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PICOBSD(8)		  BSD 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).	 The most important options for common operations are
     -src, -init, -n -and -v.

	     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),

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

     --cfg file
	     Specify a file that contains additional config commands.

     --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).  Note that this
	     option is overridden by the content of the config files (config
	     in the image tree, or the one specified with --cfg)

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

     --iso   Generate an ISO image, picobsd.iso, in addition to the disk image

	     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.

	     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).

	     Omit /boot/loader, just rely on boot2 to load the kernel.	This
	     saves some space but may have problems with kernels > 4MB.

     --objdir directory
	     Specify a directory with the result of a previous buildworld.
	     This saves the need for an --init call before creating an image.

     --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).  The source files are unmod‐
	     ified by the picobsd script.  However the source tree is not com‐
	     pletely read-only, because config(8) expects the kernel configu‐
	     ration 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).

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

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

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

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

     ·	 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:

	     ·	 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 ...

	     ·	 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.

	     ·	 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 con‐
		 figuration, 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
     picobsd can produce an ISO image named picobsd.iso, which does not use
     “El Torito” emulation, so it has no size restrictions.  Installing means
     just burning a media with the file.

   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 ⟨abial@FreeBSD.org⟩, with subsequent work on the scripts
     by Luigi Rizzo ⟨luigi@iet.unipi.it⟩ and others.  Man page and Makefiles
     created by Greg Lehey ⟨grog@lemis.com⟩.

     Documentation is still incomplete.

BSD				 June 25, 2009				   BSD

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