Answer / mohammed sajjad

Scenario 1: Backup the boot sector (or MBR)
If the first harddisk in the system is /dev/sda, to backup the boot sector the following command can be used:

# dd if=/dev/sda of=bsbackup.bin bs=512 count=1

Essentially this command will read the first 512 bytes of /dev/sda and write it to the file bsbackup.bin.
Scenario 2: Restore the boot sector from a file:

# dd if=bsbackup.bin of=/dev/sda bs=512 count=1

This will restore the boot sector to /dev/sda that was backed up in Scenario 1.
Scenario 3: Zero out the boot sector (leaving the partition table intact)
Sometimes a virus or other issue can leave a corrupted executable code section in the MBR. I have personally seen a boot sector that would not store grub information (and thus boot linux after its installed) properly until the first 446 bytes were zeroed out and grub re-installed. The following command will do just that:

# dd if=/dev/zero of=/dev/sda bs=446 count=1

Scenario 4: Zero out the entire MBR (this will erase the partition table as well – effectively destroying the ability to easily access data on the drive)
A variation of the last dd command will wipe out the master boot record entirely. You will have to repartition and reformat your hard disk after this:

Which command causes in.named to take a snapshot of its in-memory cached data.

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