SYSCTL(7)

SYSCTL(7) | Miscellaneous Information Manual | SYSCTL(7)

# NAME

sysctl — system information variables

# DESCRIPTION

The sysctl(3) library function and the sysctl(8) utility are used to get and set values of system variables, maintained by the kernel. The variables are organized in a tree and identified by a sequence of numbers, conventionally separated by dots with the topmost identifier at the left side. The numbers have corresponding text names. The sysctlnametomib(3) function or the -M argument to the sysctl(8) utility can be used to convert the text representation to the numeric one.

The individual sysctl variables are described below, both the textual and numeric form where applicable. The textual names can be used as argument to the sysctl(8) utility and in the file /etc/sysctl.conf. The numeric names are usually defined as preprocessor constants and are intended for use by programs. Every such constant expands to one integer, which identifies the sysctl variable relative to the upper level of the tree. See the sysctl(3) manual page for programming examples.

## Top level names

The top level names are defined with a CTL_ prefix in <sys/sysctl.h>, and are as follows. The next and subsequent levels down are found in the include files listed here, and described in separate sections below.

Name | Constant | Next level names | Description
kern | CTL_KERN | <sys/sysctl.h> | High kernel limits
vm | CTL_VM | <uvm/uvm_param.h> | Virtual memory
vfs | CTL_VFS | <sys/mount.h> | Filesystem
net | CTL_NET | <sys/socket.h> | Networking
debug | CTL_DEBUG | <sys/sysctl.h> | Debugging
hw | CTL_HW | <sys/sysctl.h> | Generic CPU, I/O
machdep | CTL_MACHDEP | <sys/sysctl.h> | Machine dependent
user | CTL_USER | <sys/sysctl.h> | User-level
ddb | CTL_DDB | <sys/sysctl.h> | In-kernel debugger
proc | CTL_PROC | <sys/sysctl.h> | Per-process
vendor | CTL_VENDOR | ? | Vendor specific
emul | CTL_EMUL | <sys/sysctl.h> | Emulation settings
security | CTL_SECURITY | <sys/sysctl.h> | Security settings

## The debug.* subtree

The debugging variables vary from system to system. A debugging variable may be added or deleted without need to recompile sysctl to know about it. Each time it runs, sysctl gets the list of debugging variables from the kernel and displays their current values. The system defines twenty (struct ctldebug) variables named debug0 through debug19. They are declared as separate variables so that they can be individually initialized at the location of their associated variable. The loader prevents multiple use of the same variable by issuing errors if a variable is initialized in more than one place. For example, to export the variable dospecialcheck as a debugging variable, the following declaration would be used:

    int dospecialcheck = 1;
    struct ctldebug debug5 = { "dospecialcheck", &dospecialcheck };

Note that the dynamic implementation of sysctl currently in use largely makes this particular sysctl interface obsolete. See sysctl(8) for more information.

## The vfs.* subtree

A distinguished second level name, `vfs.generic` (`VFS_GENERIC`), is used to get general information about all file systems. It has the following third level identifiers:

`vfs.generic.maxtypenum` (`VFS_MAXTYPENUM`)
  The highest valid file system type number.
`vfs.generic.conf` (`VFS_CONF`)
  Returns configuration information about the file system type given as a fourth level identifier.
`vfs.generic.usermount` (`VFS_USERMOUNT`)
  Determines if non superuser mounts are allowed, defaults to 0.
`vfs.generic.magiclinks` (`VFS_MAGICLINKS`)
  Controls if expansion of variables is going to be performed on pathnames or not. Defaults to no variable expansion, 0. Variables are of the form `@name` and the variables supported are described in symlink(7) under “MAGIC SYMLINKS”.

A second level name for controlling the wapbl(4) (Write Ahead Physical Block Logging file system journalling) capabilities with the following third level identifiers:

`vfs.wapbl.flush_disk_cache`
  Controls whether to attempt to flush the disk cache on each commit. It defaults to 1 and it should always be on to ensure integrity of file system metadata in the event of a power loss. For slow disks, turning it off can improve performance.
`vfs.wapbl.verbose_commit`
  For each transaction log commit, print the number of bytes written and the time it took to commit as seconds.nanoseconds.

The remaining second level identifiers are the file system names, identified by the type number returned by a statvfs(2) call or from `vfs.generic.conf`.

The third level identifiers available for each file system are given in the header file that defines the mount argument structure for that file system.

## The hw.* subtree

The string and integer information available for the `hw` level is detailed below. The changeable column shows whether a process with appropriate privilege may change the value.

Second level name | Type | Changeable
hw.alignbytes | integer | no
hw.byteorder | integer | no
hw.cnmagic | string | yes
hw.disknames | string | no
hw.diskstats | struct | no
hw.machine | string | no
hw.machine_arch | string | no
hw.model | string | no
hw.ncpu | integer | no
hw.ncpuonline | integer | no
hw.pagesize | integer | no
hw.physmem | integer | no
hw.physmem64 | quad | no
hw.usermem | integer | no
hw.usermem64 | quad | no

`hw.alignbytes` (`HW_ALIGNBYTES`)
  Alignment constraint for all possible data types. This shows the value ALIGNBYTES in <machine/param.h>, at the kernel compilation time.
`hw.byteorder` (`HW_BYTEORDER`)
  The byteorder (4321, or 1234).
`hw.cnmagic` (`HW_CNMAGIC`)
  The console magic key sequence.
`hw.disknames` (`HW_DISKNAMES`)
  The list of (space separated) disk device names on the system.
`hw.iostatnames` (`HW_IOSTATNAMES`)
  A space separated list of devices that will have I/O statistics collected on them.
`hw.iostats` (`HW_IOSTATS`)
  Return statistical information on the NFS mounts, disk and tape devices on the system. An array of struct io_sysctl structures is returned, whose size depends on the current number of such objects in the system. The third level name is the size of the struct io_sysctl. The type of object can be determined by examining the type element of struct io_sysctl. Which can be IOSTAT_DISK (disk drive), IOSTAT_TAPE (tape drive), or IOSTAT_NFS (NFS mount).
`hw.machine` (`HW_MACHINE`)
  The machine class.
`hw.machine_arch` (`HW_MACHINE_ARCH`)
  The machine CPU class.
`hw.model` (`HW_MODEL`)
  The machine model.
`hw.ncpu` (`HW_NCPU`)
  The number of CPUs configured.
`hw.ncpuonline` (`HW_NCPUONLINE`)
  The number of CPUs online.
`hw.pagesize` (`HW_PAGESIZE`)
  The software page size.
`hw.physmem` (`HW_PHYSMEM`)
  The bytes of physical memory as a 32-bit integer.
`hw.physmem64` (`HW_PHYSMEM64`)
  The bytes of physical memory as a 64-bit integer.
`hw.usermem` (`HW_USERMEM`)
  The bytes of non-kernel memory as a 32-bit integer.
`hw.usermem64` (`HW_USERMEM64`)
  The bytes of non-kernel memory as a 64-bit integer.

## The kern.* subtree

This subtree includes data generally related to the kernel. The string and integer information available for the `kern` level is detailed below. The changeable column shows whether a process with appropriate privilege may change the value.

Second level name | Type | Changeable
kern.aio_listio_max | integer | yes
kern.aio_max | integer | yes
kern.arandom | integer | no
kern.argmax | integer | no
kern.boothowto | integer | no
kern.boottime | struct timeval | no
kern.buildinfo | string | no
kern.ccpu | integer | no
kern.clockrate | struct clockinfo | no
kern.consdev | integer | no
kern.coredump | node | not applicable
kern.cp_id | struct | no
kern.cp_time | uint64_t[] | no
kern.cryptodevallowsoft | integer | yes
kern.defcorename | string | yes
kern.detachall | integer | yes
kern.domainname | string | yes
kern.drivers | struct kinfo_drivers | no
kern.dump_on_panic | integer | yes
kern.file | struct file | no
kern.forkfsleep | integer | yes
kern.fscale | integer | no
kern.fsync | integer | no
kern.hardclock_ticks | integer | no
kern.hostid | integer | yes
kern.hostname | string | yes
kern.iov_max | integer | no
kern.ipc | node | not applicable
kern.job_control | integer | no
kern.labeloffset | integer | no
kern.labelsector | integer | no
kern.login_name_max | integer | no
kern.logsigexit | integer | yes
kern.mapped_files | integer | no
kern.maxfiles | integer | yes
kern.maxlwp | integer | yes
kern.maxpartitions | integer | no
kern.maxphys | integer | no
kern.maxproc | integer | yes
kern.maxptys | integer | yes
kern.maxvnodes | integer | yes
kern.mbuf | node | not applicable
kern.memlock | integer | no
kern.memlock_range | integer | no
kern.memory_protection | integer | no
kern.module | node | not applicable
kern.monotonic_clock | integer | no
kern.mqueue | node | not applicable
kern.msgbuf | integer | no
kern.msgbufsize | integer | no
kern.ngroups | integer | no
kern.ntptime | struct ntptimeval | no
kern.osrelease | string | no
kern.osrevision | integer | no
kern.ostype | string | no
kern.pipe | node | not applicable
kern.pool | struct pool_sysctl | no
kern.posix1version | integer | no
kern.posix_aio | integer | no
kern.posix_barriers | integer | no
kern.posix_reader_writer_locks | integer | no
kern.posix_semaphores | integer | no
kern.posix_spin_locks | integer | no
kern.posix_threads | integer | no
kern.posix_timers | integer | no
kern.proc | struct kinfo_proc | no
kern.proc2 | struct kinfo_proc2 | no
kern.proc_args | string | no
kern.profiling | node | not applicable
kern.rawpartition | integer | no
kern.root_device | string | no
kern.root_partition | integer | no
kern.rtc_offset | integer | yes
kern.saved_ids | integer | no
kern.sbmax | integer | yes
kern.securelevel | integer | raise only
kern.somaxkva | integer | yes
kern.synchronized_io | integer | no
kern.timecounter | node | not applicable
kern.timex | struct | no
kern.tkstat | node | not applicable
kern.tty | node | not applicable
kern.urandom | integer | no
kern.usercrypto | integer | yes
kern.userasymcrypto | integer | yes
kern.veriexec | node | not applicable
kern.version | string | no
kern.vnode | struct vnode | no

`kern.aio_listio_max`
  The maximum number of asynchronous I/O operations in a single list I/O call. Like with all variables related to aio(3), the variable may be created and removed dynamically upon loading or unloading the corresponding kernel module.
`kern.aio_max`
  The maximum number of asynchronous I/O operations.
`kern.arandom`
  This variable picks a random number each time it is queried. The used random number generator (RNG) is based on arc4random(3).
`kern.argmax` (`KERN_ARGMAX`)
  The maximum bytes of argument to execve(2).
`kern.boothowto`
  Flags passed from the boot loader; see reboot(2) for the meanings of the flags.
`kern.boottime` (`KERN_BOOTTIME`)
  A struct timeval structure is returned. This structure contains the time that the system was booted.
`kern.buildinfo`
  When the kernel is built, the build environment may optionally provide arbitrary information to be stored in this variable.
`kern.ccpu` (`KERN_CCPU`)
  The scheduler exponential decay value.
`kern.clockrate` (`KERN_CLOCKRATE`)
  A struct clockinfo structure is returned. This structure contains the clock, statistics clock and profiling clock frequencies, the number of micro-seconds per hz tick, and the clock skew rate. Refer to hz(9) for additional details.
`kern.consdev` (`KERN_CONSDEV`)
  Console device.
`kern.coredump`
  Settings related to set-id processes coredumps. By default, set-id processes do not dump core in situations where other processes would. The settings in this node allows an administrator to change this behavior.

