# proc_sys_kernel

## NAME



           /proc/sys/kernel/ - control a range of kernel parameters
    


## DESCRIPTION



           _/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/auto_msgmni_ (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 _auto_msgmni_
                  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/cap_last_cap_ (since Linux 3.2)
                  See capabilities(7).
    
           _/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).  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).
    
           _/proc/sys/kernel/core_pattern_
                  See core(5).
    
           _/proc/sys/kernel/core_pipe_limit_
                  See core(5).
    
           _/proc/sys/kernel/core_uses_pid_
                  See core(5).
    
           _/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) 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/dmesg_restrict_ (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) 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) and hostname(1), 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)
                  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/ppc_htab.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).
    
           _/proc/sys/kernel/kptr_restrict_ (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/modules_disabled_ (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/auto_msgmni_.
    
           _/proc/sys/kernel/msgmnb_ (since Linux 2.2)
                  This file defines a system-wide parameter used to
                  initialize the _msg_qbytes_ setting for subsequently created
                  message queues.  The _msg_qbytes_ setting specifies the
                  maximum number of bytes that may be written to the message
                  queue.
    
           _/proc/sys/kernel/ngroups_max_ (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/ns_last_pid_ (since Linux 3.3)
                  See pid_namespaces(7).
    
           _/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
                  _panic_timeout_.  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/panic_on_oops_ (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/pid_max_ (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 _pid_max_.  On
                  64-bit systems, _pid_max_ 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).
    
           _/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)) 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) 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/randomize_va_space_ (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) 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)) 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 _/sched_autogroup_enabled_ (since Linux 2.6.38)
                  See sched(7).
    
           _/proc/sys/kernel/sched_child_runs_first_ (since Linux 2.6.23)
                  If this file contains the value zero, then, after a
                  fork(2), 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/sched_rr_timeslice_ms_ (since Linux 3.9)
                  See sched_rr_get_interval(2).
    
           _/proc/sys/kernel/sched_rt_period_us_ (since Linux 2.6.25)
                  See sched(7).
    
           _/proc/sys/kernel/sched_rt_runtime_us_ (since Linux 2.6.25)
                  See sched(7).
    
           _/proc/sys/kernel/seccomp/_ (since Linux 4.14)
                  This directory provides additional seccomp information and
                  configuration.  See seccomp(2) 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) 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/shm_rmid_forced_ (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) **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).
    
                  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)) 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/sysctl_writes_strict_ (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) 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).
    
           _/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/ptrace_scope_ (since Linux 3.5)
                  See ptrace(2).
    
           _/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.
    


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