NAME
schroot - securely enter a chroot environment
SYNOPSIS
schroot [-h|--help | -V|--version | -l|--list | -i|--info | --config
| --location | --automatic-session | -b|--begin-session |
--recover-session | -r|--run-session | -e|--end-session] [-f|--force]
[-n session-name|--session-name=session-name] [-d
directory|--directory=directory] [-u user|--user=user]
[-p|--preserve-environment] [-q|--quiet | -v|--verbose] [-c
chroot|--chroot=chroot | --all | --all-chroots | --all-sessions]
[COMMAND [ ARG1 [ ARG2 [ ARGn]]]]
DESCRIPTION
schroot allows the user to run a command or a login shell in a chroot
environment. If no command is specified, a login shell will be started
in the user’s current working directory inside the chroot.
The command is a program, plus as many optional arguments as required.
Each argument may be separately quoted.
The directory the command or login shell is run in depends upon the
context. See --directory option below for a complete description.
All chroot usage will be logged in the system logs. Under some
circumstances, the user may be required to authenticate themselves; see
the section “Authentication”, below.
If no chroot is specified, the chroot name or alias ‘default’ will be
used as a fallback. This is equivalent to “--chroot=default”.
OVERVIEW
There is often a need to run programs in a virtualised environment
rather than on the host system directly. Unlike other virtualisation
systems such as kvm or Xen, schroot does not virtualise the entire
system; it only virtualises the filesystem, and some parts of the
filesystem may still be shared with the host. It is therefore fast,
lightweight and flexible. However, it does not virtualise other
aspects of the system, such as shared memory, networking, devices etc.,
and so may be less secure than other systems, depending upon its
intended use. Some examples of existing uses for schroot include:
· Running an untrusted program in a sandbox, so that it can’t
interfere with files on the host system; this may also be used
to limit the damage a compromised service can inflict upon the
host
· Using a defined or clean environment, to guarantee the
reproducibility and integrity of a given task
· Using different versions of an operating system, or even
different operating systems altogether, e.g. different GNU/Linux
distributions
· Running 32-bit programs using a 32-bit chroot on a 64-bit host
system
· Automatic building of Debian packages using sbuild(1), which
builds each package in a pristine chroot snapshot when using LVM
snapshots or unions
· Supporting multiple system images in a cluster setup, where
modifying the base image is time-consuming and/or supporting all
the required configurations needed by users is difficult:
different chroots can support all the different configurations
required, and cluster users may be given access to the chroots
they need (which can include root access for trusted users to
maintain their own images)
A chroot may be used directly as root by running chroot(8), but normal
users are not able to use this command. schroot allows access to
chroots for normal users using the same mechanism, but with permissions
checking and allowing additional automated setup of the chroot
environment, such as mounting additional filesystems and other
configuration tasks. This automated setup is done through the action
of setup scripts which may be customised and extended to perform any
actions required.
OPTIONS
schroot accepts the following options:
Actions
-h, --help
Show help summary.
-V, --version
Print version information.
-l, --list
List all available chroots.
-i, --info
Print detailed information about the specified chroots.
--config
Print configuration of the specified chroots. This is useful
for testing that the configuration in use is the same as the
configuration file. Any comments in the original file will be
missing.
--location
Print location (path) of the specified chroots. Note that
chroot types which can only be used within a session will not
have a location until they are active.
General options
-q, --quiet
Print only essential messages.
-v, --verbose
Print all messages.
Chroot selection
-c, --chroot=chroot
Specify a chroot or active session to use. This option may be
used multiple times to specify more than one chroot, in which
case its effect is similar to --all.
-a, --all
Select all chroots and active sessions. When a command has been
specified, the command will be run in all chroots and active
sessions. If --info has been used, display information about
all chroots. This option does not make sense to use with a
login shell (when no command has been specified). This option
is equivalent to “--all-chroots --all-sessions”.
--all-chroots
Select all chroots. Identical to --all, except that active
sessions are not considered.
--all-sessions
Select all active sessions. Identical to --all, except that
chroots are not considered.
Chroot environment
-d, --directory=directory
Change to directory inside the chroot before running the command
or login shell. If directory is not available, schroot will
exit with an error status.