  The third level name is kern.coredump.setid and fourth level variables are described below.

  Fourth level name | Type | Changeable
  kern.coredump.setid.dump | integer | yes
  kern.coredump.setid.group | integer | yes
  kern.coredump.setid.mode | integer | yes
  kern.coredump.setid.owner | integer | yes
  kern.coredump.setid.path | string | yes

  `kern.coredump.setid.dump`
    If non-zero, set-id processes will dump core.
  `kern.coredump.setid.group`
    The group-id for the set-id processes' coredump.
  `kern.coredump.setid.mode`
    The mode for the set-id processes' coredump. See chmod(1).
  `kern.coredump.setid.owner`
    The user-id that will be used as the owner of the set-id processes' coredump.
  `kern.coredump.setid.path`
    The path to which set-id processes' coredumps will be saved to. Same syntax as kern.defcorename.
`kern.cp_id` (`KERN_CP_ID`)
  Mapping of CPU number to CPU id.
`kern.cp_time` (`KERN_CP_TIME`)
  Returns an array of CPUSTATES uint64_ts. This array contains the number of clock ticks spent in different CPU states. On multi-processor systems, the sum across all CPUs is returned unless appropriate space is given for one data set for each CPU. Data for a specific CPU can also be obtained by adding the number of the CPU at the end of the MIB, enlarging it by one.
`kern.cryptodevallowsoft`
  This variable controls userland access to hardware versus software transforms in the crypto(4) system. The available values are as follows:
  < 0
    Always force userlevel requests to use software transforms.
  = 0
    If present, use hardware and grant userlevel requests for non-accelerated transforms (handling the latter in software).
  > 0
    Allow user requests only for transforms which are hardware-accelerated.
`kern.defcorename` (`KERN_DEFCORENAME`)
  Default template for the name of core dump files (see also `proc.pid.corename` in the per-process variables `proc.*`, and core(5) for format of this template). The default value is %n.core and can be changed with the kernel configuration option options DEFCORENAME (see options(4) ).
`kern.detachall`
  Detach all devices at shutdown.
`kern.domainname` (`KERN_DOMAINNAME`)
  Get or set the YP domain name.
`kern.drivers` (`KERN_DRIVERS`)
  Return an array of struct kinfo_drivers that contains the name and major device numbers of all the device drivers in the current kernel. The d_name field is always a NUL terminated string. The d_bmajor field will be set to -1 if the driver doesn't have a block device.
`kern.dump_on_panic` (`KERN_DUMP_ON_PANIC`)
  Perform a crash dump on system panic(9).
`kern.file` (`KERN_FILE`)
  Return the entire file table. The returned data consists of a single struct filelist followed by an array of struct file, whose size depends on the current number of such objects in the system.
`kern.forkfsleep` (`KERN_FORKFSLEEP`)
  If fork(2) system call fails due to limit on number of processes (either the global maxproc limit or user's one), wait for this many milliseconds before returning EAGAIN error to process. Useful to keep heavily forking runaway processes in bay. Default zero (no sleep). Maximum is 20 seconds.
`kern.fscale` (`KERN_FSCALE`)
  The kernel fixed-point scale factor.
`kern.fsync` (`KERN_FSYNC`)
  Return 1 if the IEEE Std 1003.1b-1993 (“POSIX.1b”) File Synchronization Option is available on this system, otherwise 0.
`kern.hardclock_ticks` (`KERN_HARDCLOCK_TICKS`)
  Returns the number of hardclock(9) ticks.
`kern.hostid` (`KERN_HOSTID`)
  Get or set the host identifier. This is aimed to replace the legacy gethostid(3) and sethostid(3) system calls.
`kern.hostname` (`KERN_HOSTNAME`)
  Get or set the hostname(1).
`kern.iov_max` (`KERN_IOV_MAX`)
  Return the maximum number of iovec structures that a process has available for use with preadv(2), pwritev(2), readv(2), recvmsg(2), sendmsg(2) and writev(2).
`kern.ipc` (`KERN_SYSVIPC`)
  Return information about the SysV IPC parameters. The third level names for the ipc variables are detailed below.

  Third level name | Type | Changeable
  kern.ipc.sysvmsg | integer | no
  kern.ipc.sysvsem | integer | no
  kern.ipc.sysvshm | integer | no
  kern.ipc.sysvipc_info | struct | no
  kern.ipc.shmmax | integer | yes
  kern.ipc.shmmni | integer | yes
  kern.ipc.shmseg | integer | yes
  kern.ipc.shmmaxpgs | integer | yes
  kern.ipc.shm_use_phys | integer | yes
  kern.ipc.msgmni | integer | yes
  kern.ipc.msgseg | integer | yes
  kern.ipc.semmni | integer | yes
  kern.ipc.semmns | integer | yes
  kern.ipc.semmnu | integer | yes

  `kern.ipc.sysvmsg` (`KERN_SYSVIPC_MSG`)
    Returns 1 if System V style message queue functionality is available on this system, otherwise 0.
  `kern.ipc.sysvsem` (`KERN_SYSVIPC_SEM`)
    Returns 1 if System V style semaphore functionality is available on this system, otherwise 0.
  `kern.ipc.sysvshm` (`KERN_SYSVIPC_SHM`)
    Returns 1 if System V style share memory functionality is available on this system, otherwise 0.
  `kern.ipc.sysvipc_info` (`KERN_SYSVIPC_INFO`)
    Return System V style IPC configuration and run-time information. The fourth level name selects the System V style IPC facility.

    Fourth level name | Type
    KERN_SYSVIPC_MSG_INFO | struct msg_sysctl_info
    KERN_SYSVIPC_SEM_INFO | struct sem_sysctl_info
    KERN_SYSVIPC_SHM_INFO | struct shm_sysctl_info

    `KERN_SYSVIPC_MSG_INFO`
      Return information on the System V style message facility. The msg_sysctl_info structure is defined in <sys/msg.h>.
    `KERN_SYSVIPC_SEM_INFO`
      Return information on the System V style semaphore facility. The sem_sysctl_info structure is defined in <sys/sem.h>.
    `KERN_SYSVIPC_SHM_INFO`
      Return information on the System V style shared memory facility. The shm_sysctl_info structure is defined in <sys/shm.h>.
  `kern.ipc.shmmax` (`KERN_SYSVIPC_SHMMAX`)
    Max shared memory segment size in bytes.
  `kern.ipc.shmmni` (`KERN_SYSVIPC_SHMMNI`)
    Max number of shared memory identifiers.
  `kern.ipc.shmseg` (`KERN_SYSVIPC_SHMSEG`)
    Max shared memory segments per process.
  `kern.ipc.shmmaxpgs` (`KERN_SYSVIPC_SHMMAXPGS`)
    Max amount of shared memory in pages.
  `kern.ipc.shm_use_phys` (`KERN_SYSVIPC_SHMUSEPHYS`)
    Locking of shared memory in physical memory. If 0, memory can be swapped out, otherwise it will be locked in physical memory.
  `kern.ipc.msgmni`
    Max number of message queue identifiers.
  `kern.ipc.msgseg`
    Max number of number of message segments.
  `kern.ipc.semmni`
    Max number of number of semaphore identifiers.
  `kern.ipc.semmns`
    Max number of number of semaphores in system.
  `kern.ipc.semmnu`
    Max number of undo structures in system.
`kern.job_control` (`KERN_JOB_CONTROL`)
  Return 1 if job control is available on this system, otherwise 0.
`kern.labeloffset` (`KERN_LABELOFFSET`)
  The offset within the sector specified by KERN_LABELSECTOR of the disklabel(5).
`kern.labelsector` (`KERN_LABELSECTOR`)
  The sector number containing the disklabel(5).
`kern.login_name_max` (`KERN_LOGIN_NAME_MAX`)
  The size of the storage required for a login name, in bytes, including the terminating NUL.
`kern.logsigexit` (`KERN_LOGSIGEXIT`)
  If this flag is non-zero, the kernel will log(9) all process exits due to signals which create a core(5) file, and whether the coredump was created.
`kern.mapped_files` (`KERN_MAPPED_FILES`)
  Returns 1 if the IEEE Std 1003.1b-1993 (“POSIX.1b”) Memory Mapped Files Option is available on this system, otherwise 0.
`kern.maxfiles` (`KERN_MAXFILES`)
  The maximum number of open files that may be open in the system.
`kern.maxpartitions` (`KERN_MAXPARTITIONS`)
  The maximum number of partitions allowed per disk.
`kern.maxlwp`
  The maximum number of Lightweight Processes (threads) the system allows per uid.
`kern.maxphys` (`KERN_MAXPHYS`)
  Maximum raw I/O transfer size.
`kern.maxproc` (`KERN_MAXPROC`)
  The maximum number of simultaneous processes the system will allow.
`kern.maxptys` (`KERN_MAXPTYS`)
  The maximum number of pseudo terminals. This value can be both raised and lowered, though it cannot be set lower than number of currently used ptys. See also pty(4).
`kern.maxvnodes` (`KERN_MAXVNODES`)
  The maximum number of vnodes available on the system. This can only be raised.
`kern.mbuf` (`KERN_MBUF`)
  Return information about the mbuf control variables. Mbufs are data structures which store network packets and other data structures in the networking code, see mbuf(9). The third level names for the mbuf variables are detailed below. The changeable column shows whether a process with appropriate privilege may change the value.

