NetBSD/share/man/man4/options.4

.\"	$NetBSD: options.4,v 1.32 1997/11/09 01:02:06 wrstuden Exp $
.\"
.\" Copyright (c) 1996
.\" 	Perry E. Metzger.  All rights reserved.
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.\" modification, are permitted provided that the following conditions
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.\"    must display the following acknowledgement:
.\"	This product includes software developed for the NetBSD Project
.\"	by Perry E. Metzger.
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.\"    derived from this software without specific prior written permission.
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.Dd October 20, 1996
.Os NetBSD 1.3
.Dt OPTIONS 4
.Sh NAME
.Nm options
.Nd Miscellaneous kernel configuration options
.Sh SYNOPSIS
.Cd options ...
.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 -D flags to the C
compiler.
.Ss Compatibility Options
.Bl -ohang
.It Cd options COMPAT_09
Enable binary compatibility with
.Nx 0.9 .
This enables support for
16-bit user, group, and process ids (following revisions support
32-bit identifiers),
It also allows the use of the deprecated getdomainname, setdomainname,
and uname syscalls.
This option also allows using numeric filesystem identifiers rather
than strings.  Post 0.9 versions use string identifiers.
.It Cd options COMPAT_10
Enable binary compatibility with
.Nx 1.0 .
This option allows the use of the filesystem name of 
.Dq ufs
as an alias for
.Dq ffs .
The name
.Dq ffs
should be used post 1.0 in 
.Pa /etc/fstab
and other files.  It also adds old syscalls for the
.At V
shared memory interface.  This was changed post 1.0 to work on 64-bit
architectures.  This option also enables 
.Dq sgtty
compatibility, without which programs using the old interface produce
an
.Dq inappropriate ioctl
error.
.It Cd options COMPAT_11
Enable binary compatibility with
.Nx 1.1 .
This allows binaries running on the i386 port to gain direct access to
the io ports by opening
.Pa /dev/io
read/write.  This functionality was
replaced by 
.Xr i386_iopl 2
post 1.1.
On the
.Tn Atari
port, the location of the disk label was moved after 1.1. When the
.Em COMPAT_11
option is set, the kernel will read (pre) 1.1 style disk labels as a
last resort. When a disklabel is re-written, the old style label will
be replaced with a post 1.1 style label.
.It Cd options COMPAT_12
Enable binary compatibility with
.Nx 1.2 .
This allows the use of old syscalls for
.Fn reboot
and
.Fn swapon .
The syscall numbers were changed post 1.2 to add functionality to the
reboot syscall, and the new
.Fn swapctl
interface was introduced.
.It Cd options COMPAT_43
Enables compatibility with
.Bx 4.3 .
This adds an old syscall for lseek.  It also adds the ioctls for
TIOCGETP and TIOCSETP.  The return values for getpid, getgid, and
getuid syscalls are modified as well, to return the parent's pid and
uid as well as the current process's.  It also enables the deprecated
NTTYDISC terminal line discipline.
It also provides backwards compatibility with 
.Dq old
SIOC[GS]IF{ADDR,DSTADDR,BRDADDR,NETMASK} interface ioctls, including
binary compatibility with code written before the introduction of the
sa_len field in sockaddrs.
It also enables
support for some older pre BSD 4.4 socket calls.
.It Cd options COMPAT_SVR4
On those architectures that support it, this enables binary
compatibility with
.At V.4
applications built for the same architecture.  This currently includes
the sparc and i386.
.It Cd options COMPAT_LINUX
On those architectures that support it, this enables binary
compatibility with Linux ELF and a.out
applications built for the same architecture.  This is currently limited
to the i386.
.It Cd options COMPAT_SUNOS
On those architectures that support it, this enables binary
compatibility with SunOS 4.1
applications built for the same architecture.  This currently includes
the sparc and most or all m68k platforms.
.It Cd options COMPAT_ULTRIX
On those architectures that support it, this enables binary
compatibility with Ultrix
applications built for the same architecture.  This currently is limited
to the pmax.  The functionality of this option is unknown.
.It Cd options COMPAT_FREEBSD
On those architectures that support it, this enables binary
compatibility with FreeBSD
applications built for the same architecture.  This is obviously limited
to the i386.
