1.1. Disclaimer

This article assumes that the reader using the described setup is familiar with the technical concepts and commands. If you are not comfortable issuing the commands in this document or don't understand the clone script, don't try these instructions.

Additionally, if you do not know the difference between /dev/hda and /dev/hda1, please do not use this HOWTO. This may be the difference between restoring a partition and losing all your data. Please note that I take no responsibility for anything that may go wrong, even if the error resulting from any incorrectness in this article.

For the purpose of this document, the RedHat 7.3 distribution has been used in both server and client. Although this shouldn't make great difference, no test has been made on other distributions or custom setups.

1.2. Copyright

Copyright (c) 2002 Guilherme Tupynambá

Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License (GFDL), Version 1.1 or any later version published by the Free Software Foundation; with no Invariant Sections, with no Front-Cover Texts, and with no Back-Cover Texts. The clone script is licensed in the terms of the GNU General Public License (GPL).

1.3. Feedback

Comments on this article will be highly appreciated. Direct your comments to <gtupy (at) uol.com.br>.

2.1. Why clone disk images

The main reason to clone disk images is to ease the installation of an operating system and a basic set of applications over a large number of machines. One standard machine is prepared and its pristine image is saved to be restored on other machines saving efforts and simplifying procedures.

2.2. Why boot from a network

Booting from hard disk would limit the possibilities of copying images. It wouldn't be possible, for instance, to safely copy to and from a partition mounted by the booted operating system. Also, the operating system may not be Linux, in which case free options to clone are not available.

2.3. Network boot process overview

The client machine boots from a Grub floppy disk. Then, using the Grub BOOTP support, it gets an IP address from a DHCP server. Next, the client machine downloads the kernel and initrd images from the TFTP server. Once the initrd image is mounted in memory, the initialization script is run, making use of the programs and files stored in this image. This script allows block devices contents to be saved in the TFTP server, and contents from the TFTP server to be written to the block devices.

3.1. Setting up DHCP

Details on DHCP are beyond the scope of this article. The "Linux Networking HOWTO" has a chapter on DHCP.

Setting up DHCP is very easy, but if you are in a network environment administered by someone else, it's advisable to use a preexisting DHCP server. If you "own" the network then you can follow this procedure.

Install DHCP, if not installed, from the rpm package, normally found in Linux distributions:

# rpm -ihv dhcp-*.rpm
			

Edit the /etc/dhcpd.conf file to configure DHCP service. In our setup, the server has IP address 10.0.0.1 and provides IP addresses up to 253 clients. Configure /etc/dhcpd.conf according to your environment:

#/etc/dhcpd.conf
server-identifier dhcp.clonedomain.com;
default-lease-time	172800;
max-lease-time		604800;
option domain-name	"clonedomain.com";
subnet 10.0.0.0 netmask 255.255.255.0 {
	range dynamic-bootp 10.0.0.2 10.0.0.254;
}
			

Start the dhcpd server:

/etc/rc.d/init.d/dhcpd start.

3.2. Setting up TFTP

Setting up TFTP is almost as easy as DHCP.

First install from the rpm package:

# rpm -ihv tftp-server-*.rpm
			

Create a directory for the files:

# mkdir /tftpboot
# chown nobody:nobody /tftpboot
			

The directory /tftpboot is owned by user nobody, because this is the default user id set up by tftpd to access the files.

Edit the file /etc/xinetd.d/tftp to look like the following:

service tftp
{
	socket_type	= dgram
	protocol	= udp
	wait		= yes
	user		= root
	server		= /usr/sbin/in.tftpd
	server_args	= -c -s /tftpboot
	disable		= no
	per_source	= 11
	cps		= 100 2
}
			

The changes from the default file are the parameter disable = no (to enable the service) and the server argument -c. This argument allows for the creation of files, which is necessary if you want to save boot or disk images. You may want to make TFTP read only in normal operation.

Then reload xinetd:

/etc/rc.d/init.d/xinetd reload

You can use the tftp command, available from the tftp (client) rpm package, to test the server. At the tftp prompt, you can issue the commands put and get.

3.3. Using different servers

It is possible to use different servers for DHCP and TFTP. This may be necessary if using a preexisting DHCP server. You can configure the next-server parameter in DHCP to point to the TFTP server or you can use the command tftpserver in Grub.

4.1. Kernel

Identify the compressed kernel file:

# cd /boot
# ls vmlinuz-$(uname -r)
vmlinuz-2.4.18-3
			

The version may vary, according to your system. Upload this file to the TFTP server, renaming it to vmlinuz:

# tftp 10.0.0.1
tftp> binary
tftp> put vmlinuz-2.4.18-3 vmlinuz
Sent 1030147 bytes in 2.3 seconds
tftp> quit
			

You may want to keep the file name, or have different names for different hardware models or kernel versions. We use the same names for the kernel and initrd so we don't have to make another boot floppy disk after changing the kernel or the initrd images.

4.2. Files on initrd

Next, make the root file system image for the client. The full listing of the files is in Appendix A.

These files have been taken from a working system as a minimum configuration for having powerful shell (bash), client network utilities (dhcpcd and tftp), and copying and compressing utilities (dd, gzip). Administrative commands (mknod, mount, fdisk and insmod) are also present.

In the working directory create a file named initrd.lst and put these file names on it. To check the existence of these files in your system, run the following command:

# ls -d $(<initrd.lst) > /dev/null
			

You should get an error output like this:

ls: /bin/clone: No such file or directory
ls: /bin/tftp: No such file or directory
ls: /lib/3c59x.o: No such file or directory
			

The first error is a script to be created in the working directory. The second error is the program tftp found in the directory /usr/bin instead of /bin. The third is the network interface card module (probably not yours) found in the directory /lib/modules/$(uname -r)/kernel/drivers/net.