The default behaviour is as follows (all directory paths are
inside the chroot). A login shell is run in the current working
directory. If this is not available, it will try $HOME (when
--preserve-environment is used), then the user’s home directory,
and / inside the chroot in turn. A command is always run in the
current working directory inside the chroot. If none of the
directories are available, schroot will exit with an error
status.
-u, --user=user
Run as a different user. The default is to run as the current
user. If required, the user may be required to authenticate
themselves with a password. For further information, see the
section “Authentication”, below.
-p, --preserve-environment
Preserve the user’s environment inside the chroot environment.
The default is to use a clean environment; this option copies
the entire user environment and sets it in the session. The
environment variables allowed are subject to certain
restrictions; see the section “Environment”, below.
Session actions
--automatic-session
Begin, run and end a session automatically. This is the default
action, so does not require specifying in normal operation.
-b, --begin-session
Begin a session. A unique session identifier (session ID) is
returned on standard output. The session ID is required to use
the other session options. Note that the session identifier may
be specified with the --session-name option.
--recover-session
Recover an existing session. If an existing session has become
unavailable, for example becoming unmounted due to a reboot,
this option will make the session available for use again, for
example by remounting it. The session ID is specified with the
--chroot option.
-r, --run-session
Run an existing session. The session ID is specified with the
--chroot option.
-e, --end-session
End an existing session. The session ID is specified with the
--chroot option.
Session options
-n, --session-name=session-name
Name a session. The specified session-name replaces the default
session name containing an automatically-generated session ID.
Note that it is not currently possible to create a session with
the same name as a chroot or alias.
-f, --force
Force a session operation, even if it would otherwise fail.
This may be used to forcibly end a session, even if it has
active users. This does not guarantee that the session will be
ended cleanly; filesystems may not be unmounted, for example.
AUTHENTICATION
If the user is not an allowed user, or a member of the allowed groups
(or if changing to root, the allowed root users or allowed root groups)
for the specified chroot(s), permission will be immediately denied. If
switching users, and the user running the command has access, the user
will be required to authenticate themselves using the credentials of
the user being switched to.
On systems supporting Pluggable Authentication Modules (PAM), schroot
will use PAM for authentication and authorisation of users. If and
when required, schroot will prompt for a password. If PAM is not
available, all authentication will automatically fail (user switching
is not supported without PAM).
Note that when PAM is in use, the root user is not granted any special
privileges by default in the program. However, the default PAM
configuration permits root to log in without a password
(pam_rootok.so), but this may be disabled to prevent root from
accessing any chroots except if specifically permitted. In such a
situation, root must be added to the allowed users or groups as for any
other user or group. If PAM is not available, the root user will be
permitted to access all chroots, even when not explicitly granted
access.
EXAMPLES
List available chroots
% schroot -l␍
default
etch
sid
testing
unstable
Get information about a chroot
% schroot -i -c sid␍
——— Chroot ———
Name sid
Description Debian sid (unstable)
Type plain
Priority 3
Users rleigh
Groups sbuild
Root Users
Root Groups sbuild
Aliases unstable unstable-sbuild unstable-p
owerpc-sbuild
Environment Filter ^(BASH_ENV|CDPATH|ENV|HOSTALIASES|I
FS|KRB5_CONFIG|KRBCONFDIR|KRBTKFILE|KRB_CONF|LD_.*|LOCALDOMA
IN|NLSPATH|PATH_LOCALE|RES_OPTIONS|TERMINFO|TERMINFO_DIRS|TE
RMPATH)$
Run Setup Scripts true
Script Configuration script-defaults
Session Managed true
Personality linux32
Location /srv/chroot/sid
Use --all or -c multiple times to use all or multiple chroots,
respectively.
Running commands in a chroot
% schroot -c sid /bin/ls␍
[sid chroot] Running command: “/bin/ls”
CVS sbuild-chroot.c sbuild-session.h schroot.conf.5
Makefile sbuild-chroot.h schroot.1 schroot.conf.5.in
Makefile.am sbuild-config.c schroot.1.in
Makefile.in sbuild-config.h schroot.c
pam sbuild-session.c schroot.conf
% schroot -c sid -- ls -1 | head -n 5␍
[sid chroot] Running command: “ls -1”
ABOUT-NLS
AUTHORS
COPYING
ChangeLog
INSTALL
Use -- to allow options beginning with ‘-’ or ‘--’ in the command to
run in the chroot. This prevents them being interpreted as options for
schroot itself. Note that the top line was echoed to standard error,
and the remaining lines to standard output. This is intentional, so
that program output from commands run in the chroot may be piped and
redirected as required; the data will be the same as if the command was
run directly on the host system.