.It Cd options COMPAT_HPUX
On those architectures that support it, this enables binary
compatibility with HP/UX
applications built for the same architecture.  This is limited to the
hp300 port, and has some known bugs.  A limited set of programs do work.
.It Cd options COMPAT_IBCS2
On those architectures that support it, this enables binary
compatibility with iBCS2 
applications built for the same architecture.  This is currently
limited to the i386.
.It Cd options COMPAT_OSF1
On those architectures that support it, this enables binary
compatibility with Digital UNIX (formerly OSF/1)
applications built for the same architecture.  This is currently
limited to the alpha, and may not even work.
.It Cd options COMPAT_NOMID
Enable compatibility with a.out executables that lack a machine id.
This includes NetBSD 0.8's ZMAGIC format, and 386BSD and BSDI's
QMAGIC, NMAGIC, and OMAGIC a.out formats.
.El
.Ss Debugging Options
.Bl -ohang
.It Cd options DDB
Compiles in a kernel debugger for diagnosing kernel problems. See
.Xr ddb 4
for details. 
.Em NOTE:
not available on all architectures.
.It Cd options DDB_ONPANIC
If set to non-zero, the DDB will be entered upon kernel panic.  The
default if not specified is "1". Note that this sets the value of
the
.Em ddb.onpanic
sysctl variable which may be changed at run time -- see
.Xr sysctl 8
for details.
.It Cd options KGDB
Compiles in a remote kernel debugger stub for diagnosing kernel problems
using the
.Dq remote target
feature of gdb. See
.Xr gdb 1
for details. 
.Em NOTE:
not available on all architectures.
.It Cd makeoptions DEBUG="-g"
The -g flag causes
.Pa netbsd.gdb
to be built in addition to
.Pa netbsd .
.Pa netbsd.gdb
is useful for debugging kernel crash dumps with gdb.
The command
.Dl gdb -k
invokes gdb in kernel debugger mode.
See
.Xr gdb 1
for details. This also turns on
.Em options DEBUG
(which see).
.It Cd options DEBUG
Turns on miscellaneous kernel debugging. Since options are turned into
preprocessor defines (see above),
.Em options DEBUG
is equivalent to doing a 
.Em #define DEBUG
throughout the kernel. Much of the kernel has
.Em #ifdef DEBUG
conditionalized debugging code. Note that many parts of the kernel
(typically device drivers) include their own
.Em #ifdef XXX_DEBUG
conditionals instead.
This option also turns on certain other options,
which may decrease system performance.
.It Cd options 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 options KTRACE
Add hooks for the system call tracing facility, which allows users to
watch the system call invocation behavior of processes. See
.Xr ktrace 1
for details.
.It Cd options MSGBUFSIZE=integer
This option sets the size of the kernel message buffer. This buffer holds the
kernel output of
.Fn printf
when not (yet) read by
.Xr syslogd 8 .
This is particularly useful when the system has crashed and you wish to lookup
the kernel output from just before the crash. Also, since the autoconfig
output becomes more and more verbose, it sometimes happens that the message
buffer overflows before
.Xr syslogd 8
was able to read it.
Note that not all systems are capable of obtaining a variable sized message
buffer. There are also some systems on which memory contents are not preserved
accross reboots.
.El
.Ss File Systems
.Bl -ohang
.It Cd file-system FFS
Includes code implementing the Berkeley Fast File System
.Em ( FFS ) .
Most machines need this if they are not running diskless.
.It Cd file-system EXT2FS
Includes code implementing the Second Extended File System
.Em ( EXT2FS ) .
This is the most commonly used file system on the Linux operating system,
and is provided here for compatibility. Some of the specific features
of
.Em EXT2FS 
like the "behavior on errors" are not implemented. This file system can't be
used with UID or GID greater than 65535. See
.Xr mount_ext2fs 8
for details.
.It Cd file-system LFS
Include the log structured file system
.Em ( LFS ) .
See
.Xr mount_lfs 8
and
.Xr newlfs 8
for details.
.It Cd file-system 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
and similar file systems. See
.Xr mount_mfs 8
for details.
.It Cd file-system NFS
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
and
.Xr nfsiod 8
for details.
.It Cd file-system CD9660
Includes code for the ISO 9660 + Rock Ridge file system, which is the
standard file system on many CD-ROMs. Useful mostly if you have a CD
drive. See
.Xr mount_cd9660 8
for details.
