Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages


home | help
PICOBSD(8)              FreeBSD System Manager's Manual             PICOBSD(8)

     picobsd -- building small FreeBSD disk images

     picobsd [options] [config-name [site-name]]

     The picobsd utility is a script which produces a minimal implementation
     of FreeBSD (historically called PicoBSD) which typically fits on a small
     media such as a floppy disk, or can be downloaded as a single image file
     from some media such as CDROM, flash memory, or through etherboot.

     The picobsd utility was originally created to build simple standalone
     systems such as firewalls or bridges, but because of the ability to
     cross-build images with different source trees than the one in the
     server, it can be extremely useful to developers to test their code with-
     out having to reinstall the system.

     The boot media (historically a floppy disk, but also small CDROM or USB
     keys) contains a boot loader and a compressed kernel which includes a
     memory file system.  Depending on the media, it might also contain a num-
     ber of additional files, which can be updated at run time, and are used
     to override/update those in the memory file system.

     The system loads the kernel in the normal way, uncompresses the memory
     file system and mounts it as root.  It then updates the memory file sys-
     tem with files from the boot media (if present), and executes a special-
     ized version of /etc/rc.  The boot media (floppy, etc.) is required for
     loading only, and typically used read-only.  After the boot phase, the
     system runs entirely from RAM.

     The following options are available (but also check the picobsd script
     for more details):

     --src SRC_PATH
             Use the source tree at SRC_PATH instead the one at /usr/src.
             This can be useful for cross-building picobsd images.  When using
             this option, you must also create and initialize the subtree at
             <SRC_PATH>/../usr with the correct header files, libraries, and
             tools (such as the config(8) program) that are necessary for the
             cross-build (see the --init option below).  The source files are
             unmodified by the picobsd script.  However the source tree is not
             completely read-only, because config(8) expects the kernel con-
             figuration file to be in one of its subdirectories, and also the
             process of initializing the usr subtree touches some parts of the
             source tree (this is a bug in the release build scripts which
             might go away with time).

     --init  When used together with the --src option, this initializes the
             <SRC_PATH>/../usr subtree as necessary to subsequently build
             picobsd images.

             Also build kernel modules.  These are not stored on the picobsd
             image but are left available in the build directory.

     -n      Make the script non-interactive, skipping the initial menu and
             proceeding with the build process without requiring user input.

     -v      Make the script verbose, showing commands to be executed and
             waiting for user input before executing each of them.  Useful for

             Put the entire contents of the file system in the memory file
             system image which is contained in the kernel.  This is the
             default behaviour, and is extremely useful as the kernel itself
             can be loaded, using etherboot or pxeboot(8), as a fully func-
             tional system.

             Leaves files contained in the floppy.tree on the picobsd image,
             so they can be loaded separately from the kernel (and updated
             individually to customize the image).

     --floppy_size size
             Set the size of the disk image.  Typical values for a floppy disk
             are 1440 or 2880, but other values can be used for other media
             (flash memories, CDROM, network booted kernels).

     -c, -clean
             Clean the product of previous builds.

     As a result of extreme size limitations, the picobsd environment differs
     from the normal FreeBSD in a number of ways:

     +o   There are no dynamic libraries, and there is no directory /usr/lib.
         As a result, only static executables may be executed.

     +o   In order to reduce the size of the executables, all executables on a
         specific floppy are joined together as a single executable built with

     +o   Some programs are supplied in minimalistic versions, specifically ns,
         a cut-down version of netstat(1), and vm, a cut-down version of

     The picobsd sources reside in the hierarchy /usr/src/release/picobsd.  In
     the following discussion, all relative path names are relative to this

     The supported build script is /usr/src/release/picobsd/build/picobsd
     which can be run from anywhere, and relies on the sysutils/makefs port to
     build a filesystem without requiring mdconfig or root privileges to mount
     a filesystem.  When run in interactive mode (the default without the -n
     option), the script will let you configure the various parameters used to
     build the PicoBSD image.  An image is configured using the files and
     directories described below.  The base system contains a template, called
     bridge for historical reasons, that can be used as a base for building
     various kinds of network appliances.

