OPTIONS(4) | Device Drivers Manual | OPTIONS(4)

# NAME

`options` — kernel configuration options

# SYNOPSIS

`option ...`

# DESCRIPTION

This manual page describes a number of miscellaneous kernel configuration options that may be specified in a kernel config file. See config(8) for information on how to configure and build kernels. Note: options are passed to the compile process as `-D` flags to the C compiler.

# DEBUGGING OPTIONS

`makeoptions DEBUG=""`
  Do not build the debug kernel bsd.gdb. Normally, bsd.gdb is built (in addition to the regular bsd kernel) and is used for debugging kernels and their crash dumps with gdb(1). A crash dump can be debugged by starting gdb with bsd.gdb as an argument (no core file) and then using the gdb command “target kvm COREFILE”.
`makeoptions PROF="-pg"`
  The `-pg` flag causes the kernel to be compiled with support for profiling. The `option GPROF` is required for the kernel compile to succeed.
`option ACCOUNTING`
  Adds support for the acct(2) system call.
`option DDB`
  Compiles in a kernel debugger for diagnosing kernel problems. See ddb(4).
`option DDB_SAFE_CONSOLE`
  Allows a break into the kernel debugger during boot. Useful when debugging problems that can cause init(8) to fail.
`option DEBUG`
  Turns on miscellaneous kernel debugging. Since options are turned into preprocessor defines (see above), `option DEBUG` is equivalent to doing a #define DEBUG throughout the kernel. Much of the kernel has #ifdef DEBUG conditional debugging code. Note that many parts of the kernel (typically device drivers) include their own #ifdef XXX_DEBUG conditionals instead.
`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.
`option GPROF`
  Adds code to the kernel for kernel profiling with kgmon(8).
`option KTRACE`
  Adds hooks for the system call tracing facility, which allows users to watch the system call invocation behavior of processes. See ktrace(1).
`option KUBSAN`
  Detect undefined behavior in the kernel. See kubsan(4).
`option NO_PROPOLICE`
  Do not compile the kernel with the ProPolice stack protection. See gcc-local(1) for more information about ProPolice.
`option PTRACE`
  Adds hooks for the process tracing facility, allowing a process to control and observe another process. See ptrace(2).
`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.
`option VFSLCKDEBUG`
  Turns on debugging for the Virtual File System interface. See vfs(9).
`option WITNESS`
  Compiles in a lock checker for detecting lock order violations in the kernel. See witness(4).
`option WITNESS_COUNT=`integer
  Maximum number of lock types that are tracked by witness(4). It defaults to 1536.
`option WITNESS_LOCKTRACE`
  Enable witness(4) lock stack trace saving at boot. The feature is disabled by default and has to be enabled by setting the kern.witness.locktrace sysctl(8) variable.
`option WITNESS_WATCH`
  Enable witness(4) at boot. By default, the subsystem is disabled and has to be enabled at runtime by raising the kern.witness.watch sysctl(8) variable.

# FILE SYSTEMS

`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 mount_cd9660(8).
`option FFS`
  Includes code implementing the Berkeley Fast File System (FFS). Most machines need this if they are not running diskless.
`option FFS2`
  Includes code implementing the enhanced Fast File System (FFS2).
`option MFS`
  Include the memory file system (MFS). This file system stores files in swappable memory, and produces notable performance improvements when it is used as the file store for /tmp or similar mount points. See mount_mfs(8).
`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 mount_msdos(8) and fsck_msdos(8).
`option NFSCLIENT`
  Include the client side of the NFS (Network File System) remote file sharing protocol. Although the bulk of the code implementing NFS is kernel based, several user level daemons are needed for it to work. See mount_nfs(8) for details on NFS.
`option NTFS`
  Includes support for reading NTFS file systems. See mount_ntfs(8).
`option UDF`
  Includes code for the UDF file systems typically found on DVD discs. See mount_udf(8).
`option TMPFS`
  Includes code for the TMPFS efficient memory file system. See mount_tmpfs(8).

# FILE SYSTEM OPTIONS

`option BUFCACHEPERCENT=`integer
  The maximum percentage of DMA-reachable physical memory the buffer cache may use.
`option EXT2FS_SYSTEM_FLAGS`
  This option changes the behavior of the APPEND and IMMUTABLE flags for a file on an 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 chflags(1).
`option FIFO`
  Adds support for AT&T System V UNIX style FIFOs (i.e., “named pipes”). This option is recommended in almost all cases as many programs use these.
`option NFSSERVER`
  Include the server side of the NFS (Network File System) remote file sharing protocol. Although the bulk of the code implementing NFS is kernel based, several user level daemons are needed for it to work. See mountd(8) and nfsd(8).
`option QUOTA`
  Enables kernel support for file system quotas. See quotaon(8), edquota(8), repquota(8), and quota(1). Note that quotas only work on “ffs” file systems, although rpc.rquotad(8) permits them to be accessed over NFS.
`option UFS_DIRHASH`
  This option enables using an in memory hash table to speed lookups in large directories.

