fakechroot - Man Page

gives a fake chroot environment

Synopsis

fakechroot -h|--help

fakechroot -v|--version

fakechroot runs a command in an environment where is additional possibility to use chroot(8) command without root privileges. This is useful for allowing users to create own chrooted environment with possibility to install another packages without need for root privileges.

fakechroot replaces some C library functions (chroot(2), open(2), etc.) by ones that simulate the effect of being called with root privileges.

These wrapper functions are provided as a shared library libfakechroot.so which is loaded through the LD_PRELOAD mechanism of the dynamic loader. (See ld.so(8))

In fake chroot you can install i.e. Debian bootstrap with debootstrap(8) command. In such environment you can run i.e. apt-get(8) command to install another packages. You don't need a special privileges and you can run it on common user's account.

Options

-b|--bindir directory

Specify a directory which contains the replacement executables shipped with fakechroot. Those are script files with the extension .fakechroot. By default they are distributed over the bin and sbin directories in fakechroot's install prefix, but sometimes a fixed location at build time is not feasible.

-c|--config-dir directory

Specify a directory which contains additional configuration for fakechroot. The default directory are $HOME/.fakechroot and /etc/fakechroot.

-d ldso|--elfloader ldso

Specify an alternative dynamic linker. This dynamic linker will be invoked directly.

-e|--environment type

Load additional configuration with environment. This configuration file is a shell script which is executed before calling command. The script can set additional environment variables, like i.e.: FAKECHROOT_EXCLUDE_PATH, FAKECHROOT_CMD_SUBST or LD_LIBRARY_PATH.

The environment type is guessed based on command name with optional extension removed (e.g. running gettext.sh loads gettext environment file). If command argument is fakeroot(1) this argument is omitted and next argument is taken as environment type.

The configuration file name is type.env and is searched at $HOME/.fakechroot and /etc/fakechroot directories.

The default environment type is default and its configuration file name is default.env.

The special environment none means that no environment settings are loaded at all.

-l library|--lib library

Specify an alternative wrapper library. The default is libfakechroot.so

-h|--help

Display help.

-s|--use-system-libs

Use system libraries before chroot's libraries. This might be a workaround if system dynamic linker cannot load libc.so from fake chroot.

Try this setting if you noticed following errors:

 $ fakechroot /usr/sbin/chroot /tmp/sarge /bin/true
 /bin/true: relocation error: /srv/sarge/lib/tls/libc.so.6: symbol _dl
 _starting_up, version GLIBC_PRIVATE not defined in file ld-linux.so.2
  with link time reference

 $ fakechroot /usr/sbin/chroot /tmp/centos4 /bin/true
 Segmentation fault

-v|--version

Display version.

[--] command

Any command you want to be run as fakechroot. Use '--' if in the command you have other options that may confuse fakechroot's option parsing.

Examples

An example session with fakechroot:

  $ id
  uid=1000(dexter) gid=1000(dexter) groups=1000(dexter)

  $ fakechroot fakeroot debootstrap sid /tmp/sid
  I: Retrieving Release 
  I: Retrieving Release.gpg 
  I: Checking Release signature
  ...
  I: Base system installed successfully.

  $ fakechroot fakeroot chroot /tmp/sid apt-get install -q hello
  Reading package lists...
  Building dependency tree...
  Reading state information...
  The following NEW packages will be installed:
    hello
  0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
  Need to get 57.4 kB of archives.
  After this operation, 558 kB of additional disk space will be used.
  Get:1 http://ftp.us.debian.org/debian/ sid/main hello amd64 2.8-4 [57.4 kB]
  Fetched 57.4 kB in 0s (127 kB/s)
  Selecting previously unselected package hello.
  (Reading database ... 24594 files and directories currently installed.)
  Unpacking hello (from .../archives/hello_2.8-4_amd64.deb) ...
  Processing triggers for man-db ...
  Processing triggers for install-info ...
  Setting up hello (2.8-4) ...

  $ fakechroot chroot /tmp/sid hello
  Hello, world!

Fakeroot

fakeroot(1) is a complementary tool which emulates root environment. fakeroot and fakechroot might wrap the same C library functions, i.e. mknod(2) function. It is important to start fake environment in proper order. fakeroot should be started inside fakechroot:

  $ fakechroot fakeroot chroot /tmp/sid /bin/mknod /tmp/device c 1 2

Security Aspects

fakechroot is a regular, non-setuid program. It does not enhance a user's privileges.

fakechroot should not be used as a tool for enhancing system security i.e. by separating (sandboxing) applications. It is very easy to escape from a fake chroot environment.

fakechroot should not be run with real root privileges. It might decrease the security of the system because the fakechroot provides own version of core functions with behavior depended on some environment variables.

Files

libfakechroot.so: The shared library containing the wrapper functions.

Environment

FAKECHROOT

The value is true for fake chroot environment.

FAKECHROOT_AF_UNIX_PATH

The root directory for unix sockets. The default value is the same as FAKECHROOT_BASE and it can be set separately if the FAKECHROOT_BASE is too long and the unix socket path could exceed the limit of 108 bytes.

