February 2012 – dump.4network.org

Broken Grub menu – Debian.

Broken Grub menu: boot to the menu, drop to a consloe and boot Debian direct.
Sometimes the only thing that works when you boot is the Grub menu appears on your screen.
When you see Grub’s boot menu on your screen, instead fo selecting a boot item, press the “c” key.
That brings up a messages like this:
GNU GRUB version 1.99-14 Minimal BASH-like line editing is supported. For the firs word, TAB lists possible command completions. Anywhere else TAB lists posssible device or file completions. ESC at any time exits.

grub>
Now carry out the following commands at the grub prompt:
grub> search.file /sbin/init hd0,msdos1 grub> search.file /boot/grub/device.map hd0,msdos1 grub> search.file /vmlinuz hd0,msdos1

Whatever you use, will ok. What we looking for is:
– Grub pointer to my Debian installation on drive number 1, partition number 1, your pointer
will be different from my example (hd0,msdos1)
– UUID assigned to this partition 625b756b-737e-4502-a914-9d351d50603a

grub> ls (hd0,msdos1) Partition hd0,msdos1: Filesystem type ext2 - Last modification time 2012-01-11 10:57:44 Wednesday, UUID 625b756b-737e-4502-a914-9d351d50603a - Partition start at 2048 - Total size 40134656 sectros grub> set root='(hd0,msdos1)' grub> linux /boot/vmlinuz-{press TAB key} Possible files are: vmlinuz-2.6.32-5-686 vmlinuz-3.2.0-1-686-pae grub> linux /boot/vmlinuz-3.2.0-1-686-pae ro root=UUID=625b756b-737e-4502-a914-9d351d50603a grub> initrd /boot/initrd.img-{press TAB key} Possible files are:

initrd.img-2.6.32-5-686 initrd.img-3.2.0-1-686-pae grub> initrd /boot/initrd.img-3.2.0-1-686-pae grub> boot
and the machine boots 🙂

Yum local repository to be able to access a CDROM/DVD iso.

You do not have access to the network and you would like to install some packages from CDROM/DVD, please follow the instructions:

First take a backup of /etc/yum.repos.d/ then remove these repo files:
CentOS-Base.repo, CentOS-Debuginfo.repo and CentOS-Media.repo
and add a new file called CentOS_local.repo with the following lines:
[root@localhost yum.repos.d]# cat CentosOS_local.repo [dvd] name=Centos DVD baseurl=file:///media #mount location enabled=1 gpgcheck=0 [root@localhost yum.repos.d]#
Here baseurl=file:///media is the location of CentOS iso. Now you have to mount your CentOS iso into the media folder:
[root@localhost]# mount /dev/cdrom /media
Once this is done, you can go ahead with yum to install packages from CD/DVD.

SSH – Limit user logins.

SSH logins can be limited to only certain users who need remote access.
The ssh server is configured by the file /etc/ssh/sshd_config. If you wish you can restrict remote access to specific users. For example to only allow “john” and “julia” to login add the following:

AllowUsers john julia

and restart the sshd service.

RAID in Linux, using mdadm in Linux.

Linux software RAID has nothing to do with hardware RAID controllers. You don’t need an add-on controller, and you don’t need the onboard controllers that come on most motherboards.

The RAID devices are virtual devices created from two or more real block devices. This allows multiple devices (typically disk drives or partitions thereof) to be combined into a single device to hold (for example) a single filesystem.
Some RAID levels include redundancy and so can survive some degree of device failure.

Currently, Linux supports LINEAR md devices, RAID0 (striping), RAID1 (mirroring), RAID4, RAID5, RAID6, RAID10, MULTIPATH, FAULTY, and CONTAINER.
Continue reading “RAID in Linux, using mdadm in Linux.”