Compiling a FreeBSD Kernel

Ever since my college days, I always loved to have my own compiled kernel. It gave me that special feeling of “my machine is better than yours”. I am sure each time my new kernel boots up the feeling I have is is the same as what Super Man has when he bashs up the bad people.


A custom kernel boots up the system faster, it takes up lesser memory space and one can install drivers for hardware not supported by a default kernel. Besides these benefits, a custom kernel serves as an interesting conversation topic with geeks. Just ask them “What is your kernel size ?”. This would keep the Geek busy for sometime where he would explain all the great things he did to the kernel options to optimize his box.
If you happen to be a geek yourself, this is a great question to put forward to those lowly Nubies. They’ll never trouble you with stories of their nephew or kid sister after this :-D
Before we get down to compiling our own kernel, it is a wonderful idea to backup the present kernel.
# cd /boot
# cp -pvr kernel kernel.orig

The original generic kernel is now stored as kernel.orig. In case the kernel does not boot (which has happened with me many times) boot up with the generic kernel and try again. Booting any other kernel is simple. At the boot time goto to the boot prompt and simple say “boot kernel.orig” and the default kernel would boot up.

The kernel sources are located in the /usr/src/sys directory. If not installed, the kernel sources can be installed with the following commands. First insert the FreeBSD install CD in your CD drive.

# mount /cdrom
# mkdir -p /usr/src/sys
# ln -s /usr/src/sys /sys
# cat /cdrom/src/ssys.[a-d]* | tar -xzvf -

Then go to the director where the configuration files are located.

#cd /sys/i386/conf

In this directory the GENERIC kernel configuration file is located. Copy this file and it as your custom name.


First change the ident value from GENERIC to BABYBSD (or whatever name you prefer). I have given my sample configuration file at the end of this article. The options are quite easy to understand and a good starting point would be the FreeBSD Handbook which has documented most of the kernel options.
You can also go to a shell and run the command #make LINT in this directory which would create a file called LINT which has all the options supported by the kernel listed.

After this stage just run the following commands

#/usr/sbin/config BABYBSD
#cd ../compile/BABYBSD
#make depend
#make install

Please note that you must be root to run the make install. Reboot and the kernel should boot with your shining new kernel. :-)

# My Sample Kernel Configuration File for FreeBSD/i386

machine        i386
cpu                I586_CPU
cpu                I686_CPU
ident              BABYBSD

options     SCHED_4BSD        # 4BSD scheduler
options     INET            # InterNETworking
options     INET6            # IPv6 communications protocols
options     FFS            # Berkeley Fast Filesystem
options     SOFTUPDATES        # Enable FFS soft updates support
options     UFS_ACL            # Support for access control lists
options     UFS_DIRHASH        # Improve performance on big directories
options     MD_ROOT            # MD is a potential root device
options     NFSCLIENT        # Network Filesystem Client
options     NFSSERVER        # Network Filesystem Server
options     NFS_ROOT        # NFS usable as /, requires NFSCLIENT
options     MSDOSFS            # MSDOS Filesystem
options         NTFS
options     CD9660            # ISO 9660 Filesystem
options     PROCFS            # Process filesystem (requires PSEUDOFS)
options     PSEUDOFS        # Pseudo-filesystem framework
options     GEOM_GPT        # GUID Partition Tables.
options     COMPAT_43        # Compatible with BSD 4.3 [KEEP THIS!]
options     COMPAT_FREEBSD4        # Compatible with FreeBSD4
options     SCSI_DELAY=15000    # Delay (in ms) before probing SCSI
options     KTRACE            # ktrace(1) support
options     SYSVSHM            # SYSV-style shared memory
options     SYSVMSG            # SYSV-style message queues
options     SYSVSEM            # SYSV-style semaphores
options     _KPOSIX_PRIORITY_SCHEDULING # POSIX P1003_1B real-time extensions
options     KBD_INSTALL_CDEV    # install a CDEV entry in /dev
options     AHC_REG_PRETTY_PRINT    # Print register bitfields in debug
                    # output.  Adds ~128k to driver.
options     AHD_REG_PRETTY_PRINT    # Print register bitfields in debug
                    # output.  Adds ~215k to driver.
options     ADAPTIVE_GIANT        # Giant mutex is adaptive.

