Reinstalling Grub bootloader in Fedora

When a system is too damaged to permit booting from the hard disk drive, it's necessary to boot from another medium. The Fedora installation discs support a "Rescue mode" in which the system is booted from the CD and the hard disk partitions are optionally mounted for access

To access this mode, boot from your Fedora install media and select "Rescue installed system" from the boot menu using the arrow keys and Enter or by pressing the R key

The kerenel will boot from CD and the system will prompt you to select a keyboard style and language from scrollable lists of options. You will then be given the opportunity to enable the network interfaces on the system, either by entering the IP information or by using DHCP.

The system will then present a dialog stating that the rescue environment is about to find and mount the filesystems from your hard disk Fedora installation, and asks if you wish to continue. This is a critical question: if your filesystems are intact and you wish to access the data that is in them, you can select Continue, the default option. If you are concerned about the state of your filesystems and want to ensure that they will not be altered, but still want to access them, select Read-Only.

you will be redirected to root prompt. run this command from there

chroot /mnt/sysimage

1. Start the Grub shell with the grub command:
   
   # grub
   Probing devices to guess BIOS drives. This may take a long time.
   
   
   GNU GRUB version 0.97 (640K lower / 3072K upper memory)
   
   [ Minimal BASH-like line editing is supported. For the first word, TAB
   lists possible command completions. Anywhere else TAB lists the possible
   completions of a device/filename.]
   grub>
   
2. Use the find command to locate the partition containing the boot files by searching for /grub/grub.conf (or /boot/grub/grub.conf if that fails). Grub will report the partition using its own syntax:
   
   grub> find /grub/grub.conf
   (hd0,0)
   
3. Use the root command to configure the partition from which the boot files are to be loaded (use the partition ID from step 2):
   
   grub> root (hd0,0)
   
4. The partition ID from step 2 can be converted to a drive ID by removiing the comma and partition number -- for example, the partition (hd0,0) is on the drive (hd0). Use the setup command with this drive ID to install the Grub bootloader code:
   
   grub> setup (hd0)
   
5. Exit the Grub shell with quit:
   
   grub> quit
   #

You can also use rescue mode to set the root password, create alternate superuser accounts, or change or remove a boot password. Whether these are important recovery operations or a type of attack depends only on the context in which they are performed. You can slow down such an attack by configuring the system BIOS to boot only from the hard disk and installing a BIOS password, but that can be reset using a motherboard jumper in most cases.

The moral of the story: if you don't have physical security, you don't have system security.

When you are finished using rescue mode, type exit or press Ctrl-D twice. The system will then reboot.