     You can define your own PicoBSD configuration, by creating a directory
     with a name of your choice (e.g. FOO) which contains some of the follow-
     ing files and directories.  For more information on how to construct
     these files, look at one of the standard picobsd configurations as a ref-

             The kernel configuration file (required).  This is a mostly stan-
             dard kernel configuration file, possibly stripped down by remov-
             ing unnecessary drivers and options to reduce the kernel's size.

             To be recognised as a picobsd kernel config file, the file must
             also contain the line beginning with ``#PicoBSD'' below, and a
             matching MD_ROOT_SIZE option:

                   #marker    def_sz  init   MFS_inodes    floppy_inodes
                   #PicoBSD   4200    init   8192          32768
                   options MD_ROOT_SIZE=4200      # same as def_sz

             This informs the script of the size of the memory file system and
             provides a few other details on how to build the image.

             crunchgen(1) configuration (required).  It contains the list of
             directories containing program sources, the list of binaries to
             be built, and the list of libraries that these programs use.  See
             the crunchgen(1) manpage for the exact details on the syntax of
             this file.

             The following issues are particularly important when dealing with
             picobsd configurations:

             +o   We can pass build options to those makefiles which understand
                 that, in order to reduce the size of the programs.  This is
                 achieved with a line of the form

                       buildopts -DNO_PAM -DRELEASE_CRUNCH ...

             +o   When providing the list of directories where source files
                 are, it is convenient to list the following entry first:

                       srcdirs /usr/src/release/picobsd/tinyware

                 so that picobsd-specific versions of the programs will be
                 found there.

             +o   The string ``@__CWD__@'' is replaced with the full pathname
                 of the directory where the picobsd configuration resides
                 (i.e., the one where we find PICOBSD, crunch.conf, and so
                 on).  This can be useful to refer source code that resides
                 within a configuration, e.g.

                       srcdirs @__CWD__@/src

     config  Shell variables, sourced by the picobsd script (optional).  The
             most important variables here are:

             MY_DEVS  (Not used in FreeBSD 5.0 where we have devfs(5)).
                      Should be set to the list of devices to be created in
                      the /dev directory of the image (it is really the argu-
                      ment passed to MAKEDEV(8), so refer to that manpage for
                      the names).

             fd_size  Size (in kilobytes) of the picobsd image.  By default,
                      fd_size is set to 1440 which produces an image suitable
                      for a standard floppy.

                      If you plan to store the image on a CDROM (e.g. using
                      the ``El Torito'' floppy emulation), you can set fd_size
                      equal to 2880.  If you are planning to dump the image
                      onto a hard disk (either in a partition or on the whole
                      disk), you are not restricted to one of the standard
                      floppy sizes.  Using a large image size per se does not
                      waste RAM at runtime, because only the files that are
                      actually loaded from the image contribute to the memory

                      Contains a list of files to be imported in the floppy
                      tree.  Absolute names refer to the standard file system,
                      relative names refer to the root of the source tree
                      being used (i.e. SRC_PATH/..).  You can normally use
                      this option if you want to import files such as shared
                      libraries, or databases, without having to replicate
                      them first in your configuration under the floppy.tree/

             List of files from the standard floppy tree which we do not want
             to be copied (optional).

             Local additions to the standard floppy tree (optional).  The con-
             tent of this subtree will be copied as-is into the floppy image.

             Same as above, but site-specific (optional).

     More information on the build process can be found in the comments in the
     picobsd script.

