proc_sys_kernel(5) - Linux manual page (original) (raw)
procsyskernel(5) File Formats Manual procsyskernel(5)
NAME top
/proc/sys/kernel/ - control a range of kernel parametersDESCRIPTION top
_/proc/sys/kernel/_
This directory contains files controlling a range of kernel
parameters, as described below.
_/proc/sys/kernel/acct_
This file contains three numbers: _highwater_, _lowwater_, and
_frequency_. If BSD-style process accounting is enabled,
these values control its behavior. If free space on
filesystem where the log lives goes below _lowwater_ percent,
accounting suspends. If free space gets above _highwater_
percent, accounting resumes. _frequency_ determines how
often the kernel checks the amount of free space (value is
in seconds). Default values are 4, 2, and 30. That is,
suspend accounting if 2% or less space is free; resume it
if 4% or more space is free; consider information about
amount of free space valid for 30 seconds.
_/proc/sys/kernel/automsgmni_ (Linux 2.6.27 to Linux 3.18)
From Linux 2.6.27 to Linux 3.18, this file was used to
control recomputing of the value in _/proc/sys/kernel/msgmni_
upon the addition or removal of memory or upon IPC
namespace creation/removal. Echoing "1" into this file
enabled _msgmni_ automatic recomputing (and triggered a
recomputation of _msgmni_ based on the current amount of
available memory and number of IPC namespaces). Echoing
"0" disabled automatic recomputing. (Automatic recomputing
was also disabled if a value was explicitly assigned to
_/proc/sys/kernel/msgmni_.) The default value in _automsgmni_
was 1.
Since Linux 3.19, the content of this file has no effect
(because _msgmni_ defaults to near the maximum value
possible), and reads from this file always return the value
"0".
_/proc/sys/kernel/caplastcap_ (since Linux 3.2)
See [capabilities(7)](../man7/capabilities.7.html).
_/proc/sys/kernel/cap-bound_ (from Linux 2.2 to Linux 2.6.24)
This file holds the value of the kernel _capability bounding_
_set_ (expressed as a signed decimal number). This set is
ANDed against the capabilities permitted to a process
during [execve(2)](../man2/execve.2.html). Starting with Linux 2.6.25, the system-
wide capability bounding set disappeared, and was replaced
by a per-thread bounding set; see [capabilities(7)](../man7/capabilities.7.html).
_/proc/sys/kernel/corepattern_
See [core(5)](../man5/core.5.html).
_/proc/sys/kernel/corepipelimit_
See [core(5)](../man5/core.5.html).
_/proc/sys/kernel/coreusespid_
See [core(5)](../man5/core.5.html).
_/proc/sys/kernel/ctrl-alt-del_
This file controls the handling of Ctrl-Alt-Del from the
keyboard. When the value in this file is 0, Ctrl-Alt-Del
is trapped and sent to the [init(1)](../man1/init.1.html) program to handle a
graceful restart. When the value is greater than zero,
Linux's reaction to a Vulcan Nerve Pinch (tm) will be an
immediate reboot, without even syncing its dirty buffers.
Note: when a program (like dosemu) has the keyboard in
"raw" mode, the Ctrl-Alt-Del is intercepted by the program
before it ever reaches the kernel tty layer, and it's up to
the program to decide what to do with it.
_/proc/sys/kernel/dmesgrestrict_ (since Linux 2.6.37)
The value in this file determines who can see kernel syslog
contents. A value of 0 in this file imposes no
restrictions. If the value is 1, only privileged users can
read the kernel syslog. (See [syslog(2)](../man2/syslog.2.html) for more details.)
Since Linux 3.4, only users with the **CAP_SYS_ADMIN**
capability may change the value in this file.
_/proc/sys/kernel/domainname_
_/proc/sys/kernel/hostname_
can be used to set the NIS/YP domainname and the hostname
of your box in exactly the same way as the commands
[domainname(1)](../man1/domainname.1.html) and [hostname(1)](../man1/hostname.1.html), that is:
# **echo 'darkstar' > /proc/sys/kernel/hostname**
# **echo 'mydomain' > /proc/sys/kernel/domainname**
has the same effect as
# **hostname 'darkstar'**
# **domainname 'mydomain'**
Note, however, that the classic darkstar.frop.org has the
hostname "darkstar" and DNS (Internet Domain Name Server)
domainname "frop.org", not to be confused with the NIS
(Network Information Service) or YP (Yellow Pages)
domainname. These two domain names are in general
different. For a detailed discussion see the [hostname(1)](../man1/hostname.1.html)
man page.
_/proc/sys/kernel/hotplug_
This file contains the pathname for the hotplug policy
agent. The default value in this file is _/sbin/hotplug_.