  Third level name | Type | Changeable
  kern.mbuf.mblowat | integer | yes
  kern.mbuf.mclbytes | integer | yes
  kern.mbuf.mcllowat | integer | yes
  kern.mbuf.msize | integer | yes
  kern.mbuf.nmbclusters | integer | yes

  The variables are as follows:

  `kern.mbuf.mblowat` (`MBUF_MBLOWAT`)
    The mbuf low water mark.
  `kern.mbuf.mclbytes` (`MBUF_MCLBYTES`)
    The mbuf cluster size.
  `kern.mbuf.mcllowat` (`MBUF_MCLLOWAT`)
    The mbuf cluster low water mark.
  `kern.mbuf.msize` (`MBUF_MSIZE`)
    The mbuf base size.
  `kern.mbuf.nmbclusters` (`MBUF_NMBCLUSTERS`)
    The limit on the number of mbuf clusters. The variable can only be increased, and only increased on machines with direct-mapped pool pages.
`kern.memlock` (`KERN_MEMLOCK`)
  Returns 1 if the IEEE Std 1003.1b-1993 (“POSIX.1b”) Process Memory Locking Option is available on this system, otherwise 0.
`kern.memlock_range` (`KERN_MEMLOCK_RANGE`)
  Returns 1 if the IEEE Std 1003.1b-1993 (“POSIX.1b”) Range Memory Locking Option is available on this system, otherwise 0.
`kern.memory_protection` (`KERN_MEMORY_PROTECTION`)
  Returns 1 if the IEEE Std 1003.1b-1993 (“POSIX.1b”) Memory Protection Option is available on this system, otherwise 0.
`kern.module`
  Settings related to kernel modules. The third level names for the settings are described below.

  Third level name | Type | Changeable
  kern.module.autoload | integer | yes
  kern.module.autotime | integer | yes
  kern.module.verbose | integer | yes

  The variables are as follows:

  `kern.module.autoload`
    A boolean that controls whether kernel modules are loaded automatically. See module(7) for additional details.
  `kern.module.autotime`
    An integer that controls the delay before an attempt is made to automatically unload a module that was auto-loaded. Setting this value to zero disables the auto-unload function.
  `kern.module.verbose`
    A boolean that enables or disables verbose debug messages related to kernel modules.
`kern.monotonic_clock` (`KERN_MONOTONIC_CLOCK`)
  Returns the standard version the implementation of the IEEE Std 1003.1b-1993 (“POSIX.1b”) Monotonic Clock Option conforms to, otherwise 0.
`kern.mqueue`
  Settings related to POSIX message queues; see mqueue(3). This node is created dynamically when the corresponding kernel module is loaded. The third level names for the settings are described below.

  Third level name | Type | Changeable
  kern.mqueue.mq_open_max | integer | yes
  kern.mqueue.mq_prio_max | integer | yes
  kern.mqueue.mq_max_msgsize | integer | yes
  kern.mqueue.mq_def_maxmsg | integer | yes
  kern.mqueue.mq_max_maxmsg | integer | yes

  The variables are:

  `kern.mqueue.mq_open_max`
    The maximum number of message queue descriptors any single process can open.
  `kern.mqueue.mq_prio_max`
    The maximum priority of a message.
  `kern.mqueue.mq_max_msgsize`
    The maximum size of a message in a message queue.
  `kern.mqueue.mq_def_maxmsg`
    The default maximum message count.
  `kern.mqueue.mq_max_maxmsg`
    The maximum number of messages in a message queue.
`kern.msgbuf` (`KERN_MSGBUF`)
  The kernel message buffer, rotated so that the head of the circular kernel message buffer is at the start of the returned data. The returned data may contain NUL bytes.
`kern.msgbufsize` (`KERN_MSGBUFSIZE`)
  The maximum number of characters that the kernel message buffer can hold.
`kern.ngroups` (`KERN_NGROUPS`)
  The maximum number of supplemental groups.
`kern.ntptime` (`KERN_NTPTIME`)
  A struct ntptimeval structure is returned. This structure contains data used by the ntpd(8) program.
`kern.osrelease` (`KERN_OSRELEASE`)
  The system release string.
`kern.osrevision` (`KERN_OSREV`)
  The system revision string.
`kern.ostype` (`KERN_OSTYPE`)
  The system type string.
`kern.pipe` (`KERN_PIPE`)
  Pipe settings. The third level names for the integer pipe settings is detailed below. The changeable column shows whether a process with appropriate privilege may change the value.

  Third level name | Type | Changeable
  kern.pipe.kvasiz | integer | yes
  kern.pipe.maxbigpipes | integer | yes
  kern.pipe.maxkvasz | integer | yes
  kern.pipe.limitkva | integer | yes
  kern.pipe.nbigpipes | integer | yes

  The variables are as follows:

  `kern.pipe.kvasiz` (`KERN_PIPE_KVASIZ`)
    Amount of kernel memory consumed by pipe buffers.
  `kern.pipe.maxbigpipes` (`KERN_PIPE_MAXBIGPIPES`)
    Maximum number of “big” pipes.
  `kern.pipe.maxkvasz` (`KERN_PIPE_MAXKVASZ`)
    Maximum amount of kernel memory to be used for pipes.
  `kern.pipe.limitkva` (`KERN_PIPE_LIMITKVA`)
    Limit for direct transfers via page loan.
  `kern.pipe.nbigpipes` (`KERN_PIPE_NBIGPIPES`)
    Number of “big” pipes.
`kern.pool`
  Provides statistics about the pool(9) and pool_cache(9) subsystems.
`kern.posix1version` (`KERN_POSIX1`)
  The version of ISO/IEC 9945 (IEEE Std 1003.1 (“POSIX.1”)) with which the system attempts to comply.
`kern.posix_aio`
  The version of IEEE Std 1003.1 (“POSIX.1”) and its Asynchronous I/O option to which the system attempts to conform.
`kern.posix_barriers` (`KERN_POSIX_BARRIERS`)
  The version of IEEE Std 1003.1 (“POSIX.1”) and its Barriers option to which the system attempts to conform, otherwise 0.
`kern.posix_reader_writer_locks` (`KERN_POSIX_READER_WRITER_LOCKS`)
  The version of IEEE Std 1003.1 (“POSIX.1”) and its Read-Write Locks option to which the system attempts to conform, otherwise 0.
`kern.posix_semaphores` (`KERN_POSIX_SEMAPHORES`)
  The version of IEEE Std 1003.1 (“POSIX.1”) and its Semaphores option to which the system attempts to conform, otherwise 0.
`kern.posix_spin_locks` (`KERN_POSIX_SPIN_LOCKS`)
  The version of IEEE Std 1003.1 (“POSIX.1”) and its Spin Locks option to which the system attempts to conform, otherwise 0.
`kern.posix_threads` (`KERN_POSIX_THREADS`)
  The version of IEEE Std 1003.1 (“POSIX.1”) and its Threads option to which the system attempts to conform, otherwise 0.
`kern.posix_timers` (`KERN_POSIX_TIMERS`)
  The version of IEEE Std 1003.1 (“POSIX.1”) and its Timers option to which the system attempts to conform, otherwise 0.
`kern.proc` (`KERN_PROC`)
  Return the entire process table, or a subset of it. An array of struct kinfo_proc structures is returned, whose size depends on the current number of such objects in the system. The third and fourth level numeric names are as follows:

  Third level name | Fourth level is:
  KERN_PROC_ALL | None
  KERN_PROC_GID | A group ID
  KERN_PROC_PID | A process ID
  KERN_PROC_PGRP | A process group
  KERN_PROC_RGID | A real group ID
  KERN_PROC_RUID | A real user ID
  KERN_PROC_SESSION | A session ID
  KERN_PROC_TTY | A tty device
  KERN_PROC_UID | A user ID

`kern.proc2` (`KERN_PROC2`)
  As for KERN_PROC, but an array of struct kinfo_proc2 structures are returned. The fifth level name is the size of the struct kinfo_proc2 and the sixth level name is the number of structures to return.
`kern.proc_args` (`KERN_PROC_ARGS`)
  Return the argv or environment strings (or the number thereof) of a process. Multiple strings are returned separated by NUL characters. The third level name is the process ID. The fourth level name is as follows:

  KERN_PROC_ARGV | The argv strings
  KERN_PROC_ENV | The environ strings
  KERN_PROC_NARGV | The number of argv strings
  KERN_PROC_NENV | The number of environ strings

`kern.profiling` (`KERN_PROF`)
  Return profiling information about the kernel. If the kernel is not compiled for profiling, attempts to retrieve any of the KERN_PROF values will fail with EOPNOTSUPP. The third level names for the string and integer profiling information is detailed below. The changeable column shows whether a process with appropriate privilege may change the value.

