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.\"	$OpenBSD: options.4,v 1.271 2024/05/05 07:26:58 jsg Exp $
.\"	$NetBSD: options.4,v 1.21 1997/06/25 03:13:00 thorpej Exp $
.\"
.\" Copyright (c) 1998 Theo de Raadt
.\" Copyright (c) 1998 Todd Miller
.\" Copyright (c) 1998 Gene Skonicki
.\" Copyright (c) 1996
.\" 	Perry E. Metzger.  All rights reserved.
.\"
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.\"	This product includes software developed for the NetBSD Project
.\"	by Perry E. Metzger.
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.Dd $Mdocdate: May 5 2024 $
.Dt OPTIONS 4
.Os
.Sh NAME
.Nm options
.Nd kernel configuration options
.Sh SYNOPSIS
.Cd option ...
.Sh DESCRIPTION
This manual page describes a number of miscellaneous kernel
configuration options that may be specified in a kernel config file.
See
.Xr config 8
for information on how to configure and build kernels.
.Em Note :
options are passed to the compile process as
.Fl D
flags to the C compiler.
.Sh DEBUGGING OPTIONS
.Bl -ohang
.It Cd makeoptions DEBUG=""
Do not build the debug kernel
.Pa bsd.gdb .
Normally,
.Pa bsd.gdb
is built
(in addition to the regular
.Pa bsd
kernel)
and is used for debugging kernels and their crash dumps with
.Xr gdb 1 .
A crash dump can be debugged by starting gdb with
.Pa bsd.gdb
as an argument (no core file) and then using the gdb command
.Dq target kvm COREFILE .
.It Cd makeoptions PROF="-pg"
The
.Fl pg
flag causes the kernel to be compiled with support for profiling.
The
.Cm option GPROF
is required for the kernel compile to succeed.
.It Cd option ACCOUNTING
Adds support for the
.Xr acct 2
system call.
.It Cd option DDB
Compiles in a kernel debugger for diagnosing kernel problems.
See
.Xr ddb 4 .
.It Cd option DDB_SAFE_CONSOLE
Allows a break into the kernel debugger during boot.
Useful when debugging problems that can cause
.Xr init 8
to fail.
.It Cd option DEBUG
Turns on miscellaneous kernel debugging.
Since options are turned into preprocessor defines (see above),
.Cm option DEBUG
is equivalent to doing a
.Em #define DEBUG
throughout the kernel.
Much of the kernel has
.Em #ifdef DEBUG
conditional debugging code.
Note that many parts of the kernel (typically device drivers) include their own
.Em #ifdef XXX_DEBUG
conditionals instead.
.It Cd option DIAGNOSTIC
Adds code to the kernel that does internal consistency checks.
This code will cause the kernel to panic if corruption of internal data
structures is detected.
.It Cd option GPROF
Adds code to the kernel for kernel profiling with
.Xr kgmon 8 .
.It Cd option KTRACE
Adds hooks for the system call tracing facility, which allows users to
watch the system call invocation behavior of processes.
See
.Xr ktrace 1 .
.It Cd option KUBSAN
Detect undefined behavior in the kernel.
See
.Xr kubsan 4 .
.It Cd option NO_PROPOLICE
Do not compile the kernel with the ProPolice stack protection.
See
.Xr gcc-local 1
for more information about ProPolice.
.It Cd option PTRACE
Adds hooks for the process tracing facility, allowing a process to
control and observe another process.
See
.Xr ptrace 2 .
.It Cd option SMALL_KERNEL
Removes some features and some optimizations from the kernel to reduce the
size of the resulting kernel binary.
This option is used on some installation media and should not be used
for general purpose kernels.
.It Cd option VFSLCKDEBUG
Turns on debugging for the Virtual File System interface.
See
.Xr vfs 9 .
.It Cd option WITNESS
Compiles in a lock checker for detecting lock order violations in the kernel.
See
.Xr witness 4 .
.It Cd option WITNESS_COUNT= Ns Ar integer
Maximum number of lock types that are tracked by
.Xr witness 4 .
It defaults to 1536.
.It Cd option WITNESS_LOCKTRACE
Enable
.Xr witness 4
lock stack trace saving at boot.
The feature is disabled by default and has to be enabled by setting the
.Va kern.witness.locktrace
.Xr sysctl 8
variable.
.It Cd option WITNESS_WATCH
Enable
.Xr witness 4
at boot.
