FreeBSD Handbook
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9.4 The Configuration File

The general format of a configuration file is quite simple. Each line contains a keyword and one or more arguments. For simplicity, most lines only contain one argument. Anything following a # is considered a comment and ignored. The following sections describe each keyword, generally in the order they are listed in GENERIC, although some related keywords have been grouped together in a single section (such as Networking) even though they are actually scattered throughout the GENERIC file. An exhaustive list of options and more detailed explanations of the device lines is present in the LINT configuration file, located in the same directory as GENERIC. If you are in doubt as to the purpose or necessity of a line, check first in LINT.

Quoting numbers: In all versions of FreeBSD up to and including 3.X, config(8) required that any strings in the configuration file that contained numbers used as text had to be enclosed in double quotes.

This requirement was removed in the 4.X branch, which this book covers, so if you are on a pre-4.X system, see the /usr/src/sys/i386/conf/LINT and /usr/src/sys/i386/conf/GENERIC files on your system for examples.

The following is an example GENERIC kernel configuration file with various additional comments where needed for clarity. This example should match your copy in /usr/src/sys/i386/conf/GENERIC fairly closely. For details of all the possible kernel options, see /usr/src/sys/i386/conf/LINT.

    #
    # GENERIC -- Generic kernel configuration file for FreeBSD/i386
    #
    # For more information on this file, please read the handbook section on
    # Kernel Configuration Files:
    #
    #    http://www.FreeBSD.org/doc/en_US.ISO8859-1/books/handbook/kernelconfig-config.html
    #
    # The handbook is also available locally in /usr/share/doc/handbook
    # if you've installed the doc distribution, otherwise always see the
    # FreeBSD World Wide Web server (http://www.FreeBSD.ORG/) for the
    # latest information.
    #
    # An exhaustive list of options and more detailed explanations of the
    # device lines is also present in the ./LINT configuration file. If you are
    # in doubt as to the purpose or necessity of a line, check first in LINT.
    #
    # $FreeBSD: src/sys/i386/conf/GENERIC,v 1.246 2000/03/09 16:32:55 jlemon Exp $

The following are the mandatory keywords required in every kernel you build:

    machine        i386

This is the machine architecture. It must be either i386, alpha, or pc98.

    cpu          I386_CPU
    cpu          I486_CPU
    cpu          I586_CPU
    cpu          I686_CPU

The above specifies the type of CPU you have in your system. You may have multiple instances of the CPU line (i.e., you are not sure whether you should use I586_CPU or I686_CPU), however, for a custom kernel, it is best to specify only the CPU you have. If you are unsure of your CPU type, you can use the dmesg(8) command to view your boot up messages.

The Alpha architecture has different values for cpu. They include:

    cpu          EV4
    cpu          EV5

If you are using an Alpha machine, you should be using one of the above CPU types.

    ident          GENERIC

This is the identification of the kernel. You should change this to whatever you named your kernel, as in our previous example, MYKERNEL. The value you put in the ident string will print when you boot up the kernel, so it is useful to give the new kernel a different name if you want to keep it separate from your usual kernel (i.e., you want to build an experimental kernel).

    maxusers          n

The maxusers option sets the size of a number of important system tables. This number is supposed to be roughly equal to the number of simultaneous users you expect to have on your machine.

Starting with FreeBSD 4.5, the system will auto-tune this setting for you if you explicitly set it to 0[1]. If you are using an earlier version of FreeBSD, or you want to manage it yourself you will want to set maxusers to at least 4, especially if you are using the X Window System or compiling software. The reason is that the most important table set by maxusers is the maximum number of processes, which is set to 20 + 16 * maxusers, so if you set maxusers to 1, then you can only have 36 simultaneous processes, including the 18 or so that the system starts up at boot time, and the 15 or so you will probably create when you start the X Window System. Even a simple task like reading a manual page will start up nine processes to filter, decompress, and view it. Setting maxusers to 64 will allow you to have up to 1044 simultaneous processes, which should be enough for nearly all uses. If, however, you see the dreaded proc table full error when trying to start another program, or are running a server with a large number of simultaneous users (like ftp.FreeBSD.org), you can always increase the number and rebuild.