Switching users
% schroot -c sid -u root␍
Password:
[sid chroot] (rleigh→root) Running login shell: “/bin/bash”
#
If the user ‘rleigh’ was in root-users in /etc/schroot/schroot.conf, or
one of the groups he belonged to was in root-groups, they would be
granted root access without authentication, but the PAM authorisation
step is still applied.
Sessions
A chroot may be needed to run more than one command. In particular,
where the chroot is created on the fly from an LVM LV or a file on
disc, there is a need to make the chroot persistent while a given task
(or set of tasks) is performed. Sessions exist for this purpose. For
simple chroot types such as ‘plain’ and ‘directory’, sessions may be
created but are not strictly necessary.
Let’s start by looking at a session-capable chroot:
% schroot -i -c sid-snap␍
——— Chroot ———
Name sid-snap
Description Debian sid snapshot
Type lvm-snapshot
Priority 3
Users maks rleigh
Groups sbuild
Root Users
Root Groups root sbuild
Aliases
Environment Filter ^(BASH_ENV|CDPATH|ENV|HOSTALIASES|I
FS|KRB5_CONFIG|KRBCONFDIR|KRBTKFILE|KRB_CONF|LD_.*|LOCALDOMA
IN|NLSPATH|PATH_LOCALE|RES_OPTIONS|TERMINFO|TERMINFO_DIRS|TE
RMPATH)$
Run Setup Scripts true
Script Configuration script-defaults
Session Managed true
Personality linux
Device /dev/hda_vg/sid_chroot
Mount Options -o atime,async,user_xattr
Source Users
Source Groups root rleigh
Source Root Users
Source Root Groups root rleigh
LVM Snapshot Options --size 2G -c 128
Note that the Session Managed option is set to ‘true’. This is a
requirement in order to use session management, and is supported by
most chroot types. Next, we will create a new session:
% schroot -b -c sid-snap␍
sid-snap-46195b04-0893-49bf-beb8-0d4ccc899f0f
The session ID of the newly-created session is returned on standard
output. It is common to store it like this:
% SESSION=$(schroot -b -c sid-snap)␍
% echo $SESSION␍
sid-snap-46195b04-0893-49bf-beb8-0d4ccc899f0f
The session may be used just like any normal chroot. This is what the
session looks like:
% schroot -i -c sid-snap-46195b04-0893-49bf-beb8-0d4ccc899f0f␍
——— Session ———
Name sid-snap-46195b04-0893-49bf-beb8-0d
4ccc899f0f
Description Debian sid snapshot
Type lvm-snapshot
Priority 3
Users maks rleigh
Groups sbuild
Root Users
Root Groups root sbuild
Aliases
Environment Filter ^(BASH_ENV|CDPATH|ENV|HOSTALIASES|I
FS|KRB5_CONFIG|KRBCONFDIR|KRBTKFILE|KRB_CONF|LD_.*|LOCALDOMA
IN|NLSPATH|PATH_LOCALE|RES_OPTIONS|TERMINFO|TERMINFO_DIRS|TE
RMPATH)$
Run Setup Scripts true
Script Configuration script-defaults
Session Managed true
Personality linux
Mount Location /var/lib/schroot/mount/sid-snap-461
95b04-0893-49bf-beb8-0d4ccc899f0f
Path /var/lib/schroot/mount/sid-snap-461
95b04-0893-49bf-beb8-0d4ccc899f0f
Mount Device /dev/hda_vg/sid-snap-46195b04-0893-
49bf-beb8-0d4ccc899f0f
Device /dev/hda_vg/sid_chroot
Mount Options -o atime,async,user_xattr
Source Users
Source Groups root rleigh
Source Root Users
Source Root Groups root rleigh
LVM Snapshot Device /dev/hda_vg/sid-snap-46195b04-0893-
49bf-beb8-0d4ccc899f0f
LVM Snapshot Options --size 2G -c 128
Now the session has been created, commands may be run in it:
% schroot -r -c sid-snap-46195b04-0893-49bf-beb8-0d4ccc899f0f -- uname
-sr␍
I: [sid-snap-46195b04-0893-49bf-beb8-0d4ccc899f0f chroot] Running
command: “uname -sr”
Linux 2.6.18-3-powerpc
% schroot -r -c $SESSION -- uname -sr␍
I: [sid-snap-fe170af9-d9be-4800-b1bd-de275858b938 chroot] Running
command: “uname -sr”
Linux 2.6.18-3-powerpc
When all the commands to run in the session have been performed, the
session may be ended:
% schroot -e -c sid-snap-46195b04-0893-49bf-beb8-0d4ccc899f0f␍
% schroot -e -c $SESSION␍
Finally, the session names can be long and unwieldy. A name may be
specified instead of using the automatically generated session ID:
% schroot -b -c sid-snap -n my-session-name␍
my-session-name
TROUBLESHOOTING
If something is not working, and it’s not clear from the error messages
what is wrong, try using the --debug=level option to turn on debugging
messages. This gives a great deal more information. Valid debug
levels are ‘none’, and ‘notice’, ‘info’, ‘warning’ and ‘critical’ in
order of increasing severity. The lower the severity level, the more
output.