By default, the subsystem is disabled and has to be enabled at runtime
by raising the
.Va kern.witness.watch
.Xr sysctl 8
variable.
.El
.Sh FILE SYSTEMS
.Bl -ohang
.It Cd option CD9660
Includes code for the ISO 9660 + Rock Ridge file system, which is the
standard file system used on many CD-ROMs.
It also supports Joliet extensions.
See
.Xr mount_cd9660 8 .
.It Cd option EXT2FS
Includes code implementing the Second Extended File System
.Em ( EXT2FS ) ,
commonly used on the Linux operating system.
This option is provided here for compatibility.
Some specific features of
.Em EXT2FS
like the "behavior on errors" are not implemented.
This file system
can't be used with
.Vt uid_t
or
.Vt gid_t
values greater than 65535.
Also, the filesystem will not function correctly on architectures with
differing byte-orders.
That is, a big-endian machine will not be able to read an
ext2fs filesystem created on an i386 or other little-endian machine.
See
.Xr mount_ext2fs 8 .
.It Cd option FFS
Includes code implementing the Berkeley Fast File System
.Em ( FFS ) .
Most machines need this if they are not running diskless.
.It Cd option FFS2
Includes code implementing the enhanced Fast File System
.Em ( FFS2 ) .
.It Cd option MFS
Include the memory file system
.Em ( MFS ) .
This file system stores files in swappable memory, and produces
notable performance improvements when it is used as the file store
for
.Pa /tmp
or similar mount points.
See
.Xr mount_mfs 8 .
.It Cd option MSDOSFS
Includes support for the MS-DOS FAT file system.
The kernel also implements the Windows 95
extensions which permit the use of longer, mixed-case file names.
See
.Xr mount_msdos 8
and
.Xr fsck_msdos 8 .
.It Cd option NFSCLIENT
Include the client side of the
.Em NFS
(Network File System) remote file sharing protocol.
Although the bulk of the code implementing
.Em NFS
is kernel based, several user level daemons are needed for it to work.
See
.Xr mount_nfs 8
for details on NFS.
.It Cd option NTFS
Includes support for reading NTFS file systems.
See
.Xr mount_ntfs 8 .
.It Cd option UDF
Includes code for the UDF file systems typically found on DVD discs.
See
.Xr mount_udf 8 .
.It Cd option TMPFS
Includes code for the TMPFS efficient memory file system.
See
.Xr mount_tmpfs 8 .
.El
.Sh FILE SYSTEM OPTIONS
.Bl -ohang
.It Cd option BUFCACHEPERCENT= Ns Ar integer
The maximum percentage of DMA-reachable physical memory the buffer cache may use.
.It Cd option EXT2FS_SYSTEM_FLAGS
This option changes the behavior of the APPEND and IMMUTABLE flags
for a file on an
.Em EXT2FS
filesystem.
Without this option, the superuser or owner of the file can set and clear them.
With this option, only the superuser can set them, and they can't be cleared
if the securelevel is greater than 0.
See also
.Xr chflags 1 .
.It Cd option FIFO
Adds support for
.At V
style FIFOs (i.e.,
.Dq named pipes ) .
This option is recommended in almost all cases as many programs use these.
.It Cd option NFSSERVER
Include the server side of the
.Em NFS
(Network File System) remote file sharing protocol.
Although the bulk of the code implementing
.Em NFS
is kernel based, several user level daemons are needed for it to
work.
See
.Xr mountd 8
and
.Xr nfsd 8 .
.It Cd option QUOTA
Enables kernel support for file system quotas.
See
.Xr quotaon 8 ,
.Xr edquota 8 ,
.Xr repquota 8 ,
and
.Xr quota 1 .
Note that quotas only work on
.Dq ffs
file systems, although
.Xr rpc.rquotad 8
permits them to be accessed over
.Em NFS .
.It Cd option UFS_DIRHASH
This option enables using an in memory hash table to speed lookups
in large directories.
.El
.Sh MISCELLANEOUS OPTIONS
.Bl -ohang
.It Cd option APERTURE
Provide in-kernel support for controlling VGA framebuffer mapping
and PCI configuration registers by user-processes
(such as an X Window System server).
See
.Xr xf86 4 .
This option is supported on the
.Va alpha ,
.Va amd64 ,
.Va i386 ,
.Va macppc ,
and
.Va sparc64
architectures.
.It Cd option BOOT_CONFIG
Adds support for the
.Fl c
boot option (User Kernel Config).
