Creating a chroot environment – the script.

Table of contents for chroot

  1. Creating a chroot environment in Fedora with bash and other utils.
  2. Creating a chroot environment – the script.

Here is the script, very simple and effective 🙂

#!/bin/bash -
#===============================================================================
#
#          FILE:  mkchroot.sh
#
#         USAGE:  ./mkchroot.sh
#
#   DESCRIPTION:  Make a  chroot environ and cd to it
#
#       OPTIONS:  ---
#  REQUIREMENTS:  ---
#          BUGS:  ---
#         NOTES:  ---
#        AUTHOR: Amit Agarwal (aka), 
#       CREATED: 09/03/2011 02:53:37 PM IST
# Last modified: Sat Sep 03, 2011  03:11PM
#      REVISION:  ---
#===============================================================================

 binaries=( bash2 ls cat vi vim sudo)
#===  FUNCTION  ================================================================
#          NAME:  copy_binary
#   DESCRIPTION:  copy binary to chroot
#    PARAMETERS:
#       RETURNS:
#===============================================================================

copy_binary ()
{
   cmd=`which $1`
   echo $cmd
   cp $cmd bin/
   ldd $cmd
   while read line
   do
      while read ld
      do
         if [[ -f $ld ]]
         then
            echo copy $ld
            cp $ld lib/
            if [[ -L $ld ]]
            then
               ld1=$( ls -l $ld |sed 's/.*> //')
               echo "  copy $ld1"
               cp  /lib/$ld1 lib/
            fi
         fi
      done < <(echo $line|sed 's/.*> //'|sed 's/ .*//')
   done < <(ldd $cmd)
}	# ----------  end of function copy_binary  ----------

#===  FUNCTION  ================================================================
#          NAME:  init
#   DESCRIPTION:  Do the required initialization
#    PARAMETERS:
#       RETURNS:
#===============================================================================
init()
{
   mkdir -p {root,home,dev,etc,lib,usr,bin}
   mkdir -p usr/bin
   mkdir -p libexec/openssh



   mknod -m 666 dev/null c 1 3

   cd etc
   cp /etc/ld.so.cache .
   cp -avr /etc/ld.so.cache.d/ .
   cp -avr /etc/ld.so.conf.d/ .
   cp /etc/ld.so.conf .
   cp /etc/nsswitch.conf .
   cp /etc/passwd .
   cp /etc/group .
   cp /etc/hosts .
   cp /etc/resolv.conf .
   cd -
}
cd $1
if [[ -f .status ]]
then
   cat .status
   echo "Not running now"
else
   init
   for i in ${binaries[*]}
   do
       copy_binary $i
       cp -avr /etc/${i}* etc/
   done
   ln bin/bash2 bin/bash
   echo "complete" > .status
fi
cp -avr ~/bin/automation root/automation
sudo chroot .

.

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Creating a chroot environment in Fedora with bash and other utils.

Table of contents for chroot

  1. Creating a chroot environment in Fedora with bash and other utils.
  2. Creating a chroot environment – the script.

[ad#ad-2]

I am testing some of my scripts to work on a very old system and there the versions of the most popular applications are very old, real old :(. So, some of things that I am very used to since last couple of years, do not seem to work as expected and I need to keep verifying a lot of things on the server, very inconvinient to keep testing the script on the server (need to connect on VPN) just to test some very simple things.

But if I want to test it on my local desktop or laptop then I need to donwgrade all the applications on my system. Other option is changing the scripts or the paths to get the lower version to work. I found the third option which I liked best and gives me a very controlled environment – chroot.

So, what do we need to achieve this. Here are the steps

  • Create a empty directory
  • Create the minimum required directories under this directory
   mkdir -p {root,home,dev,etc,lib,usr,bin}
   mkdir -p usr/bin
   mkdir -p libexec/openssh
  • Copy the minimim files required for the chroot
   mknod -m 666 dev/null c 1 3
   cd etc
   cp /etc/ld.so.cache .
   cp -avr /etc/ld.so.cache.d/ .
   cp -avr /etc/ld.so.conf.d/ .
   cp /etc/ld.so.conf .
   cp /etc/nsswitch.conf .
   cp /etc/passwd .
   cp /etc/group .
   cp /etc/hosts .
   cp /etc/resolv.conf .
  • Copy the required libs for the commands that you would like to have in the chroot
ldd

This will have a output like this:
linux-gate.so.1 => (0x00e77000)
libselinux.so.1 => /lib/libselinux.so.1 (0x46bf1000)
librt.so.1 => /lib/librt.so.1 (0x46bba000)
libcap.so.2 => /lib/libcap.so.2 (0x46020000)
libacl.so.1 => /lib/libacl.so.1 (0x47fb3000)
libc.so.6 => /lib/libc.so.6 (0x46a09000)
libdl.so.2 => /lib/libdl.so.2 (0x46bb3000)
/lib/ld-linux.so.2 (0x469e8000)
libpthread.so.0 => /lib/libpthread.so.0 (0x46b97000)
libattr.so.1 => /lib/libattr.so.1 (0x46960000)
Here you would need to copy all the files mentioned after the => sign in the output to your lib directory for the chrooted environment. Also make sure that if the above are links then you copy the actual files also to ensure that you have the actual libs for the commands to work.

  • Copy the actual command to the chroot environment.

Copy the command with cp command to the bin directory of your chroot environment.

Once all this is done, you can simply chroot to this directory with the command

chroot

Voila, you are done.

Check that you have all the utils/commands that you have copied and nothing less/nothing more.

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