# MISCELLANEOUS OPTIONS

`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 xf86(4). This option is supported on the alpha, amd64, i386, macppc, and sparc64 architectures.
`option BOOT_CONFIG`
  Adds support for the `-c` boot option (User Kernel Config). Allows modification of kernel settings (e.g., device parameters) before booting the system. See boot_config(8).
`option CRYPTO`
  Enables support for the kernel cryptographic framework. See crypto(9). While not IP specific, this option is usually used in conjunction with option IPSEC.
`option EISAVERBOSE`
  Makes the boot process more verbose for EISA peripherals. See eisa(4).
`option KMEMSTATS`
  The kernel memory allocator, malloc(9), will keep statistics on its performance if this option is enabled.
`option MULTIPROCESSOR`
  On those architectures that have it, this enables multiprocessor support.
`option PCIVERBOSE`
  Makes the boot process more verbose for PCI peripherals (vendor names and other information is printed, etc.). See pci(4).
`option PCMCIAVERBOSE`
  Makes the boot process more verbose for PCMCIA peripherals. See pcmcia(4).
`option USER_PCICONF`
  Enables the user level access to the PCI bus configuration space through ioctls on the /dev/pci device. It's used by Xorg(1) and pcidump(8). See pci(4).
`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 vm.swapencrypt.enable controls its behaviour. See sysctl(8) and sysctl(2).

# NETWORKING OPTIONS

`option ENCDEBUG`
  This option permits the conditional logging of IPsec debugging information, and requires the IPSEC option. Debug logging can be turned on/off through the use of the net.inet.ip.encdebug sysctl variable. If net.inet.ip.encdebug is 1, debug logging is on. See sysctl(8) and sysctl(2).
`option INET6`
  Includes support for the IPv6 protocol stack. See inet6(4). INET6 enables multicast routing code as well.
`option IPSEC`
  This option enables IP security protocol support. See ipsec(4) for more details.
`option MROUTING`
  Includes support for IP multicast routers. Multicast routing is controlled by the mrouted(8) daemon.
`option PIPEX`
  Includes pipex in-kernel acceleration for PPPoE, L2TP or PPTP. See pipex(4).
`option PPP_BSDCOMP`
  Enables BSD compressor for PPP connections.
`option PPP_DEFLATE`
  For use in conjunction with PPP_BSDCOMP; provides an interface to zlib for PPP for deflate compression/decompression.
`option PPPOE_TERM_UNKNOWN_SESSIONS`
  Send PADT to terminate open sessions before connecting. See pppoe(4).
`option SOCKET_SPLICE`
  Enables zero-copy socket splicing in the kernel. See `SO_SPLICE` in setsockopt(2) and sosplice(9).
`option TCP_ECN`
  Turns on Explicit Congestion Notification (RFC 3168). 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 ECN functionality at the TCP connection establishment.
`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.

# OPERATION RELATED OPTIONS

`option BUFPAGES=`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.

`option NKMEMPAGES=`value
  Size of kernel malloc area in PAGE_SIZE-sized logical pages. This area is covered by the kernel submap 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 sysctl(8) variable vm.nkmempages.

# SCSI SUBSYSTEM OPTIONS

See scsi(4).

`option SCSI_DELAY=`value
  Delay for value seconds before starting to probe the first SCSI bus. This can be used if a SCSI device needs extra time to get ready.
`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.

  Additional information can be requested by setting SCSIDEBUG_BUSES, SCSIDEBUG_LEVEL, SCSIDEBUG_LUNS and SCSIDEBUG_TARGETS.

`option SCSIDEBUG_BUSES=`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.
`option SCSIDEBUG_LEVEL=`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.

      0x0010	(SDEV_DB1) SCSI commands, errors, and data
      0x0020	(SDEV_DB2) routine flow
      0x0040	(SDEV_DB3) routine internals
      0x0080	(SDEV_DB4) miscellaneous addition debugging

  SCSIDEBUG_LEVEL defaults to a value of 0x0030 (SDEV_DB1|SDEV_DB2).

`option SCSIDEBUG_LUNS=`value
  Define which SCSI luns will print debug info. Each bit enables debugging info for the corresponding lun.
`option SCSIDEBUG_TARGETS=`value
  Define which SCSI targets will print debug info. Each bit enables debugging info for the corresponding target.
`option SCSITERSE`
  Terser SCSI error messages. This omits the table for decoding ASC/ASCQ info, saving about 30KB.

# SYSTEM V IPC OPTIONS

`option SEMMNI=`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.
`option SEMMNS=`value
  Maximum number of semaphores in all sets in the system. Default value is 60.
`option SEMMNU=`value
  Maximum number of semaphore undo structures in the system. Default value is 30.
`option SEMUME=`value
  Maximum number of per-process undo operation entries in the system. Semaphore undo operations are invoked by the kernel when semop(2) is called with the SEM_UNDO flag and the process holding the semaphores terminates unexpectedly. Default value is 10.
`option SHMMAXPGS=`value
  Sets the maximum number of AT&T System V UNIX style shared memory pages that are available through the shmget(2) system call. Default value is 1024 on most architectures. See /usr/include/machine/vmparam.h for the default.
`option SYSVMSG`
  Includes support for AT&T System V UNIX style message queues. See msgctl(2), msgget(2), msgrcv(2), msgsnd(2).
`option SYSVSEM`
  Includes support for AT&T System V UNIX style semaphores. See semctl(2), semget(2), semop(2).
`option SYSVSHM`
  Includes support for AT&T System V UNIX style shared memory. See shmat(2), shmctl(2), shmdt(2), shmget(2).

# SEE ALSO

intro(4), files.conf(5), config(8), sysctl(8)

# HISTORY

The `options` man page first appeared in OpenBSD 2.3.

June 1, 2025 | Debian