Remote Linux Explained

Whether you use the two-step PXE boot process (network loader and kernel) or the one-step Etherboot process (just a kernel), eventually the kernel is read into memory on the client and control is transferred to it. You might be wondering, what are the differences between a network boot kernel and a typical kernel built to boot from the local hard drive? The first decision you have to make is whether the kernel is modular or monolithic—meaning no loadable modules. If you've ever built a Linux kernel, you're aware that features are either selected with “Y” (include the feature), “N” (do not include the feature) or “M” (load on demand). If your kernel is monolithic, you cannot have any M features.

Several flags have to be turned on for a monolithic network boot kernel. First, if you're creating a monolithic kernel, you need to turn off modules support. In the .config file, you will see

#CONFIG_MODULES is not set

and if you're doing a monolithic kernel, you would turn it off, as in

CONFIG_MODULES=n

CONFIG_EXT2_FS=y

CONFIG_NFS_FS=y
CONFIG_ROOT_FS=y

CONFIG_NETDEVICES=y
CONFIG_EEXPRESS_PRO100=y

CONFIG_IP_PNP_RARP=y
CONFIG_IP_PNP_BOOTP=y
CONFIG_IP_PNP_DHCP=y

CONFIG_BLK_DEV_RAM=y
CONFIG_BLK_DEV_INITRD=y

In the modular kernel, you are allowed to use M to select modules to be loaded on demand by the kernel. Since the modules are not statically bound in the kernel, there must be a place to load the modules from when they are needed. The place provided by Linux to load the kernel modules is the RAM disk. This RAM disk is used only during the boot process and only to provide the kernel modules. Another use of the RAM disk, to provide the remote root filesystem, is outside the scope of this article.

To demonstrate the use of the modular kernel, we can make support for the MINIX filesystem modular, to support a MINIX root filesystem that will be loaded from diskette. To create a modular kernel, the first thing to do is turn on modular support and rebuild the kernel:

CONFIG_MODULES=y

In our example, we're going to provide MINIX support via a dynamic module, so we'll make that option an M:

CONFIG_MINIX_FS=M

DEFAULT bzImage
APPEND vga=extended initrd=minix.gz root=/dev/fd0 ro
PROMPT 1
TIMEOUT 50

We will have to create a RAM disk that has the MINIX module (minix.o) on it, as well as insmod (the command to load the MINIX module), the console device (/dev/console) and linuxrc, the command that gets called when the RAM disk is invoked. (Refer to the initrd.txt file, written by Werner Almesberger and Hans Lerman and distributed as part of the kernel RPM, for a complete description of how RAM disks work in Linux.) To create the custom RAM disk that will mount the MINIX modular dynamically, and MINIX root filesystem from diskette, you must create a file and zero it out using dd:

dd if=/dev/zero of=minixroot bs=1k count=4096

Next, associate the file with a loopback device, /dev/loop0:

losetup /dev/loop0 minixroot

mkfs.ext2 /dev/loop0

mount /dev/loop0 /mnt

mkdir /mnt/dev /mnt/lib /mnt/sbin

mkdev /mnt/dev/console c 5 1

cp /usr/src/linux/fs/minix/minix.o /mnt/lib/minix.o
cp /sbin/sash /mnt/sbin/sash
cp /sbin/insmod /mnt/sbin/insmod

ldd /sbin/insmod

cp /lib/libc.so.6 /mnt/lib
cp /lib/ld-linux.so.2 /mnt/lib

#!/sbin/sash
/sbin/insmod /lib/minix.o

chmod 777 linuxrc

umount /mnt
losetup -d /dev/loop0

gzip minixroot