--- name: "options" section: "4" platform: "openbsd" package: "man" version: "8.0-current-20261101" release: "OpenBSD 8.0-current (2026-11-01)" platforms: ["openbsd"] url: "https://new.carta.tech/options/4" --- # options(4) kernel configuration options ## 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)](/config/8.md) 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)](/gdb/1.md). 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)](/acct/2.md) system call. - **option** **DDB** Compiles in a kernel debugger for diagnosing kernel problems. See [ddb(4)](/ddb/4.md). - **option** **DDB\_SAFE\_CONSOLE** Allows a break into the kernel debugger during boot. Useful when debugging problems that can cause [init(8)](/init/8.md) 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)](/kgmon/8.md). - **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)](/ktrace/1.md). - **option** **KUBSAN** Detect undefined behavior in the kernel. See [kubsan(4)](/kubsan/4.md). - **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)](/ptrace/2.md). - **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)](/witness/4.md). - **option** **WITNESS\_COUNT=***integer* Maximum number of lock types that are tracked by [witness(4)](/witness/4.md). It defaults to 1536. - **option** **WITNESS\_LOCKTRACE** Enable [witness(4)](/witness/4.md) 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)](/sysctl/8.md) variable. - **option** **WITNESS\_WATCH** Enable [witness(4)](/witness/4.md) at boot. By default, the subsystem is disabled and has to be enabled at runtime by raising the *kern.witness.watch* [sysctl(8)](/sysctl/8.md) 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)](/mount_cd9660/8.md). - **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)](/mount_mfs/8.md). - **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)](/mount_msdos/8.md) and [fsck\_msdos(8)](/fsck_msdos/8.md). - **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)](/mount_nfs/8.md) for details on NFS. - **option** **NTFS** Includes support for reading NTFS file systems. See [mount\_ntfs(8)](/mount_ntfs/8.md). - **option** **UDF** Includes code for the UDF file systems typically found on DVD discs. See [mount\_udf(8)](/mount_udf/8.md). - **option** **TMPFS** Includes code for the TMPFS efficient memory file system. See [mount\_tmpfs(8)](/mount_tmpfs/8.md). ## 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)](/chflags/1.md). - **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)](/mountd/8.md) and [nfsd(8)](/nfsd/8.md). - **option** **QUOTA** Enables kernel support for file system quotas. See [quotaon(8)](/quotaon/8.md), [edquota(8)](/edquota/8.md), [repquota(8)](/repquota/8.md), and [quota(1)](/quota/1.md). Note that quotas only work on “ffs” file systems, although [rpc.rquotad(8)](/rpc.rquotad/8.md) 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)](/xf86/4.md). 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)](/boot_config/8.md). - **option** **CRYPTO** Enables support for the kernel cryptographic framework. See [crypto(9)](/crypto/9.md). 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)](/eisa/4.md). - **option** **KMEMSTATS** The kernel memory allocator, [malloc(9)](/malloc/9.md), 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)](/pci/4.md). - **option** **PCMCIAVERBOSE** Makes the boot process more verbose for PCMCIA peripherals. See [pcmcia(4)](/pcmcia/4.md). - **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)](/Xorg/1.md) and [pcidump(8)](/pcidump/8.md). See [pci(4)](/pci/4.md). - **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)](/sysctl/8.md) and [sysctl(2)](/sysctl/2.md). ## 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)](/sysctl/8.md) and [sysctl(2)](/sysctl/2.md). - **option** **INET6** Includes support for the IPv6 protocol stack. See [inet6(4)](/inet6/4.md). *INET6* enables multicast routing code as well. - **option** **IPSEC** This option enables IP security protocol support. See [ipsec(4)](/ipsec/4.md) for more details. - **option** **MROUTING** Includes support for IP multicast routers. Multicast routing is controlled by the [mrouted(8)](/mrouted/8.md) daemon. - **option** **PIPEX** Includes pipex in-kernel acceleration for PPPoE, L2TP or PPTP. See [pipex(4)](/pipex/4.md). - **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** **SOCKET\_SPLICE** Enables zero-copy socket splicing in the kernel. See SO\_SPLICE in [setsockopt(2)](/setsockopt/2.md) 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)](/sysctl/8.md) variable *vm.nkmempages*. ## SCSI SUBSYSTEM OPTIONS See [scsi(4)](/scsi/4.md). - **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)](/semop/2.md) 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)](/shmget/2.md) 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)](/msgctl/2.md), [msgget(2)](/msgget/2.md), [msgrcv(2)](/msgrcv/2.md), [msgsnd(2)](/msgsnd/2.md). - **option** **SYSVSEM** Includes support for AT&T System V UNIX style semaphores. See [semctl(2)](/semctl/2.md), [semget(2)](/semget/2.md), [semop(2)](/semop/2.md). - **option** **SYSVSHM** Includes support for AT&T System V UNIX style shared memory. See [shmat(2)](/shmat/2.md), [shmctl(2)](/shmctl/2.md), [shmdt(2)](/shmdt/2.md), [shmget(2)](/shmget/2.md). ## SEE ALSO [intro(4)](/intro/4.md), [files.conf(5)](/files.conf/5.md), [config(8)](/config/8.md), [sysctl(8)](/sysctl/8.md) ## HISTORY The **options** man page first appeared in OpenBSD 2.3.