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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 without 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 number 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 system with files from the boot media (if present), and executes a specialized 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
script for more details).
The most important options for common operations are
|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
When used together with the
|Generate an ISO image, picobsd.iso, in addition to the disk image picobsd.bin.|
|Also build kernel modules. These are not stored on the picobsd image but are left available in the build directory.|
|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
Use the source tree at
instead the one at
This can be useful for cross-building
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
program) that are necessary for the cross-build (see the
|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.|
The supported build script is
which can be run from anywhere, and relies on the
port to build a filesystem without requiring
or root privileges to mount a filesystem.
When run in interactive mode (the default without the
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 following files and directories. For more information on how to construct these files, look at one of the standard picobsd configurations as a reference.
The kernel configuration file (required).
This is a mostly standard
kernel configuration file, possibly stripped down by removing
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.
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.
manpage for the exact details on the syntax of this file.
The following issues are particularly important when dealing with picobsd configurations:
|Shell variables, sourced by the picobsd script (optional). The most important variables here are:|
|(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 argument passed to MAKEDEV(8), so refer to that manpage for the names).|
Size (in kilobytes) of the
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 usage.
|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/ directory.|
|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 content 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.
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 produced 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.
dd if=picobsd.bin of=/dev/rfd0
and the floppy is ready to boot.
dd if=picobsd.bin of=/dev/ada2 dd if=picobsd.bin of=/dev/ada2s3 dd if=picobsd.bin of=/dev/ada2 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 disklabel, 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 gpart(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 ada0s2 auto disklabel -e ada0s2
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 following 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:").
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 supposed 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 firewall 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 explicitly request it.
Traditionally, this file contains the IP-to-hostname mappings.
In addition to this, the
version of this file also contains
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 table.
If the MAC address is not found, the script will prompt you to enter a hostname and IP address for the system, and this information 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
operation of the system, such as interface configuration,
router setup, network service startup, etc.
For the exact list and meaning of these variables see
It is worth mentioning that some of the variables let you overwrite the contents of some files in /etc. This option is available 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 newlines as appropriate, e.g.
host_conf="# this goes into /etc/host.conf hosts bind"
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 network 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 command 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 update).
|PICOBSD (8)||October 20, 2019|
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|“|| A UNIX saleslady, Lenore,
Enjoys work, but she likes the beach more.
She found a good way
To combine work and play:
She sells C shells by the seashore.