.It Cd file-system MSDOSFS
Includes the
.Tn MS-DOS
FAT file system, which is reportedly still used
by unfortunate people who have not heard about
.Nx .
Also implements the 
.Tn Windows 95
extensions to the same, which permit the use of longer, mixed case
file names. see
.Xr mount_msdos 8
and
.Xr fsck_msdos 8
for details.
.It Cd file-system FDESC
Includes code for a file system, conventionally mounted on
.Pa /dev/fd ,
which permits access to the per-process file descriptor space via
special files in the file system.
See
.Xr mount_fdesc 8
for details.
Note that this facility is redundant, and thus unneeded on most
.Nx
systems, since the
.Xr fd 4
pseudodevice driver already provides identical functionality. On most
.Nx
systems, instances of
.Xr fd 4
are mknoded under
.Pa /dev/fd/
and on
.Pa /dev/stdin ,
.Pa /dev/stdout ,
and
.Pa /dev/stderr .
.It Cd file-system KERNFS
Includes code which permits the mounting of a special file system
(normally mounted on
.Pa /kern )
in which files representing various kernel variables and parameters
may be found.
See
.Xr mount_kernfs 8
for details.
.It Cd file-system NULLFS
Includes code for a loopback file system. This permits portions of the file
hierarchy to be re-mounted in other places. The code really exists to
provide an example of a stackable file system layer. See
.Xr mount_null 8
for details.
.It Cd file-system PORTAL
Includes the (experimental) portal filesystem. This permits
interesting tricks like opening TCP sockets by opening files in the
file system. The portal file system is conventionally mounted on
.Pa /p
and is partially implemented by a special daemon. See
.Xr mount_portal 8
for details.
.It Cd file-system PROCFS
Includes code for a special file system (conventionally mounted on
.Pa /proc )
in which the process space becomes visible in the file system. Among
other things, the memory spaces of processes running on the system are
visible as files, and signals may be sent to processes by writing to
.Pa ctl
files in the procfs namespace. See
.Xr mount_procfs 8
for details.
.It Cd file-system UMAPFS
Includes a loopback file system in which user and group ids may be
remapped -- this can be useful when mounting alien file systems with
different uids and gids than the local system.
See
.Xr mount_umap 8
for details.
.It Cd file-system UNION
Includes code for the union file system, which permits directories to
be mounted on top of each other in such a way that both file systems
remain visible -- this permits tricks like allowing writing (and the
deleting of files) on a read-only file system like a CD-ROM by
mounting a local writable file system on top of the read-only file
system. See
.Xr mount_union 8
for details.
.El
.Ss File System Options
.Bl -ohang
.It Cd options 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 
for details.
.It Cd options QUOTA
Enables kernel support for file system quotas. See
.Xr quotaon 8 ,
.Xr edquota 8 ,
and
.Xr quota 1
for details. 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 options FIFO
Augments the system to permit the use of
.At V
style FIFOs (i.e.
.Dq named pipes
). This option is recommended.
.It Cd options NVNODE=integer
This option sets the size of the cache used by the name-to-inode translation
routines, (a.k.a. the
.Fn namei
cache, though called by many other names in the kernel source).  By default,
this cache has NPROC (set as 20 + 16 * MAXUSERS) * (80 + NPROC / 8) entries.
A reasonable way to derive a value of NVNODE, should you notice a large
number of namei cache misses with a tool such as
.Xr systat 1 ,
is to examine your system's current computed value with
.Xr sysctl 1 ,
(which calls this parameter "kern.maxvnodes") and to increase this value
until either the namei cache hit rate improves or it is determined that
your system does not benefit substantially from an increase in the size of
the namei cache.
.It Cd options 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 .
.El
.Ss Miscellaneous Options
.Bl -ohang
.It Cd options LKM
Enable loadable kernel modules. See
.Xr lkm 4
for details.
.Em NOTE:
not available on all architectures.
.It Cd options INSECURE
Hardwires the kernel security level at -1. This means that the system
always runs in secure level 0 mode, even when running multiuser. See
the manual page for
.Xr init 8
for details on the implications of this. The kernel secure level may
manipulated by the superuser by altering the
.Em kern.securelevel
sysctl variable. (It should be noted that the secure level may only be
lowered by a call from process id 1, i.e.
.Em init . )
See also
.Xr sysctl 8
and
.Xr sysctl 3 .