Note: maxusers does not limit the number of users which can log into your machine. It simply sets various table sizes to reasonable values considering the maximum number of users you will likely have on your system and how many processes each of them will be running. One keyword which does limit the number of simultaneous remote logins is pseudo-device pty 16.

    # Floating point support - do not disable.
    device          npx0     at nexus? port IO_NPX irq 13

npx0 is the interface to the floating point math unit in FreeBSD, which is either the hardware co-processor or the software math emulator. This is not optional.

    # Pseudo devices - the number indicates how many units to allocate.
    pseudo-device   loop          # Network loopback

This is the generic loopback device for TCP/IP. If you telnet or FTP to localhost (a.k.a., 127.0.0.1) it will come back at you through this pseudo-device. This is mandatory.

Everything that follows is more or less optional. See the notes underneath or next to each option for more information.

    #makeoptions     DEBUG=-g          #Build kernel with gdb(1) debug symbols
    options          MATH_EMULATE      #Support for x87 emulation

This line allows the kernel to simulate a math co-processor if your computer does not have one (386 or 486SX). If you have a 486DX, or a 386 or 486SX (with a separate 387 or 487 chip), or higher (Pentium, Pentium II, etc.), you can comment this line out.

Note: The normal math co-processor emulation routines that come with FreeBSD are not very accurate. If you do not have a math co-processor, and you need the best accuracy, it is recommended that you change this option to GPL_MATH_EMULATE to use the GNU math support, which is not included by default for licensing reasons.

    options          INET          #InterNETworking

Networking support. Leave this in, even if you do not plan to be connected to a network. Most programs require at least loopback networking (i.e., making network connections within your PC), so this is essentially mandatory.

    options          INET6          #IPv6 communications protocols

This enables the IPv6 communication protocols.

    options          FFS          #Berkeley Fast Filesystem
    options          FFS_ROOT     #FFS usable as root device [keep this!]

This is the basic hard drive filesystem. Leave it in if you boot from the hard disk.

    options          UFS_DIRHASH  #Improve performance on big directories

This option includes some code to speed up disk operations on large directories, at the expense of using a some additional memory. You would normally keep this for a large server, or interactive workstation, and remove it if you are using FreeBSD on a smaller system where memory is at a premium and disk access speed is less important, such as a firewall.

    options          SOFTUPDATES  #Enable FFS soft updates support

This option enables Soft Updates in the kernel, this will help speed up write access on the disks. They are enabled by default in the 4.X branch but may not be turned on. Review the output from mount(8) to see if you have them enabled. If you do not see the soft-updates option then you will need to activate it using the tunefs(8) or newfs(8) for new filesystems.

    options          MFS          #Memory Filesystem
    options          MD_ROOT      #MD is a potential root device

This is the memory-mapped filesystem. This is basically a RAM disk for fast storage of temporary files, useful if you have a lot of swap space that you want to take advantage of. A perfect place to mount an MFS partition is on the /tmp directory, since many programs store temporary data here. To mount an MFS RAM disk on /tmp, add the following line to /etc/fstab:

    /dev/ad1s2b    /tmp mfs rw 0 0

Now you simply need to either reboot, or run the command mount /tmp.

    options          NFS          #Network Filesystem
    options          NFS_ROOT     #NFS usable as root device, NFS required

The network filesystem. Unless you plan to mount partitions from a Unix file server over TCP/IP, you can comment these out.

    options          MSDOSFS      #MSDOS Filesystem

The MS-DOS filesystem. Unless you plan to mount a DOS formatted hard drive partition at boot time, you can safely comment this out. It will be automatically loaded the first time you mount a DOS partition, as described above. Also, the excellent mtools software (in the ports collection) allows you to access DOS floppies without having to mount and unmount them (and does not require MSDOSFS at all).

    options          CD9660       #ISO 9660 Filesystem
    options          CD9660_ROOT  #CD-ROM usable as root, CD9660 required

The ISO 9660 filesystem for CDROMs. Comment it out if you do not have a CDROM drive or only mount data CDs occasionally (since it will be dynamically loaded the first time you mount a data CD). Audio CDs do not need this filesystem.

    options          PROCFS       #Process filesystem

The process filesystem. This is a ``pretend'' filesystem mounted on /proc which allows programs like ps(1) to give you more information on what processes are running.

    options          COMPAT_43    #Compatible with BSD 4.3 [KEEP THIS!]

