Saturday, March 14, 2009

RHCE Notes - Troubleshooting booting issue

booting issue is optional question in section I,The proctor will re-image your PC to introduce booting issue, You will be given rescue CD to fix it.

It is easy to troubleshoot Linux boot issue, if you break it intentionally at each step, observe the symptom and find the fix.

#==Linux boot order
The BIOS ->MBR->Boot Loader->Kernel->/sbin/init->
/etc/inittab->
/etc/rc.d/rc.sysinit->
/etc/rc.d/rcX.d/ #where X is run level in /etc/inittab
run script with K then script with S

#==Linux rescue env
boot first linux cd then type linux rescue
TIP:
linux rescue will try to mount all partions, however if there is error only some partions are mounted, run choot /mnt/sysimage now will lost /dev /proc mounts, here is how to transfer these mounts.
mount -o bind /dev /mnt/sysimage/dev
mount -o bind /proc /mnt/sysimage/proc

Linux rescue env supports both software RAID and LVM. normal LVM commands e.g vgdisplay are not availiable,but it can be accssed by LVM "master" command e.g "lvm vgdisplay"

#== Grub boot manager
= go to grub cmd prompt by pressing c at boot menu
=find root partition, 2 methords
grub> root
(hd0,0) Filesystem type is ext2fs, partition type 0x83
grub> find /grub/stage1
(hd0,0)
=list files/dirs in current drive
cat / #type cat SPACE / TAB, it will list all fies/dir just like ls
= display contents of the file
cat /grub/grub.conf
= now you can boot interactively by type kernel and initrd commands from grub.conf


#==Restore missed file from RPM
#cd /tmp
#rpm2cpio initscripts-7.93.11.EL-1.i386.rpm cpio -icumvd ./etc/inittab
or
#rpm2cpio initscripts-7.93.11.EL-1.i386.rpm >init.cpio /* file is ./etc/inittab not /etc/initab

List contents: cpio -tv
or

install file to alternative location the copy the file
rpm --root-directory /var/tmp/a X.rpm

#== MBR corrupted.
MBR has 512 byte in total
446 Executable code section
4 Optional Disk signature
2 Usually nulls
64 Partition table #if this is overwritten, no way to recover unless you backuped the partion table or re-partion using #exact same layout
2 MBR signature

Corrupt MBR intentionaly:dd if=/dev/zero of=/dev/hda bs=446 count=1 #MBR should be at the start whole disk(not partition hda1), it has 512, the first 446 byte is exec code. DON'T overwrite whole 512 byte because it has partion table data.
ERR: no bootable media found,Missing operating system" or "Operating System Not Found
boot from cd run "linux rescue", let it mount linux partions automaticlly.
chroot /mnt/sysimage then grub-install /dev/hda
boot from cd run "linux rescue", if linux partions failed to mout
mount mannually. sfdisk -l; e2label find the boot partition
mkdir /a; mount /dev/hda1 /a; ln -s /usr/sbin/grub /sbin/grub; grub-install --root-direcotry=/a /dev/hda #it is hda not hda1



#= root (/)was not mounted
mount couldn't find file system /dev/root
switchroot mount faild...
Error 2 mounting none;exec of init ((null)) failed!!!
kernel /vmlinuz-test ro root=LABEL=/
/* root=LABEL=/ mout using label, or root=/dev/sda3 mount with direct dev-name */


#= not loading initrd image
VFS: Cannot open root device "Label=/1" or unknow-block(0,0)
Please append a correct "root=" boot option
Kernel panic: VFS: Unable to mount roof fs on unknow-block(0,0)
1) Kernel doesn't Support for the file system .compile kernel with FS support NOT as a module
2) initrd was not loaded. Add initrd=... in grub.conf
linux rescue, then chroot /mnt/syimage and create initrd file
mkinitrd /boot/initrd-filename `uname -r` #make initrd file mannually


#==/sbin/init problem.
Switching to new root
kernel panic -not syncing :Attepmted to kill init
switching to new root
/bin/sh: ro : no such file or directory
/* boot to rescue, check /sbin/init. restore from rpm package*/



#== /etc/inittab not found
"enter run level" prompt enter s. or at grub menu append s or init=/bin/sh or emergency, then restore initab from source RPM


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