.It Cd options UCONSOLE
Normally, only the superuser can execute the TIOCCONS ioctl, which
redirects console output to a non-console tty. (See
.Xr tty 4
for details).
This option permits any user to execute the ioctl. This is useful on
machines such as personal workstations which run X servers, where one
would prefer to permit console output to be viewed in a window without
requiring a suid root program to do it.
.It Cd options MACHINE_NONCONTIG
This option changes part of the VM/pmap interface, to allow for
non-contiguous memory.  On some ports it is not an option.  These
ports typically only use one of the interfaces.
.It Cd options MEMORY_DISK_HOOKS
This option allows for some machine dependent functions to be called when
the ramdisk driver is configured. This can result in automatically loading
a ramdisk from floppy on open (among other things).
.It Cd options MEMORY_DISK_IS_ROOT
Forces the ramdisk to be the root device. This can only be overridden when
the kernel is booted in the 'ask-for-root' mode.
.It Cd options NTP
Turns on in-kernel precision timekeeping support used by software
implementing
.Em NTP
(Network Time Protocol, RFC1305).
The
.Em NTP
option adds an in-kernel phase-locked loop (PLL) for normal
.Em NTP
operation, and a frequency-locked loop (FLL) for intermittently-connected
operation.
.Xr xntpd 8
will employ a user-level PLL when kernel support is unavailable,
but the in-kernel version has lower latency and more precision, and
so typically keeps much better time.
The interface to the kernel
.Em NTP
support is provided by the
.Xr ntp_adjtime 2
and
.Xr ntp_gettime 2
system calls, which are intended for use by
.Xr xntpd 8
and are enabled by the option.
On systems with sub-microsecond resolution timers, or where (HZ / 100000)
is not an integer, the
.Em NTP
option also enables extended-precision arithmetic to keep track of
fractional clock ticks at NTP time-format precision.
.It Cd options PPS
This option enables a kernel serial line discipline for receiving time
phase signals from an external reference clock such as a radio clock.
(The
.Em NTP
option (which see) must be on if the
.Em PPS
option is used.)
Some reference clocks generate a pulse per second (PPS) signal in
phase with their time source. The
.Em PPS
line discipline receives this signal on either the data leads
or the DCD control lead of a serial port.
.Em NTP
uses the PPS signal to discipline the local clock oscillator to a high
degree of precision (typically less than 50 microseconds in time and
0.1 ppm in accuracy).
.Em PPS
can also generate a serial output pulse when the system receives a PPS
interrupt.  This can be used to measure the system interrupt latency
and thus calibrate
.Em NTP
to account for it.
Using
.Em PPS
usually requires a
gadget box
to convert from TTL to RS-232 signal levels.
The gadget box and PPS are described in more detail in the HTML documentation
shipped with the xntpd distribution.
.It Cd option SETUIDSCRIPTS
Allows scripts with the setuid bit set to execute as the effective
user rather than the real user, just like binary executables.
NOTE: Using this option will also enable 
.Em option FDSCRIPTS
.It Cd option FDSCRIPTS
Allows execution of scripts with the execute bit set, but not the
read bit, by opening the file and passing the file descriptor to
the shell, rather than the filename.  NOTE: Execute only (non-readable)
scripts will have 
.Va argv[0]
set to
.Pa /dev/fd/* .
What this option allows as far as security is
concerned, is the ability to safely ensure that the correct script
is run by the interpreter, as it is passed as an already open file.
.It Cd options RTC_OFFSET=integer
The kernel (and typically the hardware battery backed-up clock on
those machines that have one) keeps time in
.Em UTC
(Universal Coordinated Time, once known as
.Em GMT ,
or Greenwich Mean Time)
and not in the time of the local time zone.
The
.Em RTC_OFFSET
option is used on some ports (such as the i386) to tell the kernel
that the hardware clock is offset from
.Em UTC
by the specified number of minutes. This is typically used when a
machine boots several operating systems and one of them wants the
hardware clock to run in the local time zone and not in
.Em UTC ,
e.g.
.Em RTC_OFFSET=300
means
the hardware clock is set to US Eastern Time (300 minutes behind 
.Em UTC ) ,
and not 
.Em UTC .
(Note:
.Em RTC_OFFSET
is used to initialize a kernel variable named
.Va rtc_offset
which is the source actually used to determine the clock offset, and
which may be accessed via the kern.rtc_offset sysctl variable.
