Diskless booting operation overview

Network Boot and Exotic Root HOWTO
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2. Diskless booting operation overview

Hey, you think it's time to start with the real stuff, right? Here we go.

2.1. Obtaining IP parameters

One could wonder how a station may boot over an IP network if it doesn't even know its own IP address. In fact, three protocols enable the client to obtain this information and some additional configuration parameters:

RARP: this is the simplest of these protocols. However I guess it does not enable the server to specify how the client should download the kernel, so we won't use it (In fact, there is a convention that uses the IP address of the workstation as filename, e.g. a client getting the address 192.168.42.12 by RARP might ask for /tftpboot/192.168.42.12 by TFTP, as the linux kernel does. The filename might also be the hex form of the IP address, this is implementation dependant, and is not mandatory.).
BOOTP: this protocol allows a server to provide the client (identified by its hardware MAC address) with much information, in particular its IP address, subnet mask, broadcast address, network address, gateway address, host name, and kernel loading path. This is the one we will use.
DHCP: this is an extension of BOOTP.

2.2. Loading the kernel

When the client has got its IP parameters, if the kernel is not on a local support (like a floppy, a cdrom, or a hard drive), the client will start to download it via TFTP. Its location is given by the BOOTP/DHCP server. A server (not necessarily the BOOTP/DHCP server) will also have to run a TFTP daemon for non local kernels. The kernel one obtains after compilation can not be used "as is" for BOOTP/DHCP operation, its binary image has to be modified with the mknbi utility (and then modified again with the imggen utility if you use LanWorks EPROMs). The mknbi utility should also be used to modify kernels that will be written in a ROM.

2.3. Mounting the root filesystem

After the kernel has started, it will try to mount its root filesystem. The location of this filesystem is also obtained through BOOTP/DHCP, and it is mounted via NFS. It means a client may use BOOTP twice for booting: the first time to get its kernel, and the second time to learn the location of the root filesystem (which may be on a third server).

Another solution is to use a ramdisk as root filesystem. In this case, the ramdisk image is obtained with the kernel via TFTP.

2.4. Terminating the boot process

When the root filesystem is mounted, you can start breathing: you can at least use your swiss army knife with its sh, sed, and awk blades. In fact, you will have to customize the initialization scripts of the client's filesystem: for instance, you will have to remove all hard drive, floppy or cdrom related stuff from /etc/fstab (when your stations are not equipped with these devices), you may also have to inhibit swap partitions activation (note there is a way to swap over NFS or network block devices). You also will have to automagically generate all network configuration files at boot time if several clients use the same remote root filesystem.

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