Linux: Creating a Rescue Ramdisk Image

Here I write about a very simple and crude method of creating a rescue initrd image for the purpose of fixing or restoring Linux systems. I use this method when it's inconvenient to creating a sophisticated Rescue CD or when the CD-ROM is inaccessible. In short, this method involves creating a ramdisk image, saving it on a hard disk partition, and using a boot loader like GRUB to boot Linux into a Ramdisk system.

1. Let's create an initrd image first. Create a text file that lists the contents of the ramdisk image. For example, my rescue.txt file has the following contents:
   
   

   /bin/bzip2
   /bin/cat
   /bin/chattr
   /bin/cp
   /bin/dash
   /bin/dd
   /bin/dmesg
   /bin/gzip
   /bin/ls
   /bin/lsattr
   /bin/mkdir
   /bin/mount
   /bin/mv
   /bin/rm
   /bin/tar
   /bin/umount
   /etc/modprobe.d
   /lib/ld-linux.so.2
   /lib/libacl.so.1
   /lib/libattr.so.1
   /lib/libblkid.so.1
   /lib/libbz2.so.1.0
   /lib/libc.so.6
   /lib/libcom_err.so.2
   /lib/libdevmapper.so.1.02.1
   /lib/libdl.so.2
   /lib/libe2p.so.2
   /lib/libext2fs.so.2
   /lib/libncurses.so.5
   /lib/libpthread.so.0
   /lib/librt.so.1
   /lib/libselinux.so.1
   /lib/libsepol.so.1
   /lib/libuuid.so.1
   /lib/modules/2.6.18.8/kernel/drivers/ide/ide-disk.ko
   /lib/modules/2.6.18.8/kernel/drivers/ide/ide-generic.ko
   /lib/modules/2.6.18.8/kernel/drivers/scsi/scsi_mod.ko
   /lib/modules/2.6.18.8/kernel/drivers/scsi/sd_mod.ko
   /lib/modules/2.6.18.8/kernel/drivers/usb/host/ehci-hcd.ko
   /lib/modules/2.6.18.8/kernel/drivers/usb/host/ohci-hcd.ko
   /lib/modules/2.6.18.8/kernel/drivers/usb/host/uhci-hcd.ko
   /lib/modules/2.6.18.8/kernel/drivers/usb/storage/usb-storage.ko
   /lib/modules/2.6.18.8/modules.dep
   /lib/terminfo/l/linux
   /sbin/fdisk
   /sbin/losetup
   /sbin/mke2fs
   /sbin/mkswap
   /sbin/modprobe
   /usr/bin/less
   /usr/lib/libz.so.1
   /usr/sbin/mkcramfs

   
   

   Save this file as, for example, rescue.txt.

   
   
2. Create a temporary directory, for example, /tmp/rescue.
   
   

   mkdir /tmp/rescue

   
   
3. Use tar to copy files specified in rescue.txt.
   
   

   cd /
   tar cvhf - -T /home/jocelyn/rescue.txt | (cd /tmp/rescue; tar xf -)

   
   
4. Create device files.
   
   

   cd /tmp/rescue
   mkdir dev mnt tmp
   cd dev
   MAKEDEV std hda sda consoleonly

   
   

   Create an InitRD image. I like to use mkcramfs for this purpose.
   
   

   mkcramfs /tmp/rescue /boot/rescue.bin

   
   
5. When you start your computer, use GRUB commands like the following to boot Linux into the ramdisk system.
   
   

   kernel (hd0,5)/boot/vmlinuz-2.6.18.8 root=/dev/ram0 init=/bin/dash
   initrd (hd0,5)/boot/rescue.bin

   
   You'll arrive at a very simple rescue console.
   

Examples of Possible Commands in Rescue Console

• Edit the partition table with fdisk.
  
  

  fdisk /dev/hda

  
  
• Display the partition table.
  
  

  fdisk -l -u /dev/hda

  
  
• Format a partition as Linux EXT2.
  
  

  mke2fs -L LINUX -I 128 /dev/hda6

  
  
• Mount a Linux partition on /mnt.
  
  

  mount -n -t ext2 /dev/hda6 /mnt

  
  
• Change the current directory and then display the current path.
  
  

  cd /mnt
  pwd

  
  
• Display the contents of the current directory.
  
  

  ls

  
  
• Mount a FAT32 partition on /tmp.
  
  

  mount -n -t vfat -o codepage=437,iocharset=iso8859-1,shortname=winnt /dev/hda7 /tmp

  
  
• Restore a backup from a tarball onto the current folder.
  
  

  tar xzf /tmp/backup-072499.tgz

  
  
• Unmount a partition.
  
  

  umount /mnt