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CHOS is a kernel module and utilities that facilitates supporting multiple Linux releases simultaneously on a single system. With chos, you can have a base OS (the true root tree for the system) and allows users to select from other accessible OS trees. The trees must be compatible with the booted kernel. For example, you could install Fedora Core 2 as your base operating system and allows users to transparently use RedHat 7.3, RedHat 8, and SuSE 9.0. This allows users to select the distribution that works for them (or their collaboration).

Quick Start

  1. Install and configure the following prerequisites
  • A compatible kernel. For CHOS < 0.13.1, the kernel must be rebuilt with CONFIG_DEBUG_RODATA=n. Kernels based on the following are supported:

    • EL5 2.6.18 family
    • EL6 2.6.32 family. This is the most widely tested option.
    • Experimental support for the EL7 3.10.0 family.
  • The associated kernel-devel package.

  1. Download the CHOS code:

    git clone https://github.com/scanon/chos; cd chos
  2. Select a CHOS release to build. The master branch (and development branches) provide access to the latest features; however, the most recent tagged release is recommended for stability:

    git tag
    git checkout <tag>
  3. Build CHOS with kernel support:

    ./configure --enable-kernel [ --with-kernel-source=/path/ ]
    make install
  4. Create a suitable /chos/ directory, which will serve as the shared chroot directory. This should include mountpoints for any shared filesystems which should be visible inside the CHOS environments.

    A suggested layout is:

    /chos/bin     -> /proc/chos/link/bin
    /chos/boot    -> /proc/chos/link/boot
    /chos/chos    -> /
    /chos/etc     -> /proc/chos/link/etc
    /chos/lib     -> /proc/chos/link/lib
    /chos/lib32   -> /proc/chos/link/lib32
    /chos/lib64   -> /proc/chos/link/lib64
    /chos/opt     -> /proc/chos/link/opt
    /chos/sbin    -> /proc/chos/link/sbin
    /chos/usr     -> /proc/chos/link/usr
    /chos/var     -> /proc/chos/link/var
  5. Edit /etc/sysconfig/chos. Change the value of $BINDMOUNT to be the list of directories in the root environment which should be bind-mounted into their respective paths in /chos/.

  6. Edit /etc/chos, the primary CHOS configuration file. This file should contain a list of environment names and path mappings in the %SHELLS section. The following example file describes two environments: "root" (the real / environment), and "debian6", located at /export/os/debian6/.

  7. Create the CHOS environments using an appropriate tool (e.g., "debootstrap").

  8. Install the "chos" utility inside the new CHOS environments. It is not necessary to build it with kernel support:

     git clone https://github.com/scanon/chos; cd chos
     make install
  9. Install the following symlinks in the CHOS environment:

     etc/chos                -> /local/etc/chos
     etc/chos.conf           -> /local/etc/chos.conf
     etc/group               -> /local/etc/group
     etc/gshadow             -> /local/etc/gshadow
     etc/hosts               -> /local/etc/hosts
     etc/ldap.conf           -> /local/etc/ldap.conf
     etc/localtime           -> /local/etc/localtime
     etc/motd                -> /local/etc/motd
     etc/nsswitch.conf       -> /local/etc/nsswitch.conf
     etc/openldap/ldap.conf  -> /local/etc/openldap/ldap.conf
     etc/passwd              -> /local/etc/passwd
     etc/resolv.conf         -> /local/etc/resolv.conf
     etc/shadow              -> /local/etc/shadow
     etc/ssh/ssh_known_hosts -> /local/etc/ssh/ssh_known_hosts
     var/tmp                 -> /local/var/tmp/
     var/cache               -> /local/var/cache/
     var/db                  -> /local/var/db/
     var/run                 -> /local/var/run/
     var/lock                -> /local/var/lock/
     var/spool               -> /local/var/spool/
  10. Start CHOS using the provided initscript:

     service chos start
  11. As a non-root user, enter and exit the new CHOS environment:

     env CHOS=debian6 chos
     cat /etc/debian_version
     env CHOS=root chos
     cat /etc/redhat-release

Detailed Installation

See the INSTALL file for more detailed installation instructions.


In the end, CHOS is just doing a chroot into the OS tree. The kernel module is only needed to simply the framework and allow the framework to be used for multiple OS trees. For example, if you installed a system as follows...

  • / (base OS - OS1)
  • /OS-2 (second OS)
  • /OS-3 (third OS)

You could chroot into /OS-2 and it would function if everything was installed correctly. However, for it to fully work you would need some things to be available in the chroot tree. For example, /proc would have to be mounted under each tree (i.e. /OS-2/proc, /OS-3/proc). Also, if NFS mounts are used, they would need to be mounted under each tree. This makes it awkward to support more than a handful of OS trees. The kernel module provides a process dependent symbolic link. This link will resolve differently for different processes. Also, children processes automatically inherit the value of their parent. Using the kernel modules, you only need this...

  • / (base OS - OS1)
  • /chos
  • /chos/proc
  • /chos/ostrees/OS2
  • /chos/ostrees/OS3

The link, which is accessed through /proc/chos/link, points to the base of the OS. The rest of the chos directory contains the subdirectories that would normally be present in an OS (/bin, /sbin, /usr, etc), but they are all symlinks that traverse through the special link. So, /chos/bin would point to /proc/chos/link/bin, which could translate to /ostrees/OS2/bin for one user and /ostrees/OS3/bin for another user.