Ubuntu Manpage: rootstrap — Construct a root filesystem image in a file

name
synopsis
description
options
configuration
modules
files
see also
author

NAME

       rootstrap — Construct a root filesystem image in a file

SYNOPSIS

       rootstrap [options ...]  imagefile

DESCRIPTION

       rootstrap is a tool for creating root filesystem images.  It was written primarily for use with User-Mode
       Linux, but may be useful for other purposes as well.

       Because it uses User-Mode Linux to bootstrap itself, rootstrap can be used without root privileges on the
       host  system,  which  are normally required for this task in order to use chroot(2) and mount and unmount
       filesystems.  Rootstrap boots UML and uses tools from the host filesystem  to  perform  installation  and
       configuration tasks.

       Note:  since  roostrap needs to access the host filesystem it requires an UML kernel with hostfs built-in
       (CONFIG_HOSTFS=y), not as module.

OPTIONS

       -s, --image-size
                 Sets the initial size of  the  image,  in  megabytes,  overriding  the  initialsize  option  in
                 rootstrap.conf.

       -o logfile
                 Log the output of the creation process to logfile instead of to standard output.

       -u args, --umlargs args
                 Pass additional arguments to user-mode linux when booting to create the image.  Supplements the
                 'umlargs' option in rootstrap.conf.

Configuration

       The  configuration  file  consists  of  sections,  starting  with  a  "[section]"  header,  followed   by
       "option=value"  entries.   Long  values  are  allowed  to  span  multiple lines if each continued line is
       indented with whitespace.

       With the exception of the special section "global", sections are  named  after  a  corresponding  module.
       When that module is executed, it receives the option/value pairs in its section as environment variables.
       All modules inherit global options as environment variables, except where overridden  by  options  within
       their  specific  section.   At least PATH must be set here, in addition to the required parameters listed
       below under [global].

       Documentation for configurable modules distributed with rootstrap is included  below.   This  section  is
       currently incomplete.

       Note  that  the  modules  shipped  with rootstrap have a recommended ordering due to their cooperation in
       building the guest OS image, equivalent modules can be used for each of these tasks:

          1. setup guest network (if you need it for OS installation)

          2. create the target filesystem

          3. mount target filesystem in the guest Linux

          4. OS installation

          5. UML basic setup (device nodes, kernel modules, etc.)

          6. Tweaking (here you should have a fully working OS to play with)

          7. unmount the target filesystem

Modules

[global]
Contains configuration options that affect the operation of rootstrap itself, or several modules.

fstype The type of filesystem to create (ext2, ext3, reiserfs, etc.) This parameter is mandatory.

umlargs Additional arguments to user-mode linux passed when booting to create the image.

PATH The PATH environment variable to pass to modules. This parameter is mandatory.

initialsize
The initial size of the filesystem image (in megabytes). This must be large enough to contain
a complete installed system as produced by the selected modules. It will be created sparsely,
so additional space is not entirely allocated until it is used. This parameter is mandatory.

freespace The amount of free space to leave on the filesystem (in megabytes). The filesystem will be
resized, if possible, to adjust the amount of free space to approximately this amount. This
parameter is optional.

modules The list of modules to invoke, in order. Each module will be searched for in several
directories, listed in the FILES section below, and passed environment variables based on the
options set in the corresponding section of the configuration file. This parameter is
mandatory.

debug When set to "true" make rootstrap spawn a shell when a module script fails allowing further
debugging actions. The shell will have the same environment as the failing script so you could
simply invoke sh -x /path/to/module to see what's going wrong. Moreover the shell
exit value will be used to tell rootstrap to: re-evaluate the script when 1 (with exit 1) or
continue with the next one when 2 (with exit 2); all other values will make rootstrap fail with
an error. This parameter is optional.

[network]
Network configuration. This module configures virtual networking with the user-mode Linux system used to
build the filesystem image. Its presence is not strictly necessary, if the selected modules do not
require network access (for example, with a local package mirror) and the network module is not included
in the global "modules" list.

interface The name of the network interface to configure inside the UML virtual machine, typically
"eth0". This parameter is mandatory.

host The IP address of the host side of the interface (the host on which rootstrap is run), as
visible to the UML virtual machine. This parameter is mandatory.

uml The IP address of the UML side of the interface (where the system is being built). The value
"dhcp" is also accepted and makes rootstrap try to configure the network interface through
DHCP. This parameter is mandatory.

transport The type of virtual networking interface to be used. Typically "tuntap". Other available
transports are "slirp" and "daemon". This parameter is mandatory.

netmask The netmask for the network interface (applies to both sides). This parameter is mandatory.

hostname The host name to use for the created UML virtual machine. This parameter is optional.

gateway A default gateway to be used by the user-mode Linux system. This parameter is optional.

domain The local domain name.

nameserver
The DNS server to use for domain name resolution inside the UML virtual machine. This parameter
is optional, but useful if you do not have a name server running on the host.