     The build script can be instructed to use an alternate source tree using
     the --src SRC_PATH option.  The tree that you specify must contain full
     sources for the kernel and for all programs that you want to include in
     your image.  As an example, to cross-build the bridge floppy using
     RELENG_4 sources, you can do the following:

           cd <some_empty_directory>
           mkdir FOO
           (cd FOO; cvs -d<my_repository> co -rRELENG_4 src)
           picobsd --src FOO/src --init    # this is needed only once
           picobsd --src FOO/src -n -v bridge

     If the build is successful, the directory build_dir-bridge/ will contain
     a kernel that can be downloaded with etherboot, a floppy image called
     picobsd.bin, plus the products of the compilation in other directories.
     If you want to modify the source tree in FOO/src, a new image can be pro-
     duced by simply running

           picobsd --src FOO/src -n -v bridge

     whereas if the change affects include files or libraries you first need
     to update them, e.g. by re-running

           picobsd --src FOO/src --init  # this is needed only once

     as you would normally do for any change of this kind.

   Floppy Install
     Historically, picobsd is run from a floppy disk, where it can be
     installed with a simple

           dd if=picobsd.bin of=/dev/rfd0

     and the floppy is ready to boot.

   Hard Disk Install
     The same process can be used to store the image on a hard disk (entire
     volume or one of the slices):

           dd if=picobsd.bin of=/dev/ad2
           dd if=picobsd.bin of=/dev/ad2s3
           dd if=picobsd.bin of=/dev/ad2 oseek=NN

     The first form will install the image on the entire disk, and it should
     work in the same way as for a floppy.

     The second form will install the image on slice number 3 (which should be
     large enough to store the contents of the image).  However, the process
     will only have success if the partition does not contain a valid diskla-
     bel, otherwise the kernel will likely prevent overwriting the label.  In
     this case you can use the third form, replacing NN with the actual start
     of the partition (which you can determine using fdisk(8)).  Note that
     after saving the image to the slice, it will not yet be recognised.  You
     have to use the disklabel(8) command to properly initialize the label (do
     not ask why!).  One way to do this is

           disklabel -w ad0s2 auto
           disklabel -e ad0s2

     and from the editor enter a line corresponding to the actual partition,
     e.g. if the image has 2.88MB (5760 sectors) you need to enter the follow-
     ing line for the partition:

           a: 5760   0    4.2BSD   512   4096

     At this point the partition is bootable.  Note that the image size can be
     smaller than the slice size (indicated as partition ``c:'').

   CDROM Install
     Another option is to put the image on a CDROM.  Assuming your image for
     disk type foo is in the directory build_dir-foo then you can produce a
     bootable ``El Torito'' image (and burn it) with the following command:

           mkisofs -b picobsd.bin -c boot.catalog -d -N -D -R -T \
               -o cd.img build_dir-foo
           burncd -f /dev/acd0c -s 4 data cd.img fixate

     Note that the image size is restricted to 1.44MB or 2.88MB, other sizes
     most likely will not work.

   Booting From The Network
     Yet another way to use picobsd is to boot the image off the network.  For
     this purpose you should use the uncompressed kernel which is available as
     a byproduct of the compilation.  Refer to the documentation for network
     booting for more details, the picobsd kernel is bootable as a standard
     FreeBSD kernel.

     To boot picobsd, insert the floppy and reset the machine.  The boot pro-
     cedure is similar to the standard FreeBSD boot.  Booting from a floppy is
     normally rather slow (in the order of 1-2 minutes), things are much
     faster if you store your image on a hard disk, Compact Flash, or CDROM.

     You can also use etherboot to load the preloaded, uncompressed kernel
     image which is a byproduct of the picobsd build.  In this case the load
     time is a matter of a few seconds, even on a 10Mbit/s ethernet.

     After booting, picobsd loads the root file system from the memory file
     system, starts /sbin/init, and passes control to a first startup script,
     /etc/rc.  The latter populates the /etc and /root directories with the
     default files, then tries to identify the boot device (floppy, hard disk
     partition) and possibly override the contents of the root file system
     with files read from the boot device.  This allows you to store local
     configuration on the same media.  After this phase the boot device is no
     longer used, unless the user specifically does it.