  Third level name | Type | Changeable
  kern.profiling.count | u_short[] | yes
  kern.profiling.froms | u_short[] | yes
  kern.profiling.gmonparam | struct gmonparam | no
  kern.profiling.state | integer | yes
  kern.profiling.tos | struct tostruct | yes

  The variables are as follows:

  `kern.profiling.count` (`GPROF_COUNT`)
    Array of statistical program counter counts.
  `kern.profiling.froms` (`GPROF_FROMS`)
    Array indexed by program counter of call-from points.
  `kern.profiling.gmonparams` (`GPROF_GMONPARAM`)
    Structure giving the sizes of the above arrays.
  `kern.profiling.state` (`GPROF_STATE`)
    Profiling state. If set to GMON_PROF_ON, starts profiling. If set to GMON_PROF_OFF, stops profiling.
  `kern.profiling.tos` (`GPROF_TOS`)
    Array of struct tostruct describing destination of calls and their counts.
`kern.rawpartition` (`KERN_RAWPARTITION`)
  The raw partition of a disk (a == 0).
`kern.root_device` (`KERN_ROOT_DEVICE`)
  The name of the root device (e.g., “wd0”).
`kern.root_partition` (`KERN_ROOT_PARTITION`)
  The root partition on the root device (a == 0).
`kern.rtc_offset` (`KERN_RTC_OFFSET`)
  Return the offset of real time clock from UTC in minutes.
`kern.saved_ids` (`KERN_SAVED_IDS`)
  Returns 1 if saved set-group and saved set-user ID is available.
`kern.sbmax` (`KERN_SBMAX`)
  Maximum socket buffer size.
`kern.securelevel` (`KERN_SECURELVL`)
  See secmodel_securelevel(9).
`kern.somaxkva` (`KERN_SOMAXKVA`)
  Maximum amount of kernel memory to be used for socket buffers.
`kern.synchronized_io` (`KERN_SYNCHRONIZED_IO`)
  Returns 1 if the IEEE Std 1003.1b-1993 (“POSIX.1b”) Synchronized I/O Option is available on this system, otherwise 0.
`kern.timecounter` (`dynamic`)
  Display and control the timecounter source of the system.

  Third level name | Type | Changeable
  kern.timecounter.choice | string | no
  kern.timecounter.hardware | string | yes
  kern.timecounter.timestepwarnings | integer | yes

  The variables are as follows:

  `kern.timecounter.choice` (`dynamic`)
    The list of available timecounters with their quality and frequency.
  `kern.timecounter.hardware` (`dynamic`)
    The currently selected timecounter source.
  `kern.timecounter.timestepwarnings` (`dynamic`)
    If non-zero display a message each time the time is stepped.
`kern.timex` (`KERN_TIMEX`)
  Not available.
`kern.tkstat` (`KERN_TKSTAT`)
  Return information about the number of characters sent and received on ttys. The third level names for the tty statistic variables are detailed below. The changeable column shows whether a process with appropriate privilege may change the value.

  Third level name | Type | Changeable
  kern.tkstat.cancc | quad | no
  kern.tkstat.nin | quad | no
  kern.tkstat.nout | quad | no
  kern.tkstat.rawcc | quad | no

  The variables are as follows:

  `kern.tkstat.cancc` (`KERN_TKSTAT_CANCC`)
    The number of canonical input characters.
  `kern.tkstat.nin` (`KERN_TKSTAT_NIN`)
    The total number of input characters.
  `kern.tkstat.nout` (`KERN_TKSTAT_NOUT`)
    The total number of output characters.
  `kern.tkstat.rawcc` (`KERN_TKSTAT_RAWCC`)
    The number of raw input characters.
`kern.tty`
  The third level names for the tty setup variables are detailed below. The changeable column shows whether a process with appropriate privilege may change the value.

  Third level name | Type | Changeable
  kern.tty.qsize | int | yes

  The variables are as follows:

  `kern.tty.qsize`
    Control/display the size of the default input and output queues selected during tty creation. Is converted to a power of two and its range is between 1024 and 65536.
`kern.uidinfo`
  Resource usage for the current user.

  Third level name | Type | Changeable
  kern.uidinfo.proccnt | integer | no
  kern.uidinfo.lwpcnt | integer | no
  kern.uidinfo.lockcnt | integer | no
  kern.uidinfo.sbsize | integer | no

  `kern.uidinfo.proccnt`
    Returns the number of active processes for the current user.
  `kern.uidinfo.lwpcnt`
    Returns the number of active threads for the current user; the first thread of each process is not counted.
  `kern.uidinfo.lockcnt`
    Number of locks held by the current user.
  `kern.uidinfo.sbsize`
    Number of bytes in socket buffers allocated to the current user.
`kern.urandom` (`KERN_URND`)
  Random integer value.
`kern.usercrypto`
  When enabled, allows userland to open(2) the /dev/crypto special device, used by the crypto(4) system.
`kern.userasymcrypto`
  Enables or disables the use of software asymmetric crypto support in the crypto(4) system.
`kern.veriexec`
  Runtime information for veriexec(8).

  Third level name | Type | Changeable
  kern.veriexec.algorithms | string | no
  kern.veriexec.count | node | not applicable
  kern.veriexec.strict | integer | yes
  kern.veriexec.verbose | integer | yes

  `kern.veriexec.algorithms`
    Returns a string with the supported algorithms in Veriexec.
  `kern.veriexec.count`
    Sub-nodes are added to this node as new mounts are monitored by Veriexec. Each mount will be under its own tableN node. Under each node there will be three variables, indicating the mount point, the file system type, and the number of entries.
  `kern.veriexec.strict`
    Controls the strict level of Veriexec. See security(7) for more information on each level's implications.
  `kern.veriexec.verbose`
    Controls the verbosity level of Veriexec. If 0, only the minimal indication required will be given about what's happening - fingerprint mismatches, removal of entries from the tables, modification of a fingerprinted file. If 1, more messages will be printed (ie., when a file with a valid fingerprint is accessed). Verbose level 2 is debug mode.
`kern.version` (`KERN_VERSION`)
  The system version string.
`kern.vnode` (`KERN_VNODE`)
  Return the entire vnode table. Note, the vnode table is not necessarily a consistent snapshot of the system. The returned data consists of an array whose size depends on the current number of such objects in the system. Each element of the array contains the kernel address of a vnode struct vnode * followed by the vnode itself struct vnode.

## The machdep.* subtree

The set of variables defined is architecture dependent. Most architectures define at least the following variables.

Second level name | Type | Changeable
`machdep.booted_kernel` | string | no

## The net.* subtree

The string and integer information available for the `net` level is detailed below. The changeable column shows whether a process with appropriate privilege may change the value. The second and third levels are typically the protocol family and protocol number, though this is not always the case.

Second level name | Type | Changeable
net.route | routing messages | no
net.inet | IPv4 values | yes
net.inet6 | IPv6 values | yes
net.key | IPsec key management values | yes

`net.route` (`PF_ROUTE`)
  Return the entire routing table or a subset of it. The data is returned as a sequence of routing messages (see route(4) for the header file, format and meaning). The length of each message is contained in the message header.

  The third level name is a protocol number, which is currently always 0. The fourth level name is an address family, which may be set to 0 to select all address families. The fifth and sixth level names are as follows:

  Fifth level name | Sixth level is:
  NET_RT_FLAGS | rtflags
  NET_RT_DUMP | None
  NET_RT_IFLIST | None

`net.inet` (`PF_INET`)
  Get or set various global information about the IPv4 (Internet Protocol version 4). The third level name is the protocol. The fourth level name is the variable name. The currently defined protocols and names are:

  Protocol name | Variable name | Type | Changeable
  arp | down | integer | yes
  arp | keep | integer | yes
  arp | log_movements | integer | yes
  arp | log_permanent_modify | integer | yes
  arp | log_wrong_iface | integer | yes
  arp | prune | integer | yes
  arp | refresh | integer | yes
  carp | allow | integer | yes
  carp | preempt | integer | yes
  carp | log | integer | yes
  carp | arpbalance | integer | yes
  icmp | errppslimit | integer | yes
  icmp | maskrepl | integer | yes
  icmp | rediraccept | integer | yes
  icmp | redirtimeout | integer | yes
  icmp | bmcastecho | integer | yes
  ip | allowsrcrt | integer | yes
  ip | anonportalgo.selected | string | yes
  ip | anonportalgo.available | string | yes
  ip | anonportalgo.reserve | struct | yes
  ip | anonportmax | integer | yes
  ip | anonportmin | integer | yes
  ip | checkinterface | integer | yes
  ip | directed-broadcast | integer | yes
  ip | do_loopback_cksum | integer | yes
  ip | forwarding | integer | yes
  ip | forwsrcrt | integer | yes
  ip | gifttl | integer | yes
  ip | grettl | integer | yes
  ip | hashsize | integer | yes
  ip | hostzerobroadcast | integer | yes
  ip | lowportmin | integer | yes
  ip | lowportmax | integer | yes
  ip | maxflows | integer | yes
  ip | maxfragpackets | integer | yes
  ip | mtudisc | integer | yes
  ip | mtudisctimeout | integer | yes
  ip | random_id | integer | yes
  ip | redirect | integer | yes
  ip | subnetsarelocal | integer | yes
  ip | ttl | integer | yes
  tcp | rfc1323 | integer | yes
  tcp | sendspace | integer | yes
  tcp | recvspace | integer | yes
  tcp | mssdflt | integer | yes
  tcp | syn_cache_limit | integer | yes
  tcp | syn_bucket_limit | integer | yes
  tcp | syn_cache_interval | integer | yes
  tcp | init_win | integer | yes
  tcp | init_win_local | integer | yes
  tcp | mss_ifmtu | integer | yes
  tcp | win_scale | integer | yes
  tcp | timestamps | integer | yes
  tcp | compat_42 | integer | yes
  tcp | cwm | integer | yes
  tcp | cwm_burstsize | integer | yes
  tcp | ack_on_push | integer | yes
  tcp | keepidle | integer | yes
  tcp | keepintvl | integer | yes
  tcp | keepcnt | integer | yes
  tcp | slowhz | integer | no
  tcp | keepinit | integer | yes
  tcp | log_refused | integer | yes
  tcp | rstppslimit | integer | yes
  tcp | ident | struct | no
  tcp | drop | struct | no
  tcp | sack.enable | integer | yes
  tcp | sack.globalholes | integer | no
  tcp | sack.globalmaxholes | integer | yes
  tcp | sack.maxholes | integer | yes
  tcp | ecn.enable | integer | yes
  tcp | ecn.maxretries | integer | yes
  tcp | congctl.selected | string | yes
  tcp | congctl.available | string | yes
  tcp | abc.enable | integer | yes
  tcp | abc.aggressive | integer | yes
  udp | checksum | integer | yes
  udp | do_loopback_cksum | integer | yes
  udp | recvspace | integer | yes
  udp | sendspace | integer | yes

  The variables are as follows:

  `arp.down`
    Failed ARP entry lifetime.
  `arp.keep`
    Valid ARP entry lifetime.
  `arp.prune`
    ARP cache pruning interval.
  `arp.refresh`
    ARP entry refresh interval.
  `carp.allow`
    If set to 0, incoming carp(4) packets will not be processed. If set to any other value, processing will occur. Enabled by default.
  `carp.arpbalance`
    If set to any value other than 0, the ARP balancing functionality of carp(4) is enabled. When ARP requests are received for an IP address which is part of any virtual host, carp will hash the source IP in the ARP request to select one of the virtual hosts from the set of all the virtual hosts which have that IP address. The master of that host will respond with the correct virtual MAC address. Disabled by default.
  `carp.log`
    If set to any value other than 0, carp(4) will log errors. Disabled by default.
  `carp.preempt`
    If set to 0, carp(4) will not attempt to become master if it is receiving advertisements from another active master. If set to any other value, carp will become master of the virtual host if it believes it can send advertisements more frequently than the current master. Disabled by default.
  `ip.allowsrcrt`
    If set to 1, the host accepts source routed packets.
  `ip.anonportalgo.available`
    The available RFC 6056 port randomization algorithms.
  `ip.anonportalgo.reserve`
    A bitmask of ports that will not be used during anonymous or privileged port selection.
  `ip.anonportalgo.selected`
    The currently selected RFC 6056 port randomization algorithm.
  `ip.anonportmax`
    The highest port number to use for TCP and UDP ephemeral port allocation. This cannot be set to less than 1024 or greater than 65535, and must be greater than `ip.anonportmin`.
  `ip.anonportmin`
    The lowest port number to use for TCP and UDP ephemeral port allocation. This cannot be set to less than 1024 or greater than 65535.
  `ip.checkinterface`
    If set to non-zero, the host will reject packets addressed to it that arrive on an interface not bound to that address. Currently, this must be disabled if ipnat is used to translate the destination address to another local interface, or if addresses are added to the loopback interface instead of the interface where the packets for those packets are received.
  `ip.directed-broadcast`
    If set to 1, enables directed broadcast behavior for the host.
  `ip.do_loopback_cksum`
    Perform IP checksum on loopback.
  `ip.forwarding`
    If set to 1, enables IP forwarding for the host, meaning that the host is acting as a router.
  `ip.forwsrcrt`
    If set to 1, enables forwarding of source-routed packets for the host. This value may only be changed if the kernel security level is less than 1.
  `ip.gifttl`
    The maximum time-to-live (hop count) value for an IPv4 packet generated by gif(4) tunnel interface.
  `ip.grettl`
    The maximum time-to-live (hop count) value for an IPv4 packet generated by gre(4) tunnel interface.
  `ip.hashsize`
    The size of IPv4 Fast Forward hash table. This value must be a power of 2 (64, 256...). A larger hash table size results in fewer collisions. Also see `ip.maxflows`.
  `ip.hostzerobroadcast`
    All zeroes address is broadcast address.
  `ip.lowportmax`
    The highest port number to use for TCP and UDP reserved port allocation. This cannot be set to less than 0 or greater than 1024, and must be greater than `ip.lowportmin`.
  `ip.lowportmin`
    The lowest port number to use for TCP and UDP reserved port allocation. This cannot be set to less than 0 or greater than 1024, and must be smaller than `ip.lowportmax`.
  `ip.maxflows`
    IPv4 Fast Forwarding is enabled by default. If set to 0, IPv4 Fast Forwarding is disabled. `ip.maxflows` controls the maximum amount of flows which can be created. The default value is 256.
  `ip.maxfragpackets`
    The maximum number of fragmented packets the node will accept. 0 means that the node will not accept any fragmented packets. -1 means that the node will accept as many fragmented packets as it receives. The flag is provided basically for avoiding possible DoS attacks.
  `ip.mtudisc`
    If set to 1, enables Path MTU Discovery (RFC 1191). When Path MTU Discovery is enabled, the transmitted TCP segment size will be determined by the advertised maximum segment size (MSS) from the remote end, as constrained by the path MTU. If MTU Discovery is disabled, the transmitted segment size will never be greater than `tcp.mssdflt` (the local maximum segment size).
  `ip.mtudisctimeout`
    The number of seconds in which a route added by the Path MTU Discovery engine will time out. When the route times out, the Path MTU Discovery engine will attempt to probe a larger path MTU.
  `ip.random_id`
    Assign random ip_id values.
  `ip.redirect`
    If set to 1, ICMP redirects may be sent by the host. This option is ignored unless the host is routing IP packets, and should normally be enabled on all systems.
  `ip.subnetsarelocal`
    If set to 1, subnets are to be considered local addresses.
  `ip.ttl`
    The maximum time-to-live (hop count) value for an IP packet sourced by the system. This value applies to normal transport protocols, not to ICMP.
  `icmp.errppslimit`
    The variable specifies the maximum number of outgoing ICMP error messages, per second. ICMP error messages that exceeded the value are subject to rate limitation and will not go out from the node. Negative value disables rate limitation.
  `icmp.maskrepl`
    If set to 1, ICMP network mask requests are to be answered.
  `icmp.rediraccept`
    If set to non-zero, the host will accept ICMP redirect packets. Note that routers will never accept ICMP redirect packets, and the variable is meaningful on IP hosts only.
  `icmp.redirtimeout`
    The variable specifies lifetime of routing entries generated by incoming ICMP redirect. This defaults to 600 seconds.
  `icmp.returndatabytes`
    Number of bytes to return in an ICMP error message.
  `icmp.bmcastecho`
    If set to 1, enables responding to ICMP echo or timestamp request to the broadcast address.
  `tcp.ack_on_push`
    If set to 1, TCP is to immediately transmit an ACK upon reception of a packet with PUSH set. This can avoid losing a round trip time in some rare situations, but has the caveat of potentially defeating TCP's delayed ACK algorithm. Use of this option is generally not recommended, but the variable exists in case your configuration really needs it.
  `tcp.compat_42`
    If set to 1, enables work-arounds for bugs in the 4.2BSD TCP implementation. Use of this option is not recommended, although it may be required in order to communicate with extremely old TCP implementations.
  `tcp.cwm`
    If set to 1, enables use of the Hughes/Touch/Heidemann Congestion Window Monitoring algorithm. This algorithm prevents line-rate bursts of packets that could otherwise occur when data begins flowing on an idle TCP connection. These line-rate bursts can contribute to network and router congestion. This can be particularly useful on World Wide Web servers which support HTTP/1.1, which has lingering connections.
  `tcp.cwm_burstsize`
    The Congestion Window Monitoring allowed burst size, in terms of packet count.
  `tcp.delack_ticks`
    Number of ticks to delay sending an ACK.
  `tcp.do_loopback_cksum`
    Perform TCP checksum on loopback.
  `tcp.init_win`
    A value indicating the TCP initial congestion window. The valid range is 0 to 10 (maximum specified by draft-ietf-tcpm-initcwnd-08.txt), with a default of 4 (approximately 4K per RFC3390).
  `tcp.init_win_local`
    Like `tcp.init_win`, but used when communicating with hosts on a local network.
  `tcp.keepcnt`
    Number of keepalive probes sent before declaring a connection dead. If set to zero, there is no limit; keepalives will be sent until some kind of response is received from the peer.
  `tcp.keepidle`
    Time a connection must be idle before keepalives are sent (if keepalives are enabled for the connection). See also tcp.slowhz.
  `tcp.keepintvl`
    Time after a keepalive probe is sent until, in the absence of any response, another probe is sent. See also tcp.slowhz.
  `tcp.log_refused`
    If set to 1, refused TCP connections to the host will be logged.
  `tcp.keepinit`
    Timeout in seconds during connection establishment.
  `tcp.mss_ifmtu`
    If set to 1, TCP calculates the outgoing maximum segment size based on the MTU of the appropriate interface. If set to 0, it is calculated based on the greater of the MTU of the interface, and the largest (non-loopback) interface MTU on the system.
  `tcp.mssdflt`
    The default maximum segment size both advertised to the peer and to use when either the peer does not advertise a maximum segment size to us during connection setup or Path MTU Discovery (`ip.mtudisc`) is disabled. Do not change this value unless you really know what you are doing.
  `tcp.recvspace`
    The default TCP receive buffer size.
  `tcp.rfc1323`
    If set to 1, enables RFC 1323 extensions to TCP.
  `tcp.rstppslimit`
    The variable specifies the maximum number of outgoing TCP RST packets, per second. TCP RST packet that exceeded the value are subject to rate limitation and will not go out from the node. Negative value disables rate limitation.
  `tcp.ident`
    Return the user ID of a connected socket pair. (RFC1413 Identification Protocol lookups.)
  `tcp.drop`
    Drop a TCP socket pair connection.
  `tcp.sack.enable`
    If set to 1, enables RFC 2018 Selective ACKnowledgement.
  `tcp.sack.globalholes`
    Global number of TCP SACK holes.
  `tcp.sack.globalmaxholes`
    Global maximum number of TCP SACK holes.
  `tcp.sack.maxholes`
    Maximum number of TCP SACK holes allowed per connection.
  `tcp.ecn.enable`
    If set to 1, enables RFC 3168 Explicit Congestion Notification.
  `tcp.ecn.maxretries`
    Number of times to retry sending the ECN-setup packet.
  `tcp.sendspace`
    The default TCP send buffer size.
  `tcp.slowhz`
    The units for tcp.keepidle and tcp.keepintvl; those variables are in ticks of a clock that ticks tcp.slowhz times per second. (That is, their values must be divided by the tcp.slowhz value to get times in seconds.)
  `tcp.syn_bucket_limit`
    The maximum number of entries allowed per hash bucket in the TCP compressed state engine.
  `tcp.syn_cache_limit`
    The maximum number of entries allowed in the TCP compressed state engine.
  `tcp.timestamps`
    If rfc1323 is enabled, a value of 1 indicates RFC 1323 time stamp options, used for measuring TCP round trip times, are enabled.
  `tcp.win_scale`
    If rfc1323 is enabled, a value of 1 indicates RFC 1323 window scale options, for increasing the TCP window size, are enabled.
  `tcp.congctl.available`
    The available TCP congestion control algorithms.
  `tcp.congctl.selected`
    The currently selected TCP congestion control algorithm.
  `tcp.abc.enable`
    If set to 1, use RFC 3465 Appropriate Byte Counting (ABC). If set to 0, use traditional Packet Counting.
  `tcp.abc.aggressive`
    Choose the L parameter found in RFC 3465. L is the maximum cwnd increase for an ack during slow start. If set to 1, use L=2*SMSS. If set to 0, use L=1*SMSS. It has no effect unless tcp.abc.enable is set to 1.
  `udp.checksum`
    If set to 1, UDP checksums are being computed. Received non-zero UDP checksums are always checked. Disabling UDP checksums is strongly discouraged.
  `udp.recvspace`
    The default UDP receive buffer size.
  `udp.sendspace`
    The default UDP send buffer size.