_/proc/sys/kernel/htab-reclaim_ (before Linux 2.4.9.2)
(PowerPC only) If this file is set to a nonzero value, the
PowerPC htab (see kernel file
_Documentation/powerpc/ppchtab.txt_) is pruned each time the
system hits the idle loop.
_/proc/sys/kernel/keys/_
This directory contains various files that define
parameters and limits for the key-management facility.
These files are described in [keyrings(7)](../man7/keyrings.7.html).
_/proc/sys/kernel/kptrrestrict_ (since Linux 2.6.38)
The value in this file determines whether kernel addresses
are exposed via _/proc_ files and other interfaces. A value
of 0 in this file imposes no restrictions. If the value is
1, kernel pointers printed using the _%pK_ format specifier
will be replaced with zeros unless the user has the
**CAP_SYSLOG** capability. If the value is 2, kernel pointers
printed using the _%pK_ format specifier will be replaced
with zeros regardless of the user's capabilities. The
initial default value for this file was 1, but the default
was changed to 0 in Linux 2.6.39. Since Linux 3.4, only
users with the **CAP_SYS_ADMIN** capability can change the
value in this file.
_/proc/sys/kernel/l2cr_
(PowerPC only) This file contains a flag that controls the
L2 cache of G3 processor boards. If 0, the cache is
disabled. Enabled if nonzero.
_/proc/sys/kernel/modprobe_
This file contains the pathname for the kernel module
loader. The default value is _/sbin/modprobe_. The file is
present only if the kernel is built with the **CONFIG_MODULES**
(**CONFIG_KMOD** in Linux 2.6.26 and earlier) option enabled.
It is described by the Linux kernel source file
_Documentation/kmod.txt_ (present only in Linux 2.4 and
earlier).
_/proc/sys/kernel/modulesdisabled_ (since Linux 2.6.31)
A toggle value indicating if modules are allowed to be
loaded in an otherwise modular kernel. This toggle
defaults to off (0), but can be set true (1). Once true,
modules can be neither loaded nor unloaded, and the toggle
cannot be set back to false. The file is present only if
the kernel is built with the **CONFIG_MODULES** option enabled.
_/proc/sys/kernel/msgmax_ (since Linux 2.2)
This file defines a system-wide limit specifying the
maximum number of bytes in a single message written on a
System V message queue.
_/proc/sys/kernel/msgmni_ (since Linux 2.4)
This file defines the system-wide limit on the number of
message queue identifiers. See also
_/proc/sys/kernel/automsgmni_.
_/proc/sys/kernel/msgmnb_ (since Linux 2.2)
This file defines a system-wide parameter used to
initialize the _msgqbytes_ setting for subsequently created
message queues. The _msgqbytes_ setting specifies the
maximum number of bytes that may be written to the message
queue.
_/proc/sys/kernel/ngroupsmax_ (since Linux 2.6.4)
This is a read-only file that displays the upper limit on
the number of a process's group memberships.
_/proc/sys/kernel/nslastpid_ (since Linux 3.3)
See [pid_namespaces(7)](../man7/pid%5Fnamespaces.7.html).
_/proc/sys/kernel/ostype_
_/proc/sys/kernel/osrelease_
These files give substrings of _/proc/version_.
_/proc/sys/kernel/overflowgid_
_/proc/sys/kernel/overflowuid_
These files duplicate the files _/proc/sys/fs/overflowgid_
and _/proc/sys/fs/overflowuid_.
_/proc/sys/kernel/panic_
This file gives read/write access to the kernel variable
_panictimeout_. If this is zero, the kernel will loop on a
panic; if nonzero, it indicates that the kernel should
autoreboot after this number of seconds. When you use the
software watchdog device driver, the recommended setting is
60.
_/proc/sys/kernel/paniconoops_ (since Linux 2.5.68)
This file controls the kernel's behavior when an oops or
BUG is encountered. If this file contains 0, then the
system tries to continue operation. If it contains 1, then
the system delays a few seconds (to give klogd time to
record the oops output) and then panics. If the
_/proc/sys/kernel/panic_ file is also nonzero, then the
machine will be rebooted.
_/proc/sys/kernel/pidmax_ (since Linux 2.5.34)
This file specifies the value at which PIDs wrap around
(i.e., the value in this file is one greater than the
maximum PID). PIDs greater than this value are not
allocated; thus, the value in this file also acts as a
system-wide limit on the total number of processes and
threads. The default value for this file, 32768, results
in the same range of PIDs as on earlier kernels. On 32-bit
platforms, 32768 is the maximum value for _pidmax_. On
64-bit systems, _pidmax_ can be set to any value up to 2^22
(**PID_MAX_LIMIT**, approximately 4 million).
_/proc/sys/kernel/powersave-nap_ (PowerPC only)
This file contains a flag. If set, Linux-PPC will use the
"nap" mode of powersaving, otherwise the "doze" mode will
be used.