Compatibility with 4.3BSD. Leave this in; some programs will act strangely if you comment this out.

    options          SCSI_DELAY=15000    #Delay (in ms) before probing SCSI

This causes the kernel to pause for 15 seconds before probing each SCSI device in your system. If you only have IDE hard drives, you can ignore this, otherwise you will probably want to lower this number, perhaps to 5 seconds, to speed up booting. Of course, if you do this, and FreeBSD has trouble recognizing your SCSI devices, you will have to raise it back up.

    options          UCONSOLE            #Allow users to grab the console

Allow users to grab the console, which is useful for X users. For example, you can create a console xterm by typing xterm -C, which will display any write(1), talk(1), and any other messages you receive, as well as any console messages sent by the kernel.

    options          USERCONFIG          #boot -c editor

This option allows you to boot the configuration editor from the boot menu.

    options          VISUAL_USERCONFIG   #visual boot -c editor

This option allows you to boot the visual configuration editor from the boot menu.

    options          KTRACE              #ktrace(1) support

This enables kernel process tracing, which is useful in debugging.

    options          SYSVSHM             #SYSV-style shared memory

This option provides for System V shared memory. The most common use of this is the XSHM extension in X, which many graphics-intensive programs will automatically take advantage of for extra speed. If you use X, you will definitely want to include this.

    options          SYSVSEM             #SYSV-style semaphores

Support for System V semaphores. Less commonly used but only adds a few hundred bytes to the kernel.

    options          SYSVMSG             #SYSV-style message queues

Support for System V messages. Again, only adds a few hundred bytes to the kernel.

Note: The ipcs(1) command will list any processes using each of these System V facilities.

    options    P1003_1B        #Posix P1003_1B real-time extensions
    options     _KPOSIX_PRIORITY_SCHEDULING

Real-time extensions added in the 1993 POSIX. Certain applications in the ports collection use these (such as StarOffice).

    options        ICMP_BANDLIM        #Rate limit bad replies

This option enables ICMP error response bandwidth limiting. You typically want this option as it will help protect the machine from denial of service packet attacks.

    # To make an SMP kernel, the next two are needed
    #options        SMP                     # Symmetric MultiProcessor Kernel
    #options        APIC_IO                 # Symmetric (APIC) I/O

The above are both required for SMP support.

    device          isa

All PCs supported by FreeBSD have one of these. If you have an IBM PS/2 (Micro Channel Architecture), FreeBSD provides some limited support at this time. For more information about the MCA support, see /usr/src/sys/i386/conf/LINT.

    device          eisa

Include this if you have an EISA motherboard. This enables auto-detection and configuration support for all devices on the EISA bus.

    device          pci

Include this if you have a PCI motherboard. This enables auto-detection of PCI cards and gatewaying from the PCI to ISA bus.

    # Floppy drives
    device          fdc0        at isa? port IO_FD1 irq 6 drq 2
    device          fd0         at fdc0 drive 0
    device          fd1         at fdc0 drive 1

This is the floppy drive controller. fd0 is the A: floppy drive, and fd1 is the B: drive.

    device          ata

This driver supports all ATA and ATAPI devices. You only need one device ata line for the kernel to detect all PCI ATA/ATAPI devices on modern machines.

    device          atadisk                 # ATA disk drives

This is needed along with device ata for ATA disk drives.

    
    device          atapicd                 # ATAPI CDROM drives

This is needed along with device ata for ATAPI CDROM drives.

    device          atapifd                 # ATAPI floppy drives

This is needed along with device ata for ATAPI floppy drives.

    device          atapist                 # ATAPI tape drives

This is needed along with device ata for ATAPI tape drives.

    options         ATA_STATIC_ID           #Static device numbering

This makes the controller number static (like the old driver) or else the device numbers are dynamically allocated.

