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Q. I've upgrade my CentOS / RHEL (Red Hat Enterprise Linux) 4.7 on HP ProLiant DL580 G5 and it is showing unknown NMI errors in the logs:

Uhhuh. NMI received for unknown reason 30.
Dazed and confused, but trying to continue
Do you have a strange power saving mode enabled?

Uhhuh. NMI received for unknown reason 20.
Dazed and confused, but trying to continue
Do you have a strange power saving mode enabled?

How do I fix this error?
A. This is caused when the system is hanging under load. Add any one of the following to you /boot/grub.conf file:

  1. Disable the NMI watchdog by adding "nmi_watchdog=0"
  2. Disable the high precision event timer (HPET) by adding "nohpet"

Open grub.conf, type:
vi grub.conf
Make modification to kernel line as follows:

title Red Hat Enterprise Linux AS (2.6.9-78.0.8.EL)
        root (hd0,0)
        kernel /vmlinuz-2.6.9-78.0.8.EL ro nohpet root=/dev/VolGroup00/LogVol00 nohpet
        initrd /initrd-2.6.9-78.0.8.EL.img

Save and close the file. Reboot the server:
# reboot

Boot Ubuntu Linux into Rescue mode to fix system

Q. How do I boot my Ubuntu Linux server into Rescue mode to fix system?

A. You can boot Ubuntu Linux into rescue mode when things go wrong, there are several ways to work on fixing them. However, you must understand your system and what to fix in case of emergency.

Use this mode only if your system is unbootable. It is also recommended that you make backup of important data.

=> Start Computer / reboot computer

=> Put your Ubuntu install CD

=> Make sure BIOS is set to boot from CD

=> Now booting will start from Ubuntu install CD

=> You will see various option screen - select Recover a broken system

You will system is booting in to rescue mode; system will ask about Network and other information. This is required to setup rescue environment.

Now you will prompted to select root partition or device.

Next it will try to mount root device and it will notify the same.

Now you will see common rescue mode option such as Reinstall GRUB or open / execute a shell etc.

Now you are in rescue mode and you need to use your own knowledge to rescue the system. When things go wrong, there are several ways to work on fixing them.

Few things to remember....

The rescue-mode CD mounts your root filesystem at /target directory. Open or execute a shell and change directory to /target
# ls /target
# cd /target
# mount

If you need to run fsck command use fdisk -l command to find out partition name. Usually they are located at /dev/discs/discX/partY. For example repair 2nd partition (not mounted as /target) you may type command:
# ls -l /dev/discs/disc0
# fsck.ext3 /dev/discs/disc0/part1

To exit type command :
# sync ;sync
# exit
# reboot

As I said earlier in rescue mode, there is no single command that I can recommend to fix problem. You need to use your own knowledge to rescue the system and don't forget to read command documentation before using any command.

Linux reset forgotten root password

Q. I forgot my root password, how can I get into my system?

A. You can reset forgotten root password under Linux by booting system into single user mode or emergency mode (also known as rescue mode).

My boot loader is GRUB (see LILO boot loader below)

)Following is the procedure to reset root password if you are using GRUB as a boot loader:

  1. Select the kernel
  2. Press the e key to edit the entry
  3. Select second line (the line starting with the word kernel)
  4. Press the e key to edit kernel entry so that you can append single user mode
  5. Append the letter S (or word Single) to the end of the (kernel) line
  6. Press ENTER key
  7. Now press the b key to boot the Linux kernel into single user mode
  8. At prompt type passwd command to reset password:

You need to mount at least / and other partitions:
# mount -t proc proc /proc
# mount -o remount,rw /

Change the root password, enter:
# passwd
Finally reboot system:
# sync
# reboot


My boot loader is LILO

At LILO boot loader type linux single and press [ENTER] key:
Boot: linux single
When you get the # prompt you will need to type passwd root to reset password:
# passwd
Reboot system:
# sync
# reboot

See also:

Boot Linux Grub Into Single User Mode

Q. How do I boot a Linux system into single user mode?
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