag.
This option conflicts with the
.Fl r
option.
.Pp
.El
The
.Ql proc
top-level MIB has a special semantic: it represent per-process values
and as such may differ from one process to another.
The second-level name is the pid of the process (in decimal form),
or the special word
.Ql curproc .
For variables below
.Ql proc. Ns Ao pid Ac Ns .rlimit ,
the integer value may be replaced
with the string
.Ql unlimited
if it matches the magic value used to disable
a limit.
.Pp
The information available from
.Nm sysctl
consists of integers, strings, and tables.
The tabular information can only be retrieved by special
purpose programs such as
.Nm ps ,
.Nm systat ,
and
.Nm netstat .
See
.Xr sysctl 7
for description of available MIBs.
.Sh CREATION AND DELETION
New nodes are allowed to be created by the superuser when the kernel
is running at security level 0.
These new nodes may refer to existing kernel data or to new data that
is only instrumented by
.Xr sysctl 3
itself.
.Pp
The syntax for creating new nodes is
.Dq //create=new.node.path
followed by one or more of the following attributes separated by
commas.
The use of a double separator (both
.Sq /
and
.Sq \&.
can be used as
separators) as the prefix tells sysctl that the first series of tokens
is not a MIB name, but a command.
It is recommended that the double separator preceding the command not
be the same as the separator used in naming the MIB entry so as to
avoid possible parse conflicts.
The
.Dq value
assigned, if one is given, must be last.
.Pp
.Bl -bullet -compact
.It
.Ar type= Ns Aq Ar T
where
.Ar T
must be one of
.Dq node ,
.Dq int ,
.Dq string ,
.Dq quad ,
or
.Dq struct .
If the type is omitted, the
.Dq node
type is assumed.
.It
.Ar size= Ns Aq Ar S
here,
.Ar S
asserts the size of the new node.
Nodes of type
.Dq node
should not have a size set.
The size may be omitted for nodes of types
.Dq int
or
.Dq quad .
If the size is omitted for a node of type
.Dq string ,
the size will be determined by the length of the given value, or by
the kernel for kernel strings.
Nodes of type
.Dq struct
must have their size explicitly set.
.It
.Ar addr= Ns Aq Ar A
or
.Ar symbol= Ns Aq Ar A
The kernel address of the data being instrumented.
If
.Dq symbol
is used, the symbol must be globally visible to the in-kernel
.Xr ksyms 4
driver.
.It
.Ar n= Ns Aq Ar N
The MIB number to be assigned to the new node.
If no number is specified, the kernel will assign a value.
.It
.Ar flags= Ns Aq Ar F
A concatenated string of single letters that govern the behavior of
the node.
Flags currently available are:
.Bl -tag -width www
.It a
Allow anyone to write to the node, if it is writable.
.It h
.Dq Hidden .
.Nm
must be invoked with
.Fl A
or the hidden node must be specifically requested in order to see it
.It i
.Dq Immediate .
Makes the node store data in itself, rather than allocating new space
for it.
This is the default for nodes of type
.Dq int
and
.Dq quad .
This is the opposite of owning data.
.It o
.Dq Own .
When the node is created, separate space will be allocated to store
the data to be instrumented.
This is the default for nodes of type
.Dq string
and
.Dq struct
where it is not possible to guarantee sufficient space to store the
data in the node itself.
.It p
.Dq Private .
Nodes that are marked private, and children of nodes so marked, are
only viewable by the superuser.
Be aware that the immediate data that some nodes may store is not
necessarily protected by this.
.It x
.Dq Hexadecimal .
Make
.Nm
default to hexadecimal display of the retrieved value
.It r
.Dq Read-only .
The data instrumented by the given node is read-only.
Note that other mechanisms may still exist for changing the data.
This is the default for nodes that instrument data.
.It w
.Dq Writable .
The data instrumented by the given node is writable at any time.
This is the default for nodes that can have children.
.It 1
.Dq Read-only at securelevel 1 .
The data instrumented by this node is writable until the securelevel
reaches or passes securelevel 1.
Examples of this include some network tunables.
.It 2
.Dq Read-only at securelevel 2 .
The data instrumented by this node is writable until the securelevel
reaches or passes securelevel 2.
An example of this is the per-process core filename setting.