  For variables net.*.ipsec, please refer to ipsec(4).

`net.inet6` (`PF_INET6`)
  Get or set various global information about the IPv6 (Internet Protocol version 6). The third level name is the protocol. The fourth level name is the variable name. The currently defined protocols and names are:

  Protocol name | Variable name | Type | Changeable
  icmp6 | errppslimit | integer | yes
  icmp6 | mtudisc_hiwat | integer | yes
  icmp6 | mtudisc_lowat | integer | yes
  icmp6 | nd6_debug | integer | yes
  icmp6 | nd6_delay | integer | yes
  icmp6 | nd6_maxnudhint | integer | yes
  icmp6 | nd6_mmaxtries | integer | yes
  icmp6 | nd6_prune | integer | yes
  icmp6 | nd6_umaxtries | integer | yes
  icmp6 | nd6_useloopback | integer | yes
  icmp6 | nodeinfo | integer | yes
  icmp6 | rediraccept | integer | yes
  icmp6 | redirtimeout | integer | yes
  ip6 | accept_rtadv | integer | yes
  ip6 | anonportalgo.selected | string | yes
  ip6 | anonportalgo.available | string | yes
  ip6 anonportalgo.reserve | struct | yes
  ip6 | anonportmax | integer | yes
  ip6 | anonportmin | integer | yes
  ip6 | auto_flowlabel | integer | yes
  ip6 | dad_count | integer | yes
  ip6 | defmcasthlim | integer | yes
  ip6 | forwarding | integer | yes
  ip6 | gifhlim | integer | yes
  ip6 | hashsize | integer | yes
  ip6 | hlim | integer | yes
  ip6 | hdrnestlimit | integer | yes
  ip6 | kame_version | string | no
  ip6 | keepfaith | integer | yes
  ip6 | log_interval | integer | yes
  ip6 | lowportmax | integer | yes
  ip6 | lowportmin | integer | yes
  ip6 | maxdynroutes | integer | yes
  ip6 | maxifprefixes | integer | yes
  ip6 | maxifdefrouters | integer | yes
  ip6 | maxflows | integer | yes
  ip6 | maxfragpackets | integer | yes
  ip6 | maxfrags | integer | yes
  ip6 | neighborgcthresh | integer | yes
  ip6 | redirect | integer | yes
  ip6 | rr_prune | integer | yes
  ip6 | use_deprecated | integer | yes
  ip6 | v6only | integer | yes
  udp6 | do_loopback_cksum | integer | yes
  udp6 | recvspace | integer | yes
  udp6 | sendspace | integer | yes

  The variables are as follows:

  `ip6.accept_rtadv`
    If set to non-zero, the node will accept ICMPv6 router advertisement packets and autoconfigures address prefixes and default routers. The node must be a host (not a router) for the option to be meaningful.
  `ip6.anonportalgo.available`
    The available RFC 6056 port randomization algorithms.
  `ip6.anonportalgo.reserve`
    A bitmask of ports that will not be used during anonymous or privileged port selection.
  `ip6.anonportalgo.selected`
    The currently selected RFC 6056 port randomization algorithm.
  `ip6.anonportmax`
    The highest port number to use for TCP and UDP ephemeral port allocation. This cannot be set to less than 1024 or greater than 65535, and must be greater than `ip6.anonportmin`.
  `ip6.anonportmin`
    The lowest port number to use for TCP and UDP ephemeral port allocation. This cannot be set to less than 1024 or greater than 65535.
  `ip6.auto_flowlabel`
    On connected transport protocol packets, fill IPv6 flowlabel field to help intermediate routers to identify packet flows.
  `ip6.dad_count`
    The variable configures number of IPv6 DAD (duplicated address detection) probe packets. The packets will be generated when IPv6 interface addresses are configured.
  `ip6.defmcasthlim`
    The default hop limit value for an IPv6 multicast packet sourced by the node. This value applies to all the transport protocols on top of IPv6. There are APIs to override the value, as documented in ip6(4).
  `ip6.forwarding`
    If set to 1, enables IPv6 forwarding for the node, meaning that the node is acting as a router. If set to 0, disables IPv6 forwarding for the node, meaning that the node is acting as a host. IPv6 specification defines node behavior for “router” case and “host” case quite differently, and changing this variable during operation may cause serious trouble. It is recommended to configure the variable at bootstrap time, and bootstrap time only.
  `ip6.gifhlim`
    The maximum hop limit value for an IPv6 packet generated by gif(4) tunnel interface.
  `ip6.hdrnestlimit`
    The number of IPv6 extension headers permitted on incoming IPv6 packets. If set to 0, the node will accept as many extension headers as possible.
  `ip6.hashsize`
    The size of IPv6 Fast Forward hash table. This value must be a power of 2 (64, 256, ...). A larger hash table size results in fewer collisions. Also see `ip6.maxflows`.
  `ip6.hlim`
    The default hop limit value for an IPv6 unicast packet sourced by the node. This value applies to all the transport protocols on top of IPv6. There are APIs to override the value, as documented in ip6(4).
  `ip6.kame_version`
    The string identifies the version of KAME IPv6 stack implemented in the kernel.
  `ip6.keepfaith`
    If set to non-zero, it enables “FAITH” TCP relay IPv6-to-IPv4 translator code in the kernel. Refer faith(4) and faithd(8) for detail.
  `ip6.log_interval`
    The variable controls amount of logs generated by IPv6 packet forwarding engine, by setting interval between log output (in seconds).
  `ip6.lowportmax`
    The highest port number to use for TCP and UDP reserved port allocation. This cannot be set to less than 0 or greater than 1024, and must be greater than `ip6.lowportmin`.
  `ip6.lowportmin`
    The lowest port number to use for TCP and UDP reserved port allocation. This cannot be set to less than 0 or greater than 1024, and must be smaller than `ip6.lowportmax`.
  `ip6.maxdynroutes`
    Maximum number of routes created by redirect. Set it to negative to disable. The default value is 4096.
  `ip6.maxifprefixes`
    Maximum number of prefixes created by route advertisements per interface. Set it to negative to disable. The default value is 16.
  `ip6.maxifdefrouters 16`
    Maximum number of default routers created by route advertisements per interface. Set it to negative to disable. The default value is 16.
  `ip6.maxflows`
    IPv6 Fast Forwarding is enabled by default. If set to 0, IPv6 Fast Forwarding is disabled. `ip6.maxflows` controls the maximum amount of flows which can be created. The default value is 256.
  `ip6.maxfragpackets`
    The maximum number of fragmented packets the node will accept. 0 means that the node will not accept any fragmented packets. -1 means that the node will accept as many fragmented packets as it receives. The flag is provided basically for avoiding possible DoS attacks.
  `ip6.maxfrags`
    The maximum number of fragments the node will accept. 0 means that the node will not accept any fragments. -1 means that the node will accept as many fragments as it receives. The flag is provided basically for avoiding possible DoS attacks.
  `ip6.neighborgcthresh`
    Maximum number of entries in neighbor cache. Set to negative to disable. The default value is 2048.
  `ip6.redirect`
    If set to 1, ICMPv6 redirects may be sent by the node. This option is ignored unless the node is routing IP packets, and should normally be enabled on all systems.
  `ip6.rr_prune`
    The variable specifies interval between IPv6 router renumbering prefix babysitting, in seconds.
  `ip6.use_deprecated`
    The variable controls use of deprecated address, specified in RFC 2462 5.5.4.
  `ip6.v6only`
    The variable specifies initial value for IPV6_V6ONLY socket option for AF_INET6 socket. Please refer to ip6(4) for detail.
  `icmp6.errppslimit`
    The variable specifies the maximum number of outgoing ICMPv6 error messages, per second. ICMPv6 error messages that exceeded the value are subject to rate limitation and will not go out from the node. Negative value disables rate limitation.
  `icmp6.mtudisc_hiwat`
  `icmp6.mtudisc_lowat`
    The variables define the maximum number of routing table entries, created due to path MTU discovery (prevents denial-of-service attacks with ICMPv6 too big messages). When IPv6 path MTU discovery happens, we keep path MTU information into the routing table. If the number of routing table entries exceed the value, the kernel will not attempt to keep the path MTU information. `icmp6.mtudisc_hiwat` is used when we have verified ICMPv6 too big messages. `icmp6.mtudisc_lowat` is used when we have unverified ICMPv6 too big messages. Verification is performed by using address/port pairs kept in connected pcbs. Negative value disables the upper limit.
  `icmp6.nd6_debug`
    If set to non-zero, kernel IPv6 neighbor discovery code will generate debugging messages. The debug outputs are useful to diagnose IPv6 interoperability issues. The flag must be set to 0 for normal operation.
  `icmp6.nd6_delay`
    The variable specifies DELAY_FIRST_PROBE_TIME timing constant in IPv6 neighbor discovery specification (RFC 2461), in seconds.
  `icmp6.nd6_maxnudhint`
    IPv6 neighbor discovery permits upper layer protocols to supply reachability hints, to avoid unnecessary neighbor discovery exchanges. The variable defines the number of consecutive hints the neighbor discovery layer will take. For example, by setting the variable to 3, neighbor discovery layer will take 3 consecutive hints in maximum. After receiving 3 hints, neighbor discovery layer will perform normal neighbor discovery process.
  `icmp6.nd6_mmaxtries`
    The variable specifies MAX_MULTICAST_SOLICIT constant in IPv6 neighbor discovery specification (RFC 2461).
  `icmp6.nd6_prune`
    The variable specifies interval between IPv6 neighbor cache babysitting, in seconds.
  `icmp6.nd6_umaxtries`
    The variable specifies MAX_UNICAST_SOLICIT constant in IPv6 neighbor discovery specification (RFC 2461).
  `icmp6.nd6_useloopback`
    If set to non-zero, kernel IPv6 stack will use loopback interface for local traffic.
  `icmp6.nodeinfo`
    The variable enables responses to ICMPv6 node information queries. If you set the variable to 0, responses will not be generated for ICMPv6 node information queries. Since node information queries can have a security impact, it is possible to fine tune which responses should be answered. Two separate bits can be set.
    1
      Respond to ICMPv6 FQDN queries, e.g. `ping6 -w`.
    2
      Respond to ICMPv6 node addresses queries, e.g. `ping6 -a`.
  `icmp6.rediraccept`
    If set to non-zero, the host will accept ICMPv6 redirect packets. Note that IPv6 routers will never accept ICMPv6 redirect packets, and the variable is meaningful on IPv6 hosts (non-router) only.
  `icmp6.redirtimeout`
    The variable specifies lifetime of routing entries generated by incoming ICMPv6 redirect.
  `udp6.do_loopback_cksum`
    Perform UDP checksum on loopback.
  `udp6.recvspace`
    Default UDP receive buffer size.
  `udp6.sendspace`
    Default UDP send buffer size.

