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Net-installing CentOS without CD-ROM and USB

26 Dec 2011

There was an old PC in our lab, and one day I wanted to use it as a node in our Hadoop cluster. There was a Fedora 8 on it. But I'd like to change it to a new CentOS 5.7. The problem I met, was that this PC was too old, that neither the CD-ROM nor the USB ports worked well.

It seems troublesome. But I still found a way to install my new CentOS on it, without a CD-ROM, without a USB port, and without any external bootable device. I used net-installation! You may argue that even with net-installation, I still need a CD or a USB disk, with the net-install ISO image on it, to boot up the machine at the very beginning. But I was able to take the GRUB on that old Fedora 8 in use and booted the net-install kernel. Here's how.

First, I downloaded the net-install image into the old file system and checked its MD5 hash:

wget http://mirrors.163.com/centos/5.7/isos/i386/CentOS-5.7-i386-netinstall.iso \
    http://mirrors.163.com/centos/5.7/isos/i386/md5sum.txt
diff <(grep "netinstall" md5sum.txt) <(md5sum CentOS-5.7-i386-netinstall.iso) && echo OK

Then I extracted the isolinux directory in the ISO file to the /boot directory. Here I put it in the boot directory just for convenience, because this is the default root directory of GRUB after we boot into the GRUB CLI.

mkdir tmp
mount -o loop CentOS-5.7-i386-netinstall.iso tmp/
cp -Rv tmp/isolinux/ /boot/

After that, I issued a reboot and pressed c on the GRUB menu to enter the GRUB CLI. The below commands can boot the kernel in the isolinux directory.

kernel /isolinux/vmlinuz
initrd /isolinux/initrd.img
boot

If the isolinux directory was in other partitions of your hard drive, then you may need to specify the root path for GRUB with the root command.

The system booted up to the installation image just like I inserted the net-install disk into the CD-ROM. On the 'Installation Method' screen, I chose HTTP or FTP and then configured the network. Next, I chose a mirror site (a self-hosted mirror site will be faster) which host CentOS files. The remain steps were the same with an ordinary installation.

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