See
.Xr sysctl 8
and
.Xr sysctl 3
for details. Since the kernel clock is initialized from the hardware
clock very early in the boot process, it is not possible to meaningfully change
.Va rtc_offset
in system initialization scripts.  Changing this value currently may
only be done at kernel compile time or by patching the kernel and
rebooting.)
.It Cd options CCDNBUF=integer
The
.Xr ccd 4
device driver uses "component buffers" to distribute I/O requests to the
components of a concatenated disk.  It keeps a freelist of buffer headers
in order to reduce use of the kernel memory allocator.
.Em CCDNBUF
is the number of buffer headers allocated on the freelist for
each component buffer.  It defaults to 8.
.It Cd options KMEMSTATS
The kernel memory allocator,
.Xr malloc 9 ,
will keep statistics on its performance if this option is enabled.
Unfortunately, this option therefore essentially disables the
.Fn MALLOC
and
.Fn FREE
forms of the memory allocator, which are used to enhance the performance
of certain critical sections of code in the kernel.
This option therefore can
lead to a significant decrease in the performance of certain code in the kernel
if enabled.  Examples of such code include the
.Fn namei
routine, the
.Xr ccd 4
driver,
the
.Xr ncr 4
driver,
and much of the networking code.
.It Cd options MAXUPRC=integer
Sets the
.Em RLIMIT_NPROC
resource limit, which specifies the maximum number of simultaneous
processes a user is permitted to run, for process 0;
this value is inherited by its child processes.
It defaults to
.Em CHILD_MAX ,
which is currently defined to be 80.
Setting
.Em MAXUPRC
to a value less than
.Em CHILD_MAX
is not permitted, as this would result in a violation of the semantics of
.St -p1003.1-90 .
.El
.Ss Networking Options
.Bl -ohang
.It Cd options GATEWAY
Enables 
.Em IPFORWARDING
(which see) 
and (on most ports) increases the size of 
.Em NMBCLUSTERS
(which see). In general,
.Em GATEWAY
is used to indicate that a system should act as a router, and
.Em IPFORWARDING
is not invoked directly.
(Note that
.Em GATEWAY
has no impact on protocols other than IP, such as CLNP or XNS.)
.It Cd options IPFORWARDING
Enables IP routing behavior. With this option enabled, the machine
will forward IP datagrams between its interfaces that are destined for
other machines. Note that even without this option, the kernel will
still forward some packets (such as source routed packets) -- removing
.Em GATEWAY
and
.Em IPFORWARDING
is insufficient to stop all routing through a bastion host on a
firewall -- source routing is controlled independently.
To turn off source routing, use 
.Em options IPFORWSRCRT=0
(which see).
Note that IP forwarding may be turned on and off independently of the
setting of the
.Em IPFORWARDING
option through the use of the
.Em net.inet.ip.forwarding
sysctl variable. If
.Em net.inet.ip.forwarding
is 1, IP forwarding is on. See
.Xr sysctl 8
and
.Xr sysctl 3
for details.
.It Cd options IPFORWSRCRT=value
If
.Em value
is set to zero, source routing of IP datagrams is turned off. If
.Em value
is set to one (the default) or the option is absent, source routed IP
datagrams are forwarded by the machine.  Note that source routing of
IP packets may be turned on and off independently of the setting of
the
.Em IPFORWSRCRT
option through the use of the
.Em net.inet.ip.forwsrcrt
sysctl variable. If
.Em net.inet.ip.forwsrcrt
is 1, forwarding of source routed IP datagrams is on. See
.Xr sysctl 8
and
.Xr sysctl 3
for details.
.It Cd options MROUTING
Includes support for IP multicast routers. You certainly want 
.Em INET
with this. Multicast routing is controlled by the
.Xr mrouted 8
daemon.
.It Cd options INET
Includes support for the TCP/IP protocol stack. You almost certainly
want this. See
.Xr inet 4
for details.
This option is currently required.
.It Cd options MCLSHIFT=value
This option is the base-2 logarithm of the size of mbuf clusters.
The BSD networking stack keeps network packets in a linked
list, or chain, of kernel buffer objects called mbufs.  The system
provides larger mbuf clusters as an optimization for large packets,
instead of using long chains for large packets.