  We reuse net.*.tcp for TCP over IPv6, and therefore we do not have variables net.*.tcp6. Variables net.inet6.udp6 have identical meaning to net.inet.udp. Please refer to `PF_INET` section above. For variables net.*.ipsec6, please refer to ipsec(4).

`net.key` (`PF_KEY`)
  Get or set various global information about the IPsec key management. The third level name is the variable name. The currently defined variable and names are:

  Variable name | Type | Changeable
  debug | integer | yes
  enabled | integer | yes
  used | integer | no
  spi_try | integer | yes
  spi_min_value | integer | yes
  spi_max_value | integer | yes
  larval_lifetime | integer | yes
  blockacq_count | integer | yes
  blockacq_lifetime | integer | yes
  esp_keymin | integer | yes
  esp_auth | integer | yes
  ah_keymin | integer | yes

  The variables are as follows:

  `debug`
    Turn on debugging message from within the kernel. The value is a bitmap, as defined in <netkey/key_debug.h>.
  `enabled`
    Control processing of IPsec control messages.
    0
      Never allow IPsec processing
    1
      Allow IPsec processing when SPD policies are present.
    2
      Force IPsec processing even when SPD policies are not present.
  `used`
    Based on if IPsec is enabled, and SPD rule existance, show if IPsec is being used. Note that currenly once IPsec is being used, it cannot be disabled.
  `spi_try`
    The number of times the kernel will try to obtain an unique SPI when it generates it from random number generator.
  `spi_min_value`
    Minimum SPI value when generating it within the kernel.
  `spi_max_value`
    Maximum SPI value when generating it within the kernel.
  `larval_lifetime`
    Lifetime for LARVAL SAD entries, in seconds.
  `blockacq_count`
    Number of ACQUIRE PF_KEY messages to be blocked after an ACQUIRE message. It avoids flood of ACQUIRE PF_KEY from being sent from the kernel to the key management daemon.
  `blockacq_lifetime`
    Lifetime of ACQUIRE PF_KEY message.
  `esp_keymin`
    Minimum ESP key length, in bits. The value is used when the kernel creates proposal payload on ACQUIRE PF_KEY message.
  `esp_auth`
    Whether ESP authentication should be used or not. Non-zero value indicates that ESP authentication should be used. The value is used when the kernel creates proposal payload on ACQUIRE PF_KEY message.
  `ah_keymin`
    Minimum AH key length, in bits, The value is used when the kernel creates proposal payload on ACQUIRE PF_KEY message.

## The proc.* subtree

The string and integer information available for the `proc` level is detailed below. The changeable column shows whether a process with appropriate privilege may change the value. These values are per-process, and as such may change from one process to another. When a process is created, the default values are inherited from its parent. When a set-user-ID or set-group-ID binary is executed, the value of PROC_PID_CORENAME is reset to the system default value. The second level name is either the magic value PROC_CURPROC, which points to the current process, or the PID of the target process.

Third level name | Type | Changeable
proc.pid.corename | string | yes
proc.pid.rlimit | node | not applicable
proc.pid.stopfork | int | yes
proc.pid.stopexec | int | yes
proc.pid.stopexit | int | yes

`proc.pid.corename` (`PROC_PID_CORENAME`)
  The template used for the core dump file name (see core(5) for details). The base name must either be core or end with the suffix .core (the super-user may set arbitrary names). By default it points to KERN_DEFCORENAME.
`proc.pid.rlimit` (`PROC_PID_LIMIT`)
  Return resources limits, as defined for the getrlimit(2) and setrlimit(2) system calls. The fourth level name is one of:
  `proc.pid.rlimit.cputime` (`PROC_PID_LIMIT_CPU`)
    The maximum amount of CPU time (in seconds) to be used by each process.
  `proc.pid.rlimit.filesize` (`PROC_PID_LIMIT_FSIZE`)
    The largest size (in bytes) file that may be created.
  `proc.pid.rlimit.datasize` (`PROC_PID_LIMIT_DATA`)
    The maximum size (in bytes) of the data segment for a process; this defines how far a program may extend its break with the sbrk(2) system call.
  `proc.pid.rlimit.stacksize` (`PROC_PID_LIMIT_STACK`)
    The maximum size (in bytes) of the stack segment for a process; this defines how far a program's stack segment may be extended. Stack extension is performed automatically by the system.
  `proc.pid.rlimit.coredumpsize` (`PROC_PID_LIMIT_CORE`)
    The largest size (in bytes) core file that may be created.
  `proc.pid.rlimit.memoryuse` (`PROC_PID_LIMIT_RSS`)
    The maximum size (in bytes) to which a process's resident set size may grow. This imposes a limit on the amount of physical memory to be given to a process; if memory is tight, the system will prefer to take memory from processes that are exceeding their declared resident set size.
  `proc.pid.rlimit.memorylocked` (`PROC_PID_LIMIT_MEMLOCK`)
    The maximum size (in bytes) which a process may lock into memory using the mlock(2) function.
  `proc.pid.rlimit.maxproc` (`PROC_PID_LIMIT_NPROC`)
    The maximum number of simultaneous processes for this user id.
  `proc.pid.rlimit.descriptors` (`PROC_PID_LIMIT_NOFILE`)
    The maximum number of open files for this process.
  `proc.pid.rlimit.sbsize` (`PROC_PID_LIMIT_SBSIZE`)
    The maximum size (in bytes) of the socket buffers set by the setsockopt(2) SO_RCVBUF and SO_SNDBUF options.

  The fifth level name is one of `soft` (`PROC_PID_LIMIT_TYPE_SOFT`) or `hard` (`PROC_PID_LIMIT_TYPE_HARD`), to select respectively the soft or hard limit. Both are of type integer.

`proc.pid.stopfork` (`PROC_PID_STOPFORK`)
  If non zero, the process' children will be stopped after fork(2) calls. The children is created in the SSTOP state and is never scheduled for running before being stopped. This feature helps attaching a process with a debugger such as gdb(1) before it had the opportunity to actually do anything.

  This value is inherited by the process's children, and it also apply to emulation specific system calls that fork a new process, such as sproc() or clone().

`proc.pid.stopexec` (`PROC_PID_STOPEXEC`)
  If non zero, the process will be stopped on next exec(3) call. The process created by exec(3) is created in the SSTOP state and is never scheduled for running before being stopped. This feature helps attaching a process with a debugger such as gdb(1) before it had the opportunity to actually do anything.

  This value is inherited by the process's children.

`proc.pid.stopexit` (`PROC_PID_STOPEXIT`)
  If non zero, the process will be stopped on when it has cause to exit, either by way of calling exit(3), _exit(2), or by the receipt of a specific signal. The process is stopped before any of its resources or vm space is released allowing examination of the termination state of a process before it disappears. This feature can be used to examine the final conditions of the process's vmspace via pmap(1) or its resource settings with sysctl(8) before it disappears.

  This value is also inherited by the process's children.

## The user.* subtree (CTL_USER)

The string and integer information available for the `user` level is detailed below. The changeable column shows whether a process with appropriate privilege may change the value.

Second level name | Type | Changeable
user.atexit_max | integer | no
user.bc_base_max | integer | no
user.bc_dim_max | integer | no
user.bc_scale_max | integer | no
user.bc_string_max | integer | no
user.coll_weights_max | integer | no
user.cs_path | string | no
user.expr_nest_max | integer | no
user.line_max | integer | no
user.posix2_c_bind | integer | no
user.posix2_c_dev | integer | no
user.posix2_char_term | integer | no
user.posix2_fort_dev | integer | no
user.posix2_fort_run | integer | no
user.posix2_localedef | integer | no
user.posix2_sw_dev | integer | no
user.posix2_upe | integer | no
user.posix2_version | integer | no
user.re_dup_max | integer | no
user.stream_max | integer | no
user.stream_max | integer | no
user.tzname_max | integer | no

