add manual on kernel options, from netbsd, changed a bit ...

need more work, ppl look!

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+.\"	$OpenBSD: options.4,v 1.1 1997/08/18 01:27:54 mickey Exp $
+.\"	$NetBSD: options.4,v 1.21 1997/06/25 03:13:00 thorpej Exp $
+.\"
+.\" Copyright (c) 1996
+.\" 	Perry E. Metzger.  All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\" 3. All advertising materials mentioning features or use of this software
+.\"    must display the following acknowledgement:
+.\"	This product includes software developed for the NetBSD Project
+.\"	by Perry E. Metzger.
+.\" 4. The name of the author may not be used to endorse or promote products
+.\"    derived from this software without specific prior written permission.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
+.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
+.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
+.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
+.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
+.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
+.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+.\"
+.\"
+.Dd October 20, 1996
+.Os NetBSD 1.3
+.Dt OPTIONS 4
+.Sh NAME
+.Nm option
+.Nd Miscellaneous 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 -D flags to the C
+compiler.
+.Ss Compatibility Options
+.Bl -ohang
+.It Cd option 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 fileystem identifiers rather
+than strings.  Post 0.9 versions use string identifiers.
+.It Cd option 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 inapropriate ioctl
+error.
+.It Cd option 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 option 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 was changed post 1.2 to add functionality to the
+reboot syscall, and the new
+.Fn swapctl
+interface was introduced.
+.It Cd option 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 disipline.
+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 option 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 option 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 option 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 option 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 option 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 option 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 option 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 option 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 option 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 option 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 option 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 option 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 option DEBUG
+(which see).
+.It Cd option DEBUG
+Turns on miscellaneous kernel debugging. Since options are turned into
+preprocessor defines (see above),
+.Em option 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, notably the
+.Em KMEMSTATS
+option, which may decrease system performance.
+.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 KTRACE
+Add hooks for the system call tracing facility, which allows users to
+watch the system call invokation behavior of processes. See
+.Xr ktrace 1
+for details.
+.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 greather 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 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 
+for details.
+.It Cd option 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 option FIFO
+Augments the system to permit the use of
+.At V
+style FIFOs (i.e.
+.Dq named pipes
+). This option is recommended.
+.It Cd option 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 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 greather than 0. See also
+.Xr chflags 1 .
+.El
+.Ss Miscellaneous Options
+.Bl -ohang
+.It Cd option LKM
+Enable loadable kernel modules. See
+.Xr lkm 4
+for details.
+.Em NOTE:
+not available on all architectures.
+.It Cd option 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 option 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 option 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 option 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 option 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 option 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 option 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
+it run by the interpreter, as it is passed as an already open file.
+.It Cd option 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 option 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 option 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 
+.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.
+.Em N.B. This option is silently
+.Em turned on by the DEBUG option.
+.El
+.Ss Networking Options
+.Bl -ohang
+.It Cd option 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 option 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 option 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 option 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 option 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 option 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 option 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 option NS
+Include support for the Xerox XNS protocol stack. See
+.Xr ns 4
+for details.
+.It Cd option ISO,TPIP
+Include support for the ubiquitous OSI protocol stack. See
+.Xr iso 4
+for details.
+.It Cd option EON
+Include support for OSI tunneling over IP.
+.It Cd option 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 option IPX,IPXIP
+Include support for Internetwork Packet Exchange protocol.
+.It Cd option NETATALK
+Include support for AppleTalk.
+.It Cd option 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 option 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 option PFIL_HOOKS
+This option turns on the packet filter interface hooks.  See
+.Xr pfil 9
+for details.
+.It Cd option IPFILTER
+This option enables the IP filtering on the packet level using the
+ip-filter package.  This option requies that the
+.Em PFIL_HOOK
+option also be included.
+.It Cd option IPFILTER_LOG
+This option, in conjuction with
+.Em IPFILTER ,
+enables logging of IP packets using ip-filter.
+.It Cd option 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 option 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 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 .
+.It Cd option 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 option 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 option 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 option 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 option 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 option 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 option 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 option NBUF=value
+.It Cd option BUFPAGES=value
+These option set the number of pages available for the buffer cache.
+Their default value is a machine depedant value, often calculated as
+between 5% and 10% of total available RAM.
+.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.