These three files will be discussed in upcoming sections separately soon. If there are other missing files, check for lack of installation or differences in version, distribution or hardware. Adjust the list to match your system.

4.3. Packing initrd

The next step is to make the image. A size of 4 Mbytes was enough to hold the files in our setup. You may increase this size if necessary.

# dd if=/dev/zero of=initrd bs=1024 count=4096
4096+0 records in
4096+0 records out
# yes | mkfs initrd
mke2fs 1.27 (8-Mar-2002)
initrd is not a block special device.
Proceed anyway? (y,n) Filesystem label=
blah blah blah...
# mkdir mnt
# mount -o loop initrd mnt/
# egrep -v "clone|3c59x|tftp" initrd.lst | cpio -pdm mnt
4876 blocks
			

Now the three files excluded by the egrep command. Copy the tftp program from /usr/bin to the image directory:

# cp -p /usr/bin/tftp mnt/bin/
			

Identify the proper module for your network interface card. Use the output of the commands lspci and lsmod to identify the file, which resides in the directory /lib/modules/$(uname -r)/kernel/drivers/net.

Whenever you see a reference to 3c59x, use the name of the module suited for your case.

# cp -p /lib/modules/$(uname -r)/kernel/drivers/net/3c59x.o mnt/lib/
			

Edit the clone script found in Appendix B, changing the variables as explained in Section 6. Make it executable and copy it to the image directory:

# chmod +x clone
# cp -p clone mnt/bin/
			

Unmount, compress, and send the initrd image.

# umount mnt/
# gzip initrd
# tftp 10.0.0.1
tftp> binary
tftp> put initrd.gz
Sent 1155530 bytes in 2.8 seconds
tftp> quit
			

5.1. Grub menu file

In the working directory create a file named grub.conf with the following content:

default=0
timeout=1
title Clone
	bootp
	root (nd)
	kernel /vmlinuz rw root=/dev/ram ramdisk_size=4096 init=/bin/clone
	initrd /initrd.gz
			

In the last four lines are the Grub commands to boot from network:

• bootp, to get an IP address from the DHCP server.

• root (nd), to set the root in the network (TFTP server). An alternative TFTP server could be set before this command using the command tftpserver <tftp server>.

• kernel, to specify the kernel file and its parameters:

  • rw, to specify writable mounting of the root file system.

  • root, to specify where to mount the root file system (in ram memory).

  • ramdisk_size, to specify the ram disk size. 4096 (kbytes) is the default size but if you needed a greater image, change this parameter accordingly.

  • init, to specify (our script) as the first program to run in user mode (in the absence of init and sh).

• initrd to specify the file holding the image of the root file system.

5.2. Compiling Grub with network support

To compile Grub, first download the source tarball from the Grub web site and unpack it. Run configure specifying the menu file you just created and the network interface card model. Run make as usual.

# tar xzf grub-0.92.tar.gz
# cd grub-0.92
# ./configure --enable-preset-menu=../grub.conf --enable-3c90x
# make
			

Again, where you see 3c90x put the model of your network interface card. First check if it is supported by Grub.

5.3. Making the boot floppy disk

Once Grub is compiled, the image of the boot floppy disk is the concatenation of the files stage1/stage1 and stage2/stage2. To make the floppy disk run:

# cat stage1/stage1 stage2/stage2 | dd of=/dev/fd0
			

You should now have a boot floppy disk.

6.1. Saving and restoring disk images

If one of the copy operations is selected, the script asks for the name of the image to be saved or restored. The image name is limited to the elements of the array image_a. A named pipe with the same name as the image is created if doesn't exist. Finally, dd and tftp are invoked simultaneously to transfer the image. Unlike regular ftp, tftp puts and gets named pipes just like regular files.

6.2. Using fdisk

If fdisk command is selected it is invoked for the block device. fdisk is normally run for the whole disk as opposed to one partition. Note that what normally is called /dev/hda will be called /dev/hda0 by the clone script. The input of fdisk could be put in the script to make automatic creation of partitions if desired.

7.1. Saving and restoring files instead of file systems

If you don't want to save a whole disk image just the files within the file system, you can use a similar solution but with tar or cpio instead of dd. Also you need to mount the file system. More commands should be added to the clone script as shown below.

# mkdir /mnt
# mount ${device_name} /mnt
# mknod ${image} p
# tftp ${tftp_server} <<-EOT &
binary
put ${image}
EOT
tar czf ${image}
			

or

# tftp ${tftp_server} <<-EOT &
binary
get ${image}
EOT
tar xzf ${image}
			

You have to put the mkdir and tar programs in the initrd image so that the script can use them.

7.2. Setting up the master boot record

In a situation where you used this setup to reorganize and resize your partitions, you may end up with a disk that doesn't boot. Running the setup command from Grub (including the grub program in the image) should resolve the situation. See the Grub documentation for details.

7.3. Loading necessary modules

Depending on your kernel, additional modules may be necessary to access some block devices like SCSI devices. Just put the necessary modules in the /lib directory of the initrd image and the correspondent insmod commands in the clone script. The same applies for file systems. If, for instance, you want save the files instead of the image of a fat file system you will need the fat.o and vfat.o modules.

7.4. Predefined operations on grub.conf

The Grub menu file grub.conf may be customized to present a few copy options or even execute a predefined operation such as repartitioning the disk and retrieving specified images from network. Again, you can use the concepts presented here to achieve a specific application.