How to Rescue your system with Single user mode in Linux, CentOS 6 / RHEL 6, How to restore Grub on RHEL6

How to Rescue your system with Single user mode in Linux, CentOS 6 / RHEL 6, 
Rescue mode provides the ability to boot a small Red Hat Enterprise Linux environment entirely from CD-ROM, or some other boot method, instead of the system's hard drive.
As the name implies, rescue mode is provided to rescue you from something. During normal operation, your Red Hat Enterprise Linux system uses files located on your system's hard drive to do everything — run programs, store your files, and more.
However, there may be times when you are unable to get Red Hat Enterprise Linux running completely enough to access files on your system's hard drive. Using rescue mode, you can access the files stored on your system's hard drive, even if you cannot actually run Red Hat Enterprise Linux from that hard drive.
To boot into rescue mode, you must be able to boot the system using one of the following methods.

• By booting the system from an installation boot CD-ROM.
• By booting the system from other installation boot media, such as USB flash devices.
• By booting the system from the Red Hat Enterprise Linux CD-ROM #1

 Re-install bootstrap code (GRUB)

Please perform the following steps:

1. Boot from CentOS installation disc (for example, CD #1 or DVD).
2. For CentOS-4 or CentOS-5 type "linux rescue" at the "boot:" prompt. For CentOS-6 pick "Rescue installed system" from the boot menu.
3. Mount all filesystems in read-write mode.
4. Change root to real root ('/') on your hard disk:

   # chroot /mnt/sysimage
   

5. Re-install bootstrap code (GRUB).
   • If you wish to re-install GRUB to the MBR on a SCSI or SATA disk (/dev/sda):

     # grub-install /dev/sda
     

   • If you wish to re-install GRUB to the PBR of Partition #2 on a SCSI or SATA disk (/dev/sda2):

     # grub-install /dev/sda2
     

   • If you wish to re-install GRUB to the MBR on an IDE disk (/dev/hda):

     # grub-install /dev/hda
     

   • If you wish to re-install GRUB to the MBR on a HP Smart Array disk (/dev/c0d0):

     # grub-install /dev/cciss/c0d0

     How to restore Grub on RHEL6

     Red Hat Enterprise Linux 6 – How to restore Grub.
     Also works on CentOS and Fedora.
     1. Boot from RHEL6 DVD.
     2. Select “Rescue installed system”.
     3. Choose -> Language, Keyboard, and Rescue Method. Select “Local CD\DVD” for the Rescue Method.
     4. Setup Networking -> Yes or No, doesn’t matter for restoring Grub.
     5. Next on the “Rescue Screen”, select Continue and OK twice.
     6. Next on the “First Aid Kit quickstart menu”, select “shell Start shell”.
     The prompt will start:
     Starting shell…
     bash-4.1#
     7. Type: “chroot /mnt/sysimage”
     8. Type: “cd /boot/grub”
     9. Type: “fdisk -l” to see where hd0.
     10. Restore Grub, type: “grub-install /dev/sda1”
     If No error, it should display:
     # this device map was generated by anaconda
     (hd0) /dev/sda
     

     For more Information in Linux . 

     1. http://newstips24.blogspot.com/2013/11/setup-dns-server-step-by-step-in-centos.html
     2. DNS Scavenging setup & Understanding Aging and Scavenging, Windows 2000
     3. http://newstips24.blogspot.com/2013/11/dns-server-not-responding-error-how-to.html
     4. Reverse DNS and IP Lookup
     5. Error "Server Error in '/' Application.What does this mean?
     6. pathping command,Using the pathping command
     7. What Is 'Telnet'? What Does Telnet Do?
     8. ARP (Address Resolution Protocol)
     9. Configure NFS Server to share any directories in local network.
     10. Linux Internet Web Server and Domain Configuration Tutorial
     11. APIPA - Automatic Private IP Addressing, Is my computer using APIPA now?
     12. IPTables Tables, Chains, Rules Fundamentals,input and ouput in iptable.
     13. Configuring Network Address Translation, what you defined above .