_/proc/sys/kernel/printk_
See [syslog(2)](../man2/syslog.2.html).
_/proc/sys/kernel/pty_ (since Linux 2.6.4)
This directory contains two files relating to the number of
UNIX 98 pseudoterminals (see [pts(4)](../man4/pts.4.html)) on the system.
_/proc/sys/kernel/pty/max_
This file defines the maximum number of pseudoterminals.
_/proc/sys/kernel/pty/nr_
This read-only file indicates how many pseudoterminals are
currently in use.
_/proc/sys/kernel/random/_
This directory contains various parameters controlling the
operation of the file _/dev/random_. See [random(4)](../man4/random.4.html) for
further information.
_/proc/sys/kernel/random/uuid_ (since Linux 2.4)
Each read from this read-only file returns a randomly
generated 128-bit UUID, as a string in the standard UUID
format.
_/proc/sys/kernel/randomizevaspace_ (since Linux 2.6.12)
Select the address space layout randomization (ASLR) policy
for the system (on architectures that support ASLR). Three
values are supported for this file:
**0** Turn ASLR off. This is the default for
architectures that don't support ASLR, and when the
kernel is booted with the _norandmaps_ parameter.
**1** Make the addresses of [mmap(2)](../man2/mmap.2.html) allocations, the
stack, and the VDSO page randomized. Among other
things, this means that shared libraries will be
loaded at randomized addresses. The text segment of
PIE-linked binaries will also be loaded at a
randomized address. This value is the default if
the kernel was configured with **CONFIG_COMPAT_BRK**.
**2** (Since Linux 2.6.25) Also support heap
randomization. This value is the default if the
kernel was not configured with **CONFIG_COMPAT_BRK**.
_/proc/sys/kernel/real-root-dev_
This file is documented in the Linux kernel source file
_Documentation/admin-guide/initrd.rst_ (or
_Documentation/initrd.txt_ before Linux 4.10).
_/proc/sys/kernel/reboot-cmd_ (Sparc only)
This file seems to be a way to give an argument to the
SPARC ROM/Flash boot loader. Maybe to tell it what to do
after rebooting?
_/proc/sys/kernel/rtsig-max_
(Up to and including Linux 2.6.7; see [setrlimit(2)](../man2/setrlimit.2.html)) This
file can be used to tune the maximum number of POSIX real-
time (queued) signals that can be outstanding in the
system.
_/proc/sys/kernel/rtsig-nr_
(Up to and including Linux 2.6.7.) This file shows the
number of POSIX real-time signals currently queued.
_/proc/_pid_/schedautogroupenabled_ (since Linux 2.6.38)
See [sched(7)](../man7/sched.7.html).
_/proc/sys/kernel/schedchildrunsfirst_ (since Linux 2.6.23)
If this file contains the value zero, then, after a
[fork(2)](../man2/fork.2.html), the parent is first scheduled on the CPU. If the
file contains a nonzero value, then the child is scheduled
first on the CPU. (Of course, on a multiprocessor system,
the parent and the child might both immediately be
scheduled on a CPU.)
_/proc/sys/kernel/schedrrtimeslicems_ (since Linux 3.9)
See [sched_rr_get_interval(2)](../man2/sched%5Frr%5Fget%5Finterval.2.html).
_/proc/sys/kernel/schedrtperiodus_ (since Linux 2.6.25)
See [sched(7)](../man7/sched.7.html).
_/proc/sys/kernel/schedrtruntimeus_ (since Linux 2.6.25)
See [sched(7)](../man7/sched.7.html).
_/proc/sys/kernel/seccomp/_ (since Linux 4.14)
This directory provides additional seccomp information and
configuration. See [seccomp(2)](../man2/seccomp.2.html) for further details.
_/proc/sys/kernel/sem_ (since Linux 2.4)
This file contains 4 numbers defining limits for System V
IPC semaphores. These fields are, in order:
SEMMSL The maximum semaphores per semaphore set.
SEMMNS A system-wide limit on the number of semaphores in
all semaphore sets.
SEMOPM The maximum number of operations that may be
specified in a [semop(2)](../man2/semop.2.html) call.
SEMMNI A system-wide limit on the maximum number of
semaphore identifiers.
_/proc/sys/kernel/sg-big-buff_
This file shows the size of the generic SCSI device (sg)
buffer. You can't tune it just yet, but you could change
it at compile time by editing _include/scsi/sg.h_ and
changing the value of **SG_BIG_BUFF**. However, there
shouldn't be any reason to change this value.
_/proc/sys/kernel/shmrmidforced_ (since Linux 3.1)
If this file is set to 1, all System V shared memory
segments will be marked for destruction as soon as the
number of attached processes falls to zero; in other words,
it is no longer possible to create shared memory segments
that exist independently of any attached process.
