PICOBSD(8) BSD System Manager's Manual PICOBSD(8)NAMEpicobsd — building small FreeBSD disk images
SYNOPSISpicobsd [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),
Clean the product of previous builds.
Specify a file that contains additional config commands.
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
--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.
Specify a directory with the result of a previous buildworld.
This saves the need for an --init call before creating an image.
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
The following issues are particularly important when dealing with
· 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:
so that picobsd-specific versions of the programs will be
· 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‐
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
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
USING ALTERNATE SOURCE TREES
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 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.
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:”).
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
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.
CONFIGURING a PicoBSD system
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,
#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
SEE ALSOcrunchgen(1), mdconfig(8), swapon(8)AUTHORS
Andrzej Bialecki ⟨abial@FreeBSD.org⟩, with subsequent work on the scripts
by Luigi Rizzo ⟨firstname.lastname@example.org⟩ and others. Man page and Makefiles
created by Greg Lehey ⟨email@example.com⟩.
Documentation is still incomplete.
BSD June 25, 2009 BSD