If you are still having trouble, the developers may be contacted on the
mailing list:
Debian buildd-tools Developers
<buildd-tools-devel@lists.alioth.debian.org>
BUGS
On the mips and mipsel architectures, Linux kernels up to and including
at least version 2.6.17 have broken personality(2) support, which
results in a failure to set the personality. This will be seen as an
“Operation not permitted” (EPERM) error. To work around this problem,
set personality to ‘undefined’, or upgrade to a more recent kernel.
ENVIRONMENT
By default, the environment is not preserved, and the following
environment variables are defined: HOME, LOGNAME, PATH, SHELL, TERM
(preserved if already defined), and USER. The environment variables
SCHROOT_COMMAND, SCHROOT_USER, SCHROOT_GROUP, SCHROOT_UID and
SCHROOT_GID are set inside the chroot specifying the command being run,
the user name, group name, user ID and group ID, respectively.
The following, potentially dangerous, environment variables are removed
for safety by default: BASH_ENV, CDPATH, ENV, HOSTALIASES, IFS,
KRB5_CONFIG, KRBCONFDIR, KRBTKFILE, KRB_CONF, LD_.*, LOCALDOMAIN,
NLSPATH, PATH_LOCALE, RES_OPTIONS, TERMINFO, TERMINFO_DIRS, and
TERMPATH. If desired, the environment-filter configuration key will
allow the exclusion list to the modified; see schroot.conf(5) for
further details.
FILES
Configuration files
/etc/schroot/schroot.conf
The system-wide chroot definition file. This file must be owned
by the root user, and not be writable by other.
/etc/schroot/chroot.d
Additional chroot definitions may be placed in files under this
directory. They are treated in exactly that same manner as
/etc/schroot/schroot.conf. Each file may contain one or more
chroot definitions. Note that the files in this directory
follow the same naming rules as run-parts(8) when run with the
--lsbsysinit option.
/etc/schroot/setup.d
The system-wide chroot setup script directories. See schroot-
setup(5).
/etc/pam.d/schroot
PAM configuration.
System directories
/usr/lib/schroot
Directory containing helper programs used by setup scripts.
Session directories
Each directory contains a directory or file with the name of each
session. Not all chroot types make use of all the following
directories.
/var/lib/schroot/session
Directory containing the session configuration for each active
session.
/var/lib/schroot/mount
Directory used to mount the filesystems used by each active
session.
/var/lib/schroot/union/underlay
Directory used for filesystem union source (underlay).
/var/lib/schroot/union/overlay
Directory used for filesystem union writeable overlay.
/var/lib/schroot/unpack
Directory used for unpacking file chroots.
AUTHORS
Roger Leigh.
COPYRIGHT
Copyright © 2005-2010 Roger Leigh <rleigh@debian.org>
schroot is free software: you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the
Free Software Foundation, either version 3 of the License, or (at your
option) any later version.
SEE ALSO
dchroot(1), sbuild(1), chroot(2), run-parts(8), schroot-setup(5),
schroot-faq(7), schroot.conf(5).