.El
.Pp
.It
.Ar value= Ns Aq Ar V
An initial starting value for a new node that does not reference
existing kernel data.
Initial values can only be assigned for nodes of the
.Dq int ,
.Dq quad ,
and
.Dq string
types.
.El
.Pp
New nodes must fit the following set of criteria:
.Pp
.Bl -bullet -compact
.It
If the new node is to address an existing kernel object, only one of the
.Dq symbol
or
.Dq addr
arguments may be given.
.It
The size for a
.Dq struct
type node must be specified; no initial value is expected or permitted.
.It
Either the size or the initial value for a
.Dq string
node must be given.
.It
The node which will be the parent of the new node must be writable.
.El
.Pp
If any of the given parameters describes an invalid configuration,
.Nm
will emit a diagnostic message to the standard error and exit.
.Pp
Descriptions can be added by the super-user to any node that does not
have one, provided that the node is not marked with the
.Dq PERMANENT
flag.
The syntax is similar to the syntax for creating new nodes with the
exception of the keyword that follows the double separator at the
start of the command:
.Dq //describe=new.node.path=new node description .
Once a description has been added, it cannot be changed or removed.
.Pp
When destroying nodes, only the path to the node is necessary, i.e.,
.Dq //destroy=old.node.path .
No other parameters are expected or permitted.
Nodes being destroyed must have no children, and their parent must be
writable.
Nodes that are marked with the
.Dq Dv PERMANENT
flag (as assigned by the kernel) may not be deleted.
.Pp
In all cases, the initial
.Sq =
that follows the command (eg,
.Dq create ,
.Dq destroy ,
or
.Dq describe )
may be replaced with another instance of the separator character,
provided that the same separator character is used for the length of
the name specification.
.Sh FILES
.Bl -tag -width /etc/sysctl.conf -compact
.It Pa /etc/sysctl.conf
.Nm
variables set at boot time
.El
.Sh EXAMPLES
For example, to retrieve the maximum number of processes allowed
in the system, one would use the following request:
.Bd -literal -offset indent -compact
sysctl kern.maxproc
.Ed
.Pp
To set the maximum number of processes allowed
in the system to 1000, one would use the following request:
.Bd -literal -offset indent -compact
sysctl -w kern.maxproc=1000
.Ed
.Pp
Information about the system clock rate may be obtained with:
.Bd -literal -offset indent -compact
sysctl kern.clockrate
.Ed
.Pp
Information about the load average history may be obtained with:
.Bd -literal -offset indent -compact
sysctl vm.loadavg
.Ed
.Pp
To view the values of the per-process variables of the current shell,
the request:
.Bd -literal -offset indent -compact
sysctl proc.$$
.Ed
can be used if the shell interpreter replaces $$ with its pid (this is true
for most shells).
.Pp
To redirect core dumps to the
.Pa /var/tmp/ Ns Aq username
directory,
.Bd -literal -offset indent -compact
sysctl -w proc.$$.corename=/var/tmp/%u/%n.core
.Ed
should be used.
.Bd -literal -offset indent -compact
sysctl -w proc.curproc.corename=/var/tmp/%u/%n.core
.Ed
changes the value for the sysctl process itself, and will not have the desired
effect.
.Pp
To create the root of a new sub-tree called
.Dq local
add some children to the new node, and some descriptions:
.Bd -literal -offset indent -compact
sysctl -w //create=local
sysctl -w //describe=local=my local sysctl tree
sysctl -w //create=local.esm_debug,type=int,symbol=esm_debug,flags=w
sysctl -w //describe=local.esm_debug=esm driver debug knob
sysctl -w //create=local.audiodebug,type=int,symbol=audiodebug,flags=w
sysctl -w //describe=local.audiodebug=generic audio debug knob
.Ed
Note that the children are made writable so that the two debug
settings in question can be tuned arbitrarily.
.Pp
To destroy that same subtree:
.Bd -literal -offset indent -compact
sysctl -w //destroy=local.esm_debug
sysctl -w //destroy=local.audiodebug
sysctl -w //destroy=local
.Ed
.Sh SEE ALSO
.Xr sysctl 3 ,
.Xr ksyms 4 ,
.Xr sysctl 7
.Sh HISTORY
.Nm sysctl
first appeared in
.Bx 4.4 .