8/13/2019 Inside GRUB

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GEEKS !!!

We all know the booting process of RHEL. You all had learned this during your RHCE

Trainingand RHCSS Training.

Lets understand once again the internals of GRUBand how it helps in booting your machine.

GRUB consists of different parts. Two actually.

The first part is installed in the master boot record (MBR)of your hard disk or in the boot sector o

the active partition.

By default, Red Hat Enterprise Linux installs it in the MBR. This part is very small, just 446 bytes,

and it is called the GRUB stage 1 boot loader. But, you should remember that, a mere, 446 bytes in

the MBR are not enough to start an entire operating system.

This is the reason we have stage 2 part of GRUB.

This part is in the first megabyte of your hard diskan area used to store metadata needed to star

the computer. With the use of the stage 2, GRUB is capable of reading files directly from the filesystem, as long as the file system is supported. You can find a list of supported file systems in

the /boot/grubdirectory. The minimal file system drivers in this directory all have a name that ends

with stage1_5.

An important file also used by GRUB is/boot/grub/device.map. This file contains the mapping

between kernel device names and BIOS device names. If youre ever having a problem recognizing

the boot disk correctly, the reason might be a mis-configuration of this file.

On a server that experiences severe problems with GRUB, it can be useful to check whether GRUB

is still present in the MBR.

To do this, the tool xxdcan be useful. xxd is a hexadecimal viewer, and if you use xxd -l 512

/dev/sda on your computer, it will show you the contents of the MBR.

Even if the contents are completely hexadecimal, youll easily recognize the GRUB error message,

which indicates that a GRUB boot loader has been installed.

INSIDE GRUB IN RHEL 6

http://en.wikipedia.org/wiki/Master_boot_recordhttp://www.gnu.org/software/grub/http://networknuts.wordpress.com/2013/10/06/grub-redhat-training-rhel6/http://linuxcommand.org/man_pages/xxd1.htmlhttp://en.wikipedia.org/wiki/Master_boot_recordhttp://www.gnu.org/software/grub/http://networknuts.net/programs-and-courses/redhat/rhs333.htmlhttp://networknuts.net/programs-and-courses/redhat/rhce.htmlhttp://in.redhat.com/

8/13/2019 Inside GRUB

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GEEKS !!

Apart from the GRUB error message that is shown. in the lines that start with 1c0 and 1d0, you can

also recognize the partition table. The second columns of these lines clearly mark the existence of

a partition of type 83 and 8e.

Inside GRUB RHEL 6

http://networknuts.files.wordpress.com/2013/10/grub-rhel-networknuts.png