    # ATA and ATAPI devices
    device          ata0        at isa? port IO_WD1 irq 14
    device          ata1        at isa? port IO_WD2 irq 15

Use the above for older, non-PCI systems.

    # SCSI Controllers
    device          ahb        # EISA AHA1742 family
    device          ahc        # AHA2940 and onboard AIC7xxx devices
    device          amd        # AMD 53C974 (Teckram DC-390(T))
    device          dpt        # DPT Smartcache - See LINT for options!
    device          isp        # Qlogic family
    device          ncr        # NCR/Symbios Logic
    device          sym        # NCR/Symbios Logic (newer chipsets)
    
    device          adv0       at isa?
    device          adw
    device          bt0        at isa?
    device          aha0       at isa?
    device          aic0       at isa?

SCSI controllers. Comment out any you do not have in your system. If you have an IDE only system, you can remove these altogether.

    # SCSI peripherals
    device          scbus      # SCSI bus (required)
    device          da         # Direct Access (disks)
    device          sa         # Sequential Access (tape etc)
    device          cd         # CD
    device          pass       # Passthrough device (direct SCSI
    access)

SCSI peripherals. Again, comment out any you do not have, or if you have only IDE hardware, you can remove them completely.

    # RAID controllers
    device          ida        # Compaq Smart RAID
    device          amr        # AMI MegaRAID
    device          mlx        # Mylex DAC960 family

Supported RAID controllers. If you do not have any of these, you can comment them out or remove them.

    # atkbdc0 controls both the keyboard and the PS/2 mouse
    device          atkbdc0    at isa? port IO_KBD

The keyboard controller (atkbdc) provides I/O services for the AT keyboard and PS/2 style pointing devices. This controller is required by the keyboard driver (atkbd) and the PS/2 pointing device driver (psm).

    device          atkbd0     at atkbdc? irq 1

The atkbd driver, together with atkbdc controller, provides access to the AT 84 keyboard or the AT enhanced keyboard which is connected to the AT keyboard controller.

    device          psm0       at atkbdc? irq 12

Use this device if your mouse plugs into the PS/2 mouse port.

    device          vga0        at isa?

The video card driver.

    # splash screen/screen saver
    pseudo-device          splash

Splash screen at start up! Screen savers require this too.

    # syscons is the default console driver, resembling an SCO console
    device          sc0          at isa?

sc0 is the default console driver, which resembles a SCO console. Since most full-screen programs access the console through a terminal database library like termcap, it should not matter whether you use this or vt0, the VT220 compatible console driver. When you log in, set your TERM variable to scoansi if full-screen programs have trouble running under this console.

    # Enable this and PCVT_FREEBSD for pcvt vt220 compatible console driver
    #device          vt0     at isa?
    #options         XSERVER          # support for X server on a vt console
    #options         FAT_CURSOR       # start with block cursor
    # If you have a ThinkPAD, uncomment this along with the rest of the PCVT lines
    #options         PCVT_SCANSET=2   # IBM keyboards are non-std

This is a VT220-compatible console driver, backward compatible to VT100/102. It works well on some laptops which have hardware incompatibilities with sc0. Also set your TERM variable to vt100 or vt220 when you log in. This driver might also prove useful when connecting to a large number of different machines over the network, where termcap or terminfo entries for the sc0 device are often not available -- vt100 should be available on virtually any platform.

    # Power management support (see LINT for more options)
    device          apm0     at nexus? disable flags 0x20  # Advanced Power Management

Advanced Power Management support. Useful for laptops.

    # PCCARD (PCMCIA) support
    device          card
    device          pcic0    at isa? irq 10 port 0x3e0 iomem 0xd0000
    device          pcic1    at isa? irq 11 port 0x3e2 iomem 0xd4000 disable

PCMCIA support. You want this if you are using a laptop.