The mbuf cluster size,
or 
.Em MCLBYTES, 
must be a power of two, and is computed as two raised to the power
.Em MCLSHIFT.
On systems with Ethernet network adaptors,
.Em	MCLSHIFT
is often set to 11, giving 2048-byte mbuf clusters, large enough to
hold a 1500-byte Ethernet frame in a single cluster. Systems with
networks supporting larger frame sizes like ATM, FDDI, or HIPPI may
perform better with MCLSHIFT set to 12 or 13, giving mbuf cluster
sizes of 4096 and 8192 bytes, respectively.
.It Cd options NS
Include support for the Xerox XNS protocol stack. See
.Xr ns 4
for details.
.It Cd options ISO,TPIP
Include support for the ubiquitous OSI protocol stack. See
.Xr iso 4
for details.
.It Cd options EON
Include support for OSI tunneling over IP.
.It Cd options CCITT,LLC,HDLC
Include support for the X.25 protocol stack.
The state of this code is currently unknown, and probably contains
bugs.
.It Cd options NETATALK
Include support for the Appletalk protocol stack. The kernel provides
provision for the
.Em Datagram Delivery Protocol (DDP),
providing SOCK_DGRAM support and Appletalk routing. This stack is used by the 
.Em Netatalk
package, which adds support for Appletalk server services via user libraries
and applications.
.It Cd options IPNOPRIVPORTS
Normally, only root can bind a socket descriptor to a so-called
.Dq privileged
TCP
port, that is, a port number in the range 0-1023.  This option
eliminates those checks from the kernel. This can be useful if there
is a desire to allow daemons without privileges to bind those ports,
e.g. on firewalls. The security tradeoffs in doing this are subtle.
This option should only be used by experts.
.It Cd options TCP_COMPAT_42
TCP bug compatibility with 4.2BSD.  In 4.2BSD, TCP sequence numbers
were 32-bit signed values.  Modern implementations of TCP use unsigned
values.  This option clamps the initial sequence number to start in
the range 2^31 rather than the full unsigned range of 2^32.  Also,
under 4.2BSD, keepalive packets must contain at least one byte or else
the remote end would not respond.
.It Cd options PFIL_HOOKS
This option turns on the packet filter interface hooks.  See
.Xr pfil 9
for details.
.It Cd options IPFILTER_LOG
This option, in conjunction with
.Em pseudo-device ipfilter ,
enables logging of IP packets using ip-filter.
.It Cd options IPFILTER_DEFAULT_BLOCK
This option sets the default policy of ip-filter.  If it is set,
ip-filter will block packets by default.
.It Cd options PPP_FILTER
This option turns on
.Xr pcap 3
based filtering for ppp connections. This option is used by
.Xr pppd 8
which needs to be compiled with 
.Em PPP_FILTER
defined (the current default).
.El
.Ss System V IPC Options
.Bl -ohang
.It Cd options SYSVMSG
Includes support for
.At V
style message queues.  See
.Xr msgctl 2 ,
.Xr msgget 2 ,
.Xr msgrcv 2 ,
.Xr msgsnd 2 .
.It Cd options SYSVSEM
Includes support for
.At V
style semaphores. See
.Xr semctl 2 ,
.Xr semget 2 ,
.Xr semop 2 .
.It Cd options SYSVSHM
Includes support for
.At V
style shared memory.  See
.Xr shmat 2 ,
.Xr shmctl 2 ,
.Xr shmdt 2 ,
.Xr shmget 2 .
.It Cd options SHMPAXPGS=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 ports. See
.Pa /usr/include/machine/vmparam.h
for the default.
.El
.Ss VM Related Options
.Bl -ohang
.It Cd options SWAPPAGER
Turns on paging.  (To be specific, this enables the virtual memory
module responsible for handling page faults for
.Dq anonymous 
objects (i.e. BSS pages)).
.Em MANDATORY
-- you cannot actually run without
this
.Dq option .
.It Cd options VNODEPAGER
Support for mmap()ing of files. (Specifically, this enables the
virtual memory module responsible for handling page faults on mapped
files (
.Dq plain file
vnodes)).
.Em MANDATORY
-- you cannot actually run without
this
.Dq option .
.It Cd options DEVPAGER
Support for mmap()ing of devices.  (Specifically, this enables the
virtual memory module responsible for handling page faults on mapped
devices (
.Dq cdev
vnodes)).