FAKECHROOT_BASE

The root directory of fake chroot environment.

FAKECHROOT_CMD_SUBST

A list of command substitutions. If a program tries to execute one of the commands given (path relative to the chroot, trailing dot is removed) then the substitute command runs instead (path to substitute command is not chrooted).

The substituted command inherits FAKECHROOT_* variables but the original FAKECHROOT_BASE variable which is saved as FAKECHROOT_BASE_ORIG. It means that substituted command runs outside fakechroot environment. Also original command name is saved as FAKECHROOT_CMD_ORIG.

For example:

  export FAKECHROOT_CMD_SUBST=/usr/bin/mkfifo=/bin/true

will substitute /bin/true for /usr/bin/mkfifo and will make possible to install sysvinit binary package.

Give as many substitute commands as you want, separated by : (colon) characters.

It is suggested to substitute at least:

/bin/mount=/bin/true
/sbin/insserv=/bin/true
/sbin/ldconfig=/bin/true
/usr/bin/env=/usr/bin/env.fakechroot
/usr/bin/ischroot=/bin/true
/usr/bin/ldd=/usr/bin/ldd.fakechroot
/usr/bin/mkfifo=/bin/true

to make debootstrap(8) working correctly.

To prevent some looping, the command substitution is done only if FAKECHROOT_CMD_ORIG variable is not set currently.

FAKECHROOT_DEBUG

The fakechroot library will dump some debugging info if this variable is set.

FAKECHROOT_DETECT

If this variable is set then fakechroot version string is printed to standard output and the current process is terminated with status taken from this variable. It can be a method to check if fakechroot is preloaded correctly.

  $ case "`FAKECHROOT_DETECT=1 /bin/echo`" in fakechroot*) echo LOADED;; esac

FAKECHROOT_ELFLOADER

A path to another dynamic linker (i.e. /lib/ld-linux.so.2 for i386 architecture, /lib64/ld-linux-x86-64.so.2 for x86_64 architecture).

This dynamic linker will be invoked directly. The dynamic linker don't allow to change argv[0] besides the file name of the executable file, so some application won't work correctly, i.e. busybox(1).

FAKECHROOT_EXCLUDE_PATH

The list of directories which are excluded from being chrooted. The elements of list are separated with colon.

The /dev, /proc and /sys directories are excluded by default if this environment variable is not set.

This list has to contain at most 100 elements.

FAKECHROOT_EXTRA_LIBRARY_PATH

The list of extra directories in fake chroot environment that are added to LD_LIBRARY_PATH variable. The directories might be used by some important commands which use libraries placed in the runpath. In that case dynamic linker can't find correct directory in a fake chroot environment without the extra library path.

The default value is /lib/systemd:/usr/lib/man-db for systemctl(1) and man(1) commands.

FAKECHROOT_VERSION

The version number of the current fakechroot library.

LD_LIBRARY_PATH, LD_PRELOAD

Fakechroot is implemented by wrapping system calls. This is accomplished by setting LD_PRELOAD=libfakechroot.so. If this library can't be found by dynamic linker, the /etc/ld.so.conf file or LD_LIBRARY_PATH variable have to be modified.

Limitations

/lib/ld-linux.so.2 and /lib64/ld-linux-x86-64.so.2 are always loaded from real environment. This path is hardcoded by linker for all binaries. You can set the FAKECHROOT_ELFLOADER environment variable or use --elfloader option.
Every command executed within fakechroot needs to be linked to the same version of the dynamic linker from real environment. If the libraries in chroot are not compatible, try to use --use-system-libs option.
You can provide symlinks to the outside. The symlink have to be created before chroot is called. It can be useful for accessing the real /proc and /dev directory. You can also set the FAKECHROOT_EXCLUDE_PATH environment variable:
```
  $ export FAKECHROOT_EXCLUDE_PATH=/tmp:/proc:/dev:/sys:/var/run:/home
```
Statically linked binaries doesn't work, especially ldconfig(8), so you have to wrap this command with dummy version and set the proper FAKECHROOT_CMD_SUBST environment variable.
ldd(1) also doesn't work. You have to use alias ldd='LD_TRACE_LOADED_OBJECTS=1' or to use a wrapper instead. The wrapper is installed as ldd.fakechroot and can be used with FAKECHROOT_CMD_SUBST environment variable.
The full screen applications hangs up if /dev/tty file is not a real device. Link /dev/tty file or whole /dev directory to the real one or remove it from fake chroot environment with FAKECHROOT_EXCLUDE_PATH variable.
lckpwdf(3) and ulckpwdf(3) are ignored so passwd(1) command should work
Your real uid should exist in /etc/passwd. Create it with adduser --uid realuid realuser inside fake chroot environment.
debuild(1) cleans environment. Use --preserve-env option to prevent this behavior.
rpmbuild(8) uses own glob(3) implementation which breaks fakechroot so buildroot directory have to be the same inside and outside fakechroot.

fakechroot - Man Page

Synopsis

Description

Options

Examples

Fakeroot

Security Aspects

Files

Environment

Limitations

See Also

Bugs

Authors

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