    # Serial (COM) ports
    device          sio0     at isa? port IO_COM1 flags 0x10 irq 4
    device          sio1     at isa? port IO_COM2 irq 3
    device          sio2     at isa? disable port IO_COM3 irq 5
    device          sio3     at isa? disable port IO_COM4 irq 9

These are the four serial ports referred to as COM1 through COM4 in the MS-DOS/Windows world.

Note: If you have an internal modem on COM4 and a serial port at COM2, you will have to change the IRQ of the modem to 2 (for obscure technical reasons, IRQ2 = IRQ 9) in order to access it from FreeBSD. If you have a multiport serial card, check the manual page for sio(4) for more information on the proper values for these lines. Some video cards (notably those based on S3 chips) use IO addresses in the form of 0x*2e8, and since many cheap serial cards do not fully decode the 16-bit IO address space, they clash with these cards making the COM4 port practically unavailable.

Each serial port is required to have a unique IRQ (unless you are using one of the multiport cards where shared interrupts are supported), so the default IRQs for COM3 and COM4 cannot be used.

    # Parallel port
    device          ppc0    at isa? irq 7

This is the ISA-bus parallel port interface.

    device          ppbus      # Parallel port bus (required)

Provides support for the parallel port bus.

    device          lpt        # Printer

Support for parallel port printers.

Note: All three of the above are required to enable parallel printer support.

    device          plip       # TCP/IP over parallel

This is the driver for the parallel network interface.

    device          ppi        # Parallel port interface device

The general-purpose I/O (``geek port'') + IEEE1284 I/O.

    #device         vpo        # Requires scbus and da

This is for an Iomega Zip drive. It requires scbus and da support. Best performance is achieved with ports in EPP 1.9 mode.

    # PCI Ethernet NICs.
    device          de         # DEC/Intel DC21x4x (``Tulip'')
    device          fxp        # Intel EtherExpress PRO/100B (82557, 82558)
    device          tx         # SMC 9432TX (83c170 ``EPIC'')
    device          vx         # 3Com 3c590, 3c595 (``Vortex'')
    device          wx         # Intel Gigabit Ethernet Card (``Wiseman'')

Various PCI network card drivers. Comment out or remove any of these not present in your system.

    # PCI Ethernet NICs that use the common MII bus controller code.
    device          miibus     # MII bus support

MII bus support is required for some PCI 10/100 Ethernet NICs, namely those which use MII-compliant transceivers or implement transceiver control interfaces that operate like an MII. Adding device miibus to the kernel config pulls in support for the generic miibus API and all of the PHY drivers, including a generic one for PHYs that are not specifically handled by an individual driver.

    device          dc         # DEC/Intel 21143 and various workalikes
    device          rl         # RealTek 8129/8139
    device          sf         # Adaptec AIC-6915 (``Starfire'')
    device          sis        # Silicon Integrated Systems SiS 900/SiS 7016
    device          ste        # Sundance ST201 (D-Link DFE-550TX)
    device          tl         # Texas Instruments ThunderLAN
    device          vr         # VIA Rhine, Rhine II
    device          wb         # Winbond W89C840F
    device          xl         # 3Com 3c90x (``Boomerang'', ``Cyclone'')

Drivers that use the MII bus controller code.

    # ISA Ethernet NICs.
    device          ed0    at isa? port 0x280 irq 10 iomem 0xd8000
    device          ex
    device          ep
    # WaveLAN/IEEE 802.11 wireless NICs. Note: the WaveLAN/IEEE really
    # exists only as a PCMCIA device, so there is no ISA attachment needed
    # and resources will always be dynamically assigned by the pccard code.
    device          wi
    # Aironet 4500/4800 802.11 wireless NICs. Note: the declaration below will
    # work for PCMCIA and PCI cards, as well as ISA cards set to ISA PnP
    # mode (the factory default). If you set the switches on your ISA
    # card for a manually chosen I/O address and IRQ, you must specify
    # those parameters here.
    device          an
    # The probe order of these is presently determined by i386/isa/isa_compat.c.
    device          ie0    at isa? port 0x300 irq 10 iomem 0xd0000
    device          fe0    at isa? port 0x300
    device          le0    at isa? port 0x300 irq 5 iomem 0xd0000
    device          lnc0   at isa? port 0x280 irq 10 drq 0
    device          cs0    at isa? port 0x300
    device          sn0    at isa? port 0x300 irq 10
    # requires PCCARD (PCMCIA) support to be activated
    #device         xe0    at isa?