Allows modification of kernel settings (e.g., device parameters) before
booting the system.
See
.Xr boot_config 8 .
.It Cd option CRYPTO
Enables support for the kernel cryptographic framework.
See
.Xr crypto 9 .
While not IP specific, this option is usually used in conjunction with option
.Em IPSEC .
.It Cd option EISAVERBOSE
Makes the boot process more verbose for EISA peripherals.
See
.Xr eisa 4 .
.It Cd option KMEMSTATS
The kernel memory allocator,
.Xr malloc 9 ,
will keep statistics on its performance if this option is enabled.
.It Cd option MULTIPROCESSOR
On those architectures that have it, this enables multiprocessor support.
.It Cd option PCIVERBOSE
Makes the boot process more verbose for PCI peripherals
(vendor names and other information is printed, etc.).
See
.Xr pci 4 .
.It Cd option PCMCIAVERBOSE
Makes the boot process more verbose for PCMCIA peripherals.
See
.Xr pcmcia 4 .
.It Cd option USER_PCICONF
Enables the user level access to the PCI bus configuration space
through ioctls on the
.Pa /dev/pci
device.
It's used by
.Xr Xorg 1
and
.Xr pcidump 8 .
See
.Xr pci 4 .
.It Cd option UVM_SWAP_ENCRYPT
Enables kernel support for encrypting pages that are written out to
swap storage.
Swap encryption prevents sensitive data from remaining
on the disk even after the operating system has been shut down.
This option should be turned on if cryptographic filesystems are used.
The sysctl variable
.Em vm.swapencrypt.enable
controls its behaviour.
See
.Xr sysctl 8
and
.Xr sysctl 2 .
.El
.Sh NETWORKING OPTIONS
.Bl -ohang
.It Cd option ENCDEBUG
This option permits the conditional logging of IPsec debugging information,
and requires the
.Em IPSEC
option.
Debug logging can be turned on/off through the use of the
.Em net.inet.ip.encdebug
sysctl variable.
If
.Em net.inet.ip.encdebug
is 1, debug logging is on.
See
.Xr sysctl 8
and
.Xr sysctl 2 .
.It Cd option INET6
Includes support for the IPv6 protocol stack.
See
.Xr inet6 4 .
.Em INET6
enables multicast routing code as well.
.It Cd option IPSEC
This option enables IP security protocol support.
See
.Xr ipsec 4
for more details.
.It Cd option MROUTING
Includes support for IP multicast routers.
Multicast routing is controlled by the
.Xr mrouted 8
daemon.
.It Cd option ND6_DEBUG
The option sets the default value of
.Em net.inet6.icmp6.nd6_debug
to 1,
for debugging IPv6 neighbor discovery protocol handling.
See
.Xr sysctl 2 .
.It Cd option PIPEX
Includes pipex in-kernel acceleration for PPPoE, L2TP or PPTP.
See
.Xr pipex 4 .
.It Cd option PPP_BSDCOMP
Enables BSD compressor for PPP connections.
.It Cd option PPP_DEFLATE
For use in conjunction with PPP_BSDCOMP; provides an interface to zlib for PPP
for deflate compression/decompression.
.It Cd option PPPOE_TERM_UNKNOWN_SESSIONS
Send PADT to terminate open sessions before connecting.
See
.Xr pppoe 4 .
.It Cd option SOCKET_SPLICE
Enables zero-copy socket splicing in the kernel.
See
.Dv SO_SPLICE
in
.Xr setsockopt 2
and
.Xr sosplice 9 .
.It Cd option TCP_ECN
Turns on Explicit Congestion Notification (RFC 3168).
.Em ECN
allows intermediate routers to use the Congestion Experienced
codepoint in the IP header as an indication of congestion, and allows
TCP to adjust the transmission rate using this signal.
Both communication endpoints negotiate enabling
.Em ECN
functionality at the TCP connection establishment.
.It Cd option TCP_SIGNATURE
Turns on support for the TCP MD5 Signature option (RFC 2385).
This is used by
Internet backbone routers to provide per-packet authentication for the TCP
packets used to communicate BGP routing information.
You will also need a
routing daemon that supports this option in order to actually use it.
.El
.Sh OPERATION RELATED OPTIONS
.Bl -ohang -compact
.It Cd option BUFPAGES= Ns Ar value
This option sets the number of pages available for the buffer cache.
The default value is machine dependent, often calculated as
between 5% and 10% of total available RAM.