     After this, control is transferred to a second script, /etc/rc1 (which
     can be overridden from the boot device).  This script tries to associate
     a hostname to the system by using the MAC address of the first ethernet
     interface as a key, and /etc/hosts as a lookup table.  Then control is
     passed to the main user configuration script, /etc/rc.conf, which is sup-
     posed to override the value of a number of configuration variables which
     have been pre-set in /etc/rc.conf.defaults.  You can use the hostname
     variable to create different configurations from the same file.  After
     taking control back, /etc/rc1 completes the initializations, and as part
     of this it configures network interfaces and optionally calls the fire-
     wall configuration script, /etc/rc.firewall, where the user can store his
     own firewall configuration.

     Note that by default picobsd runs entirely from main memory, and has no
     swap space, unless you explicitly request it.  The boot device is also
     not used anymore after /etc/rc1 takes control, again, unless you explic-
     itly request it.

     The operation of a picobsd system can be configured through a few files
     which are read at boot time, very much like a standard FreeBSD system.
     There are, however, some minor differences to reduce the number of files
     to store and/or customize, thus saving space.  Among the files to config-
     ure we have the following:

             Traditionally, this file contains the IP-to-hostname mappings.
             In addition to this, the picobsd version of this file also con-
             tains a mapping between Ethernet (MAC) addresses and hostnames,
             as follows:

                   #ethertable     start of the ethernet->hostname mapping
                   # mac_address           hostname
                   # 00:12:34:56:78:9a     pinco
                   # 12:34:56:*            pallino
                   # *                     this-matches-all

             where the line containing ``#ethertable'' marks the start of the

             If the MAC address is not found, the script will prompt you to
             enter a hostname and IP address for the system, and this informa-
             tion will be stored in the /etc/hosts file (in memory) so you can
             simply store them on disk later.

             Note that you can use wildcards in the address part, so a line
             like the last one in the example will match any MAC address and
             avoid the request.

             This file contains a number of variables which control the opera-
             tion of the system, such as interface configuration, router set-
             up, network service startup, etc.  For the exact list and meaning
             of these variables see /etc/rc.conf.defaults.

             It is worth mentioning that some of the variables let you over-
             write the contents of some files in /etc.  This option is avail-
             able at the moment for /etc/host.conf and /etc/resolv.conf, whose
             contents are generally very short and suitable for this type of
             updating.  In case you use these variables, remember to use new-
             lines as appropriate, e.g.

                   host_conf="# this goes into /etc/host.conf

             Although not mandatory, in this file you should only set the
             variables indicated in /etc/rc.conf.defaults, and avoid starting
             services which depend on having the network running.  This can be
             done at a later time: if you set firewall_enable="YES", the
             /etc/rc.firewall script will be run after configuring the network
             interfaces, so you can set up your firewall and safely start net-
             work services or enable things such as routing and bridging.

             This script can be used to configure the ipfw(4) firewall.  On
             entry, the fwcmd variable is set to the pathname of the firewall
             command, firewall_type contains the value set in /etc/rc.conf,
             and hostname contains the name assigned to the host.

     There is a small script called update which can be used to edit and/or
     save to disk a copy of the files you have modified after booting.  The
     script takes one or more absolute pathnames, runs the editor on the files
     passed as arguments, and then saves a compressed copy of the files on the
     disk (mounting and unmounting the latter around the operation).

     If invoked without arguments, update edits and saves rc.conf,
     rc.firewall, and master.passwd.

     If one of the arguments is /etc (the directory name alone), then the com-
     mand saves to disk (without editing) all the files in the directory for
     which a copy already exists on disk (e.g. as a result of a previous

     crunchgen(1), mdconfig(8), swapon(8)

     Andrzej Bialecki <>, with subsequent work on the scripts
     by Luigi Rizzo <> and others.  Man page and Makefiles
     created by Greg Lehey <>.

     Building picobsd is still a black art.  The biggest problem is determin-
     ing what will fit on the floppies, and the only practical method is trial
     and error.

FreeBSD 6.2                    January 31, 2006                    FreeBSD 6.2


Want to link to this manual page? Use this URL:

home | help