device        apic            # I/O APIC

# Bus support.  Do not remove isa, even if you have no isa slots
device        isa
device        eisa
device        pci

# Floppy drives
device        fdc

# ATA and ATAPI devices
device        ata
device        atadisk        # ATA disk drives
device        atapicd        # ATAPI CDROM drives
options     ATA_STATIC_ID    # Static device numbering

# SCSI peripherals
device        scbus        # SCSI bus (required for SCSI)
device        ch        # SCSI media changers
device        da        # Direct Access (disks)
device        cd        # CD
device        pass        # Passthrough device (direct SCSI access)
device        ses        # SCSI Environmental Services (and SAF-TE)

# atkbdc0 controls both the keyboard and the PS/2 mouse
device        atkbdc        # AT keyboard controller
device        atkbd        # AT keyboard
device        psm        # PS/2 mouse

device        vga        # VGA video card driver

device        splash        # Splash screen and screen saver support

# syscons is the default console driver, resembling an SCO console
device        sc

# Enable this for the pcvt (VT220 compatible) console driver
#device        vt
#options     XSERVER        # support for X server on a vt console
#options     FAT_CURSOR    # start with block cursor

device        agp        # support several AGP chipsets

# Floating point support – do not disable.
device        npx

# Power management support (see NOTES for more options)
#device        apm
# Add suspend/resume support for the i8254.
device        pmtimer

# PCI Ethernet NICs that use the common MII bus controller code.
# NOTE: Be sure to keep the ‘device miibus’ line in order to use these NICs!
device        miibus        # MII bus support
device        rl        # RealTek 8129/8139

# Wireless NIC cards
device        wlan        # 802.11 support

# Pseudo devices.
device        loop        # Network loopback
device        mem        # Memory and kernel memory devices
device        io        # I/O device
device        random        # Entropy device
device        ether        # Ethernet support
device        sl        # Kernel SLIP
device        ppp        # Kernel PPP
device        tun        # Packet tunnel.
device        pty        # Pseudo-ttys (telnet etc)
device        md        # Memory “disks”
device        gif        # IPv6 and IPv4 tunneling
device        faith        # IPv6-to-IPv4 relaying (translation)

# The bpf’ device enables the Berkeley Packet Filter.
# Be aware of the administrative consequences of enabling this!
# Note that ‘bpf’ is required for DHCP.
device        bpf        # Berkeley packet filter

# USB support
device        uhci        # UHCI PCI->USB interface
device        ohci        # OHCI PCI->USB interface
device        ehci        # EHCI PCI->USB interface (USB 2.0)
device        usb        # USB Bus (required)
#device        udbp        # USB Double Bulk Pipe devices
device        ugen        # Generic
device        uhid        # “Human Interface Devices”
device        ukbd        # Keyboard
device        umass        # Disks/Mass storage – Requires scbus and da
device        ums        # Mouse

# Specific to my Machine
device         atapicam    # for CD Writer to be detected by K3B and other CD writing software
device          sound          # To Enable Sound
device          “snd_ad1816”
device          “snd_als4000”
device          snd_cmi
device          “snd_cs4281”
device          snd_csa
device          “snd_ds1”
device          “snd_emu10k1”
device          “snd_es137x”
device          snd_ess
device          “snd_fm801”
device          snd_gusc
device          snd_ich
device          snd_maestro
device          “snd_maestro3”
device          snd_mss
device          snd_neomagic
device          “snd_sb16”
device          snd_sbc
device          snd_solo
device          “snd_t4dwave”
device          “snd_via8233”
device          “snd_via82c686”
device          snd_vibes
device          snd_uaudio
device pcii
device          acpi
device          acpi_asus
device          acpi_video

device          cbb             # cardbus (yenta) bridge
device          pccard          # PC Card (16-bit) bus
device          cardbus         # CardBus (32-bit) bus

options     DEVICE_POLLING
options     HZ=1000
options        PERFMON
device          apm_saver       # Requires APM

options         AUTO_EOI_1
options        AUTO_EOI_2

options         VESA