.Em MANDATORY
-- you cannot actually run without
this
.Dq option .
.It Cd options NMBCLUSTERS=value
Size of kernel mbuf cluster map,
.Em mb_map ,
in CLBYTES-sized logical
pages.  Default on most ports is 256 (512 with
.Dq options GATEWAY ).
See
.Pa /usr/include/machine/param.h
for exact default information.  Increase this value if you get 
.Dq mb_map full
messages.
.It Cd options NKMEMCLUSTERS=value
Size of kernel malloc arena in CLBYTES-sized logical pages.  This area
is covered by the kernel submap 
.Em kmem_map .
See
.Pa /usr/include/machine/param.h
for the default value, which is port specific.  Increase this value if
you get 
.Dq out of space in kmem_map
panics.
.\" , which mean you have run out of mallocable kernel memory.
.It Cd options NBUF=value
.It Cd options BUFPAGES=value
These option set the number of pages available for the buffer cache.
Their default value is a machine dependent value, often calculated as
between 5% and 10% of total available RAM.
.El
.Ss i386-specific Options
.Bl -ohang
.It Cd options I386_CPU,I486_CPU,I586_CPU,I686_CPU
Include support for a particular class of CPU (i386, i486, Pentium, or
Pentium Pro).  If the appropriate class for your CPU is not configured, the
kernel will use the highest class available that will work.  In general, using
the correct CPU class will result in the best performance.  At least one of
these options must be present.
.It Cd options MATH_EMULATE
Include the floating point emulator.  This is useful only for CPUs that lack an
internal floating point unit or coprocessor.
.It Cd options VM86
Include support for virtual 8086 mode, used by DOS emulators.
.It Cd options USER_LDT
Include i386-specific system calls for modifying the local descriptor table,
used by Windows emulators.
.It Cd options REALBASEMEM
Overrides the base memory size passed in from the boot block.  Use this option
only if the boot block reports the size incorrectly.  (Note that some BIOSes
put the extended BIOS data area at the top of base memory, and therefore report
a smaller base memory size to prevent programs overwriting it.  This is correct
behavior, and you should not use the
.Em REALBASEMEM
option to access this memory.)
.It Cd options REALEXTMEM
Overrides the extended memory size passed in from the boot block.  Use this
option only if the boot block reports the size incorrectly.
.El
.\" The following requests should be uncommented and used where appropriate.
.\" .Sh FILES
.\" .Sh EXAMPLES
.Sh SEE ALSO
.Xr config 8 ,
.Xr init 8 ,
.Xr gettimeofday 2 ,
.Xr sysctl 8 ,
.Xr sysctl 3 ,
.Xr xntpd 8 ,
.Xr ntp_adjtime 2 ,
.Xr ntp_gettime 2 ,
.Xr i386_iopl 2 ,
.Xr msgctl 2 ,
.Xr msgget 2 ,
.Xr msgrcv 2 ,
.Xr msgsnd 2 ,
.Xr semctl 2 ,
.Xr semget 2 ,
.Xr semop 2 ,
.Xr shmat 2 ,
.Xr shmctl 2 ,
.Xr shmdt 2 ,
.Xr shmget 2 ,
.Xr ddb 4 ,
.Xr lkm 4 ,
.Xr inet 4 ,
.Xr ns 4 ,
.Xr iso 4 ,
.Xr mrouted 8 ,
.Xr mount_lfs 8 ,
.Xr newlfs 8 ,
.Xr mount_mfs 8 ,
.Xr mount_nfs 8 ,
.Xr mount_cd9660 8 ,
.Xr mount_msdos 8 ,
.Xr mount_fdesc 8 ,
.Xr mount_kernfs 8 ,
.Xr mount_null 8 ,
.Xr mount_portal 8 ,
.Xr mount_procfs 8 ,
.Xr mount_umap 8 ,
.Xr mount_union 8 ,
.Xr edquota 8 ,
.Xr quotaon 8 ,
.Xr quota 1 ,
.Xr rpc.rquotad 8 ,
.Xr ktrace 1 ,
.Xr gdb 1
.Sh HISTORY
The
.Nm
man page first appeared in
.Nx 1.3 .
.Sh BUGS
The
.Em INET
and the
.Em VNODEPAGER
options should not be required.