`user.atexit_max` (`USER_ATEXIT_MAX`)
  The maximum number of functions that may be registered with atexit(3).
`user.bc_base_max` (`USER_BC_BASE_MAX`)
  The maximum ibase/obase values in the bc(1) utility.
`user.bc_dim_max` (`USER_BC_DIM_MAX`)
  The maximum array size in the bc(1) utility.
`user.bc_scale_max` (`USER_BC_SCALE_MAX`)
  The maximum scale value in the bc(1) utility.
`user.bc_string_max` (`USER_BC_STRING_MAX`)
  The maximum string length in the bc(1) utility.
`user.coll_weights_max` (`USER_COLL_WEIGHTS_MAX`)
  The maximum number of weights that can be assigned to any entry of the LC_COLLATE order keyword in the locale definition file.
`user.cs_path` (`USER_CS_PATH`)
  Return a value for the PATH environment variable that finds all the standard utilities.
`user.expr_nest_max` (`USER_EXPR_NEST_MAX`)
  The maximum number of expressions that can be nested within parenthesis by the expr(1) utility.
`user.line_max` (`USER_LINE_MAX`)
  The maximum length in bytes of a text-processing utility's input line.
`user.posix2_char_term` (`USER_POSIX2_CHAR_TERM`)
  Return 1 if the system supports at least one terminal type capable of all operations described in IEEE Std 1003.2 (“POSIX.2”), otherwise 0.
`user.posix2_c_bind` (`USER_POSIX2_C_BIND`)
  Return 1 if the system's C-language development facilities support the C-Language Bindings Option, otherwise 0.
`user.posix2_c_dev` (`USER_POSIX2_C_DEV`)
  Return 1 if the system supports the C-Language Development Utilities Option, otherwise 0.
`user.posix2_fort_dev` (`USER_POSIX2_FORT_DEV`)
  Return 1 if the system supports the FORTRAN Development Utilities Option, otherwise 0.
`user.posix2_fort_run` (`USER_POSIX2_FORT_RUN`)
  Return 1 if the system supports the FORTRAN Runtime Utilities Option, otherwise 0.
`user.posix2_localedef` (`USER_POSIX2_LOCALEDEF`)
  Return 1 if the system supports the creation of locales, otherwise 0.
`user.posix2_sw_dev` (`USER_POSIX2_SW_DEV`)
  Return 1 if the system supports the Software Development Utilities Option, otherwise 0.
`user.posix2_upe` (`USER_POSIX2_UPE`)
  Return 1 if the system supports the User Portability Utilities Option, otherwise 0.
`user.posix2_version` (`USER_POSIX2_VERSION`)
  The version of IEEE Std 1003.2 (“POSIX.2”) with which the system attempts to comply.
`user.re_dup_max` (`USER_RE_DUP_MAX`)
  The maximum number of repeated occurrences of a regular expression permitted when using interval notation.
`user.stream_max` (`USER_STREAM_MAX`)
  The minimum maximum number of streams that a process may have open at any one time.
`user.tzname_max` (`USER_TZNAME_MAX`)
  The minimum maximum number of types supported for the name of a timezone.

## The vm.* subtree (CTL_VM)

The string and integer information available for the `vm` level is detailed below. The changeable column shows whether a process with appropriate privilege may change the value.

Second level name | Type | Changeable
vm.anonmax | int | yes
vm.anonmin | int | yes
vm.bufcache | int | yes
vm.bufmem | int | no
vm.bufmem_hiwater | int | yes
vm.bufmem_lowater | int | yes
vm.execmax | int | yes
vm.execmin | int | yes
vm.filemax | int | yes
vm.filemin | int | yes
vm.loadavg | struct loadavg | no
vm.maxslp | int | no
vm.nkmempages | int | no
vm.uspace | int | no
vm.uvmexp | struct uvmexp | no
vm.uvmexp2 | struct uvmexp_sysctl | no
vm.vmmeter | struct vmtotal | no

`vm.anonmax` (`VM_ANONMAX`)
  The percentage of physical memory which will be reclaimed from other types of memory usage to store anonymous application data.
`vm.anonmin` (`VM_ANONMIN`)
  The percentage of physical memory which will be always be available for anonymous application data.
`vm.bufcache` (`VM_BUFCACHE`)
  The percentage of physical memory which will be available for the buffer cache.
`vm.bufmem` (`VM_BUFMEM`)
  The amount of kernel memory that is being used by the buffer cache.
`vm.bufmem_lowater` (`VM_BUFMEM_LOWATER`)
  The minimum amount of kernel memory to reserve for the buffer cache.
`vm.bufmem_hiwater` (`VM_BUFMEM_HIWATER`)
  The maximum amount of kernel memory to be used for the buffer cache.
`vm.execmax` (`VM_EXECMAX`)
  The percentage of physical memory which will be reclaimed from other types of memory usage to store cached executable data.
`vm.execmin` (`VM_EXECMIN`)
  The percentage of physical memory which will be always be available for cached executable data.
`vm.filemax` (`VM_FILEMAX`)
  The percentage of physical memory which will be reclaimed from other types of memory usage to store cached file data.
`vm.filemin` (`VM_FILEMIN`)
  The percentage of physical memory which will be always be available for cached file data.
`vm.loadavg` (`VM_LOADAVG`)
  Return the load average history. The returned data consists of a struct loadavg.
`vm.maxslp` (`VM_MAXSLP`)
  The value of the maxslp kernel global variable.
`vm.vmmeter` (`VM_METER`)
  Return system wide virtual memory statistics. The returned data consists of a struct vmtotal.
vm.user_va0_disable
  A flag which controls whether user processes can map virtual address 0.
`vm.uspace` (`VM_USPACE`)
  The number of bytes allocated for each kernel stack.
`vm.uvmexp` (`VM_UVMEXP`)
  Return system wide virtual memory statistics. The returned data consists of a struct uvmexp.
`vm.uvmexp2` (`VM_UVMEXP2`)
  Return system wide virtual memory statistics. The returned data consists of a struct uvmexp_sysctl.

## The ddb.* subtree (CTL_DDB)

The information available for the `ddb` level is detailed below. The changeable column shows whether a process with appropriate privilege may change the value.

Second level name | Type | Changeable
ddb.radix | integer | yes
ddb.maxoff | integer | yes
ddb.maxwidth | integer | yes
ddb.lines | integer | yes
ddb.tabstops | integer | yes
ddb.onpanic | integer | yes
ddb.fromconsole | integer | yes
ddb.tee_msgbuf | integer | yes
ddb.commandonenter | string | yes

`ddb.radix` (`DDBCTL_RADIX`)
  The input and output radix.
`ddb.maxoff` (`DDBCTL_MAXOFF`)
  The maximum symbol offset.
`ddb.maxwidth` (`DDBCTL_MAXWIDTH`)
  The maximum output line width.
`ddb.lines` (`DDBCTL_LINES`)
  Number of display lines.
`ddb.tabstops` (`DDBCTL_TABSTOPS`)
  Tab width.
`ddb.onpanic` (`DDBCTL_ONPANIC`)
  If greater than zero, DDB will be entered if the kernel panics. A value of 1 causes the system to enter DDB on panic, while a value of 2 causes the kernel to attempt to print out a stack trace before entering DDB. A value of 0 causes the kernel to attempt to print a stack trace, then reboot, while a value of -1 means neither a stack trace will be printed nor DDB entered.
`ddb.fromconsole` (`DDBCTL_FROMCONSOLE`)
  If not zero, DDB may be entered by sending a break on a serial console or by a special key sequence on a graphics console.
`ddb.tee_msgbuf`
  If not zero, DDB will output also to the kernel message buffer.
`ddb.commandonenter`
  If not empty, a command to be executed on each enter to the DDB.

Some of these MIB nodes are also available as variables from within the debugger. See ddb(4) for more details.

## The security.* subtree (CTL_SECURITY)

The `security` level contains various security-related settings for the system. The available second level names are:

Second level name | Type | Changeable
`security.curtain` | integer | yes
`security.models` | node | not applicable
`security.pax` | node | not applicable

Available settings are detailed below.

`security.curtain`
  If non-zero, will filter return objects according to the user ID requesting information about them, preventing from users any access to objects they do not own.

  At the moment, it affects ps(1), netstat(1) (for PF_INET, PF_INET6, and PF_UNIX PCBs), and w(1).

`security.models`
  NetBSD supports pluggable security models. Every security model used, whether if loaded as a module or built with the system, is required to add an entry to this node with at least one element, “name”, indicating the name of the security model.

  In addition to the name, any settings and other information private to the security model will be available under this node. See secmodel(9) for more information.

`security.pax`
  Settings for PaX -- exploit mitigation features. For more information on any of the PaX features, please see paxctl(8) and security(7). The available third and fourth level names are:

  Third and fourth level names | Type | Changeable
  `security.pax.aslr.enabled` | integer | yes
  `security.pax.aslr.global` | integer | yes
  `security.pax.mprotect.enabled` | integer | yes
  `security.pax.mprotect.global` | integer | yes
  `security.pax.segvguard.enabled` | integer | yes
  `security.pax.segvguard.expiry_timeout` | integer | yes
  `security.pax.segvguard.global` | integer | yes
  `security.pax.segvguard.max_crashes` | integer | yes
  `security.pax.segvguard.suspend_timeout` | integer | yes

  `security.pax.aslr.enabled`
    Enable PaX ASLR (Address Space Layout Randomization).

    The value of this knob must be non-zero for PaX ASLR to be enabled, even if a program is set to explicit enable.

  `security.pax.aslr.global`
    Specifies the default global policy for programs without an explicit enable/disable flag.

    When non-zero, all programs will get PaX ASLR, except those exempted with paxctl(8). Otherwise, all programs will not get PaX ASLR, except those specifically marked as such with paxctl(8).

  `security.pax.mprotect.enabled`
    Enable PaX MPROTECT restrictions.

    These are mprotect(2) restrictions to better enforce a W^X policy. The value of this knob must be non-zero for PaX MPROTECT to be enabled, even if a program is set to explicit enable.

  `security.pax.mprotect.global`
    Specifies the default global policy for programs without an explicit enable/disable flag.

    When non-zero, all programs will get the PaX MPROTECT restrictions, except those exempted with paxctl(8). Otherwise, all programs will not get the PaX MPROTECT restrictions, except those specifically marked as such with paxctl(8).

  `security.pax.segvguard.enabled`
    Enable PaX Segvguard.

    PaX Segvguard can detect and prevent certain exploitation attempts, where an attacker may try for example to brute-force function return addresses of respawning daemons.

    Note: The NetBSD interface and implementation of the Segvguard is still experimental, and may change in future releases.

  `security.pax.segvguard.expiry_timeout`
    If the max number was not reached within this timeout (in seconds), the entry will expire.
  `security.pax.segvguard.global`
    Specifies the default global policy for programs without an explicit enable/disable flag.

    When non-zero, all programs will get the PaX Segvguard, except those exempted with paxctl(8). Otherwise, no program will get the PaX Segvguard restrictions, except those specifically marked as such with paxctl(8).

  `security.pax.segvguard.max_crashes`
    The maximum number of segfaults a program can receive before suspension.
  `security.pax.segvguard.suspend_timeout`
    Number of seconds to suspend a user from running a faulting program when the limit was exceeded.

## The vendor.* subtree (CTL_VENDOR)

The `vendor` toplevel name is reserved to be used by vendors who wish to have their own private MIB tree. Intended use is to store values under “vendor.<yourname>.*”.

# SEE ALSO

sysctl(3), ipsec(4), tcp(4), security(7), sysctl(8)

# HISTORY

The sysctl variables first appeared in 4.4BSD.

August 3, 2014 | NetBSD 7.0