ISA Ethernet drivers. See /usr/src/sys/i386/conf/LINT for which cards are supported by which driver.

    pseudo-device   ether         # Ethernet support

ether is only needed if you have an Ethernet card. It includes generic Ethernet protocol code.

    pseudo-device   sl      1     # Kernel SLIP

sl is for SLIP support. This has been almost entirely supplanted by PPP, which is easier to set up, better suited for modem-to-modem connection, and more powerful. The number after sl specifies how many simultaneous SLIP sessions to support.

    pseudo-device   ppp     1     # Kernel PPP

This is for kernel PPP support for dial-up connections. There is also a version of PPP implemented as a userland application that uses tun and offers more flexibility and features such as demand dialing. The number after ppp specifies how many simultaneous PPP connections to support.

    pseudo-device   tun           # Packet tunnel.

This is used by the userland PPP software. A number after tun specifies the number of simultaneous PPP sessions to support. See the PPP section of this book for more information.

    
    pseudo-device   pty           # Pseudo-ttys (telnet etc)

This is a ``pseudo-terminal'' or simulated login port. It is used by incoming telnet and rlogin sessions, xterm, and some other applications such as Emacs. A number after pty indicates the number of ptys to create. If you need more than the default of 16 simultaneous xterm windows and/or remote logins, be sure to increase this number accordingly, up to a maximum of 256.

    pseudo-device   md            # Memory ``disks''

Memory disk pseudo-devices.

    pseudo-device   gif

    pseudo-device   gif     4     # IPv6 and IPv4 tunneling

This implements IPv6 over IPv4 tunneling, IPv4 over IPv6 tunneling, IPv4 over IPv4 tunneling, and IPv6 over IPv6 tunneling. Beginning with FreeBSD 4.4 the gif device is ``auto-cloning'', and you should use the first example (without the number after gif). Earlier versions of FreeBSD require the number.

    pseudo-device   faith   1     # IPv6-to-IPv4 relaying (translation)

This pseudo-device captures packets that are sent to it and diverts them to the IPv4/IPv6 translation daemon.

    # The `bpf' pseudo-device enables the Berkeley Packet Filter.
    # Be aware of the administrative consequences of enabling this!
    pseudo-device   bpf           # Berkeley packet filter

This is the Berkeley Packet Filter. This pseudo-device allows network interfaces to be placed in promiscuous mode, capturing every packet on a broadcast network (e.g., an Ethernet). These packets can be captured to disk and or examined with the tcpdump(1) program.

Note: The bpf pseudo-device is also used by dhclient(8) to obtain the IP address of the default router (gateway) and so on. If you use DHCP, leave this uncommented.

    # USB support
    #device         uhci          # UHCI PCI->USB interface
    #device         ohci          # OHCI PCI->USB interface
    #device         usb           # USB Bus (required)
    #device         ugen          # Generic
    #device         uhid          # ``Human Interface Devices''
    #device         ukbd          # Keyboard
    #device         ulpt          # Printer
    #device         umass         # Disks/Mass storage - Requires scbus and da
    #device         ums           # Mouse
    # USB Ethernet, requires mii
    #device         aue           # ADMtek USB ethernet
    #device         cue           # CATC USB ethernet
    #device         kue           # Kawasaki LSI USB ethernet

Support for various USB devices.

For more information and additional devices supported by FreeBSD, see /usr/src/sys/i386/conf/LINT.

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9.4 The Configuration File

Notes