The effect is as though a [shmctl(2)](../man2/shmctl.2.html) **IPC_RMID** is performed
on all existing segments as well as all segments created in
the future (until this file is reset to 0). Note that
existing segments that are attached to no process will be
immediately destroyed when this file is set to 1. Setting
this option will also destroy segments that were created,
but never attached, upon termination of the process that
created the segment with [shmget(2)](../man2/shmget.2.html).
Setting this file to 1 provides a way of ensuring that all
System V shared memory segments are counted against the
resource usage and resource limits (see the description of
**RLIMIT_AS** in [getrlimit(2)](../man2/getrlimit.2.html)) of at least one process.
Because setting this file to 1 produces behavior that is
nonstandard and could also break existing applications, the
default value in this file is 0. Set this file to 1 only
if you have a good understanding of the semantics of the
applications using System V shared memory on your system.
_/proc/sys/kernel/shmall_ (since Linux 2.2)
This file contains the system-wide limit on the total
number of pages of System V shared memory.
_/proc/sys/kernel/shmmax_ (since Linux 2.2)
This file can be used to query and set the run-time limit
on the maximum (System V IPC) shared memory segment size
that can be created. Shared memory segments up to 1 GB are
now supported in the kernel. This value defaults to
**SHMMAX**.
_/proc/sys/kernel/shmmni_ (since Linux 2.4)
This file specifies the system-wide maximum number of
System V shared memory segments that can be created.
_/proc/sys/kernel/sysctlwritesstrict_ (since Linux 3.16)
The value in this file determines how the file offset
affects the behavior of updating entries in files under
_/proc/sys_. The file has three possible values:
-1 This provides legacy handling, with no printk warnings.
Each [write(2)](../man2/write.2.html) must fully contain the value to be
written, and multiple writes on the same file
descriptor will overwrite the entire value, regardless
of the file position.
0 (default) This provides the same behavior as for -1,
but printk warnings are written for processes that
perform writes when the file offset is not 0.
1 Respect the file offset when writing strings into
_/proc/sys_ files. Multiple writes will _append_ to the
value buffer. Anything written beyond the maximum
length of the value buffer will be ignored. Writes to
numeric _/proc/sys_ entries must always be at file offset
0 and the value must be fully contained in the buffer
provided to [write(2)](../man2/write.2.html).
_/proc/sys/kernel/sysrq_
This file controls the functions allowed to be invoked by
the SysRq key. By default, the file contains 1 meaning
that every possible SysRq request is allowed (in older
kernel versions, SysRq was disabled by default, and you
were required to specifically enable it at run-time, but
this is not the case any more). Possible values in this
file are:
0 Disable sysrq completely
1 Enable all functions of sysrq
> 1 Bit mask of allowed sysrq functions, as follows:
2 Enable control of console logging level
4 Enable control of keyboard (SAK, unraw)
8 Enable debugging dumps of processes etc.
16 Enable sync command
32 Enable remount read-only
64 Enable signaling of processes (term, kill, oom-
kill)
128 Allow reboot/poweroff
256 Allow nicing of all real-time tasks
This file is present only if the **CONFIG_MAGIC_SYSRQ** kernel
configuration option is enabled. For further details see
the Linux kernel source file
_Documentation/admin-guide/sysrq.rst_ (or
_Documentation/sysrq.txt_ before Linux 4.10).
_/proc/sys/kernel/version_
This file contains a string such as:
#5 Wed Feb 25 21:49:24 MET 1998
The "#5" means that this is the fifth kernel built from
this source base and the date following it indicates the
time the kernel was built.
_/proc/sys/kernel/threads-max_ (since Linux 2.3.11)
This file specifies the system-wide limit on the number of
threads (tasks) that can be created on the system.
Since Linux 4.1, the value that can be written to
_threads-max_ is bounded. The minimum value that can be
written is 20. The maximum value that can be written is
given by the constant **FUTEX_TID_MASK** (0x3fffffff). If a
value outside of this range is written to _threads-max_, the
error **EINVAL** occurs.
The value written is checked against the available RAM
pages. If the thread structures would occupy too much
(more than 1/8th) of the available RAM pages, _threads-max_
is reduced accordingly.
_/proc/sys/kernel/yama/ptracescope_ (since Linux 3.5)
See [ptrace(2)](../man2/ptrace.2.html).
_/proc/sys/kernel/zero-paged_ (PowerPC only)
This file contains a flag. When enabled (nonzero), Linux-
PPC will pre-zero pages in the idle loop, possibly speeding
up get_free_pages.SEE ALSO top
[proc(5)](../man5/proc.5.html), [proc_sys(5)](../man5/proc%5Fsys.5.html)COLOPHON top
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