[mkfs]
This module creates the filesystem for the rootfs. It uses the fstype global parameter.

[mount]
This module mounts the root filesystem where the OS image is going to be created. It uses the fstype
global parameter.

[umount]
This module unmounts the root filesystem and /proc.

[uml]
Creates /etc/fstab and necessary device nodes, it eventually copies kernel modules into the target image.
It uses the fstype global parameter.

kernel_modules
How to deal with kernel modules for the guest Linux. It can be one of none, hostfs or copy,
defaults to the latter which copies the available modules in the target filesystem.

kernel_modules_dir
The directory where kernel_modules are located on the host filesystem. It defaults
to /usr/lib/uml/modules

install_modules
This parameter is superseeded by kernel_modules and kernel_modules_dir. It accepts a boolean
value (true/false).

[debian]
Debian installation. This module installs a basic Debian system using debootstrap.

dist The distribution to install (e.g., potato, woody, etc.)

mirror A URL for a Debian archive containing the base packages. This must be a URL understood by
debootstrap, which as of this writing includes URLs understood by wget, and file: URLs.

exclude A list of packages which should be excluded (never installed at all). It is quite possible to
produce a broken system, or fail to build a system at all, if this option is used improperly.
It is useful for excluding packages, such as pcmcia-cs, which are typically not necessary for
UML and other applications.

Corresponds to debootstrap's --exclude option

purge A list of packages which should be removed after installation is complete. Use this for
packages which are required during installation, but may be removed afterward. The same
warning applies as with the exclude option.

Packages are removed with dpkg --purge.

include A list of packages which should be included in the initial set of packages to install.

Corresponds to debootstrap's --include option.

install Extra packages to install via apt after initial debootstrap install.

See also the sources option.

debconf_preseed_file
A file containing a debconf DB to be used as default. Read debconf documentation and see your
/var/cache/debconf/config.dat for examples and caveats.

sources Sources for target's sources.list. If no value is given the default is the main section of
mirror. NOTE: you can provide multiple lines if each new line is indented with blank spaces.

preferences
Preferences for target's apt preferences file (see apt_preferences(5)). NOTE: you can provide
multiple lines if each new line is indented with blank spaces. Multiple apt_preferences stanzas
are allowed provided that they are separated by a line containing only a dot (".") and
obviously indented as specified above.

apt_conf Apt configuration for target's apt.conf file (see apt.conf(5)). The file will be created before
installing additional packages so it will be already effective then. NOTE: you can provide
multiple lines if each new line is indented with blank spaces.

apt_force_yes
Will make rootstrap use the --force-yes switch to install debian packages. This useful when you
need to install packages from sources that don't use a Release.gpg file and thus failing apt
key authentication. It can be either true or false and defaults to the former.

Be careful anyway as using this option "will cause apt to continue without prompting if it is
doing something potentially harmful" (from apt-get(8)). This option is provided standalone
instead of forcing its inclusion in apt_conf to avoid causing destructive actions later when
using apt-get from the UML instance.

Custom modules
Custom modules can very easily be used by rootstrap, in addition to (or in place of) the supplied
modules. See FILES below for locations that are searched for modules.

When a module is invoked, the filesystem being created is mounted on $TARGET, /dev, /etc and /tmp are
tmpfs filesystems internal to the UML system, while the root filesystem is a hostfs mount of the system
where rootstrap is running, to have access to the above shadowed directories the full host filesystem is
available on $HOST. /lib/modules is tmpfs too and bind mounted to the host's /usr/lib/uml/modules to let
scripts load necessary kernel modules. This means that most software on the host system should be
available and work as expected. The working directory where rootstrap is run is available as $WORKDIR.
The environment is generated from the configuration file as described above.

Be careful about modules ordering (see CONFIGURATION above), you'll mostly want to plug into the tweaking
step to perform custom configurations.

To debug modules enable the debug global parameter, see its description fro more hints.

FILES

       /etc/rootstrap/rootstrap.conf
                 System-wide default configuration

       rootstrap.conf
                 Local overrides

       /usr/lib/rootstrap/modules
                 Modules distributed with rootstrap

       /etc/rootstrap/modules
                 System-wide overrides and additional modules

       modules   Local overrides and additional modules

AUTHOR

       Rootstrap was written by Matt Zimmerman mdz@debian.org

                                                                                                    rootstrap(1)