.Pp
.It Cd option NKMEMPAGES= Ns Ar value
Size of kernel malloc area in PAGE_SIZE-sized logical pages.
This area is covered by the kernel submap
.Em kmem_map .
The kernel attempts to auto-size this map based on the amount of
physical memory in the system.
Platform-specific code may place bounds on this computed size,
which may be viewed with the
.Xr sysctl 8
variable
.Em vm.nkmempages .
.El
.Sh SCSI SUBSYSTEM OPTIONS
See
.Xr scsi 4 .
.Bl -ohang
.It Cd option SCSI_DELAY= Ns Ar value
Delay for
.Ar value
seconds before starting to probe the first SCSI bus.
This can be used if a SCSI device needs extra time to get ready.
.It Cd option SCSIDEBUG
Enable printing of SCSI subsystem debugging info to the console.
Debugging info on the probing and attachment of all SCSI devices will
be printed.
.Pp
Additional information can be requested by setting
.Em SCSIDEBUG_BUSES ,
.Em SCSIDEBUG_LEVEL ,
.Em SCSIDEBUG_LUNS
and
.Em SCSIDEBUG_TARGETS .
.It Cd option SCSIDEBUG_BUSES= Ns Ar value
Define which SCSI buses will print debug info.
Each bit enables debugging info for the corresponding bus.
e.g. a value of 0x1 enables debug info for bus 0.
.It Cd option SCSIDEBUG_LEVEL= Ns Ar value
Define which of the four levels of debugging info are printed.
Each bit enables a level, and multiple levels are specified by setting multiple
bits.
.Bd -literal -offset indent
0x0010	(SDEV_DB1) SCSI commands, errors, and data
0x0020	(SDEV_DB2) routine flow
0x0040	(SDEV_DB3) routine internals
0x0080	(SDEV_DB4) miscellaneous addition debugging
.Ed
.Pp
.Em SCSIDEBUG_LEVEL
defaults to a value of 0x0030 (SDEV_DB1|SDEV_DB2).
.It Cd option SCSIDEBUG_LUNS= Ns Ar value
Define which SCSI luns will print debug info.
Each bit enables debugging info for the corresponding lun.
.It Cd option SCSIDEBUG_TARGETS= Ns Ar value
Define which SCSI targets will print debug info.
Each bit enables debugging info for the corresponding target.
.It Cd option SCSITERSE
Terser SCSI error messages.
This omits the table for decoding ASC/ASCQ info, saving about 30KB.
.El
.Sh SYSTEM V IPC OPTIONS
.Bl -ohang
.It Cd option SEMMNI= Ns Ar value
Number of semaphore identifiers (also called semaphore handles
and semaphore sets) available in the system.
Default value is 10.
The kernel allocates memory for the control structures at startup,
so arbitrarily large values should be avoided.
.It Cd option SEMMNS= Ns Ar value
Maximum number of semaphores in all sets in the system.
Default value is 60.
.It Cd option SEMMNU= Ns Ar value
Maximum number of semaphore undo structures in the system.
Default value is 30.
.It Cd option SEMUME= Ns Ar value
Maximum number of per-process undo operation entries in the
system.
Semaphore undo operations are invoked by the kernel when
.Xr semop 2
is called with the SEM_UNDO flag and the process holding
the semaphores terminates unexpectedly.
Default value is 10.
.It Cd option SHMMAXPGS= Ns Ar value
Sets the maximum number of
.At V
style shared memory pages that are available through the
.Xr shmget 2
system call.
Default value is 1024 on most architectures.
See
.Pa /usr/include/machine/vmparam.h
for the default.
.It Cd option SYSVMSG
Includes support for
.At V
style message queues.
See
.Xr msgctl 2 ,
.Xr msgget 2 ,
.Xr msgrcv 2 ,
.Xr msgsnd 2 .
.It Cd option SYSVSEM
Includes support for
.At V
style semaphores.
See
.Xr semctl 2 ,
.Xr semget 2 ,
.Xr semop 2 .
.It Cd option SYSVSHM
Includes support for
.At V
style shared memory.
See
.Xr shmat 2 ,
.Xr shmctl 2 ,
.Xr shmdt 2 ,
.Xr shmget 2 .
.El
.Sh SEE ALSO
.Xr intro 4 ,
.Xr files.conf 5 ,
.Xr config 8 ,
.Xr sysctl 8
.Sh HISTORY
The
.Nm
man page first appeared in
.Ox 2.3 .