DISKLESS(8) BSD System Manager's Manual DISKLESS(8)NAMEdiskless — booting a system over the network
DESCRIPTION
The ability to boot a machine over the network is useful for diskless or
dataless machines, or as a temporary measure while repairing or re-
installing filesystems on a local disk. This file provides a general
description of the interactions between a client and its server when a
client is booting over the network.
OPERATION
When booting a system over the network, there are three phases of inter‐
action between client and server:
1. The stage-1 bootstrap loads a boot program, from
2. The boot program loads a kernel.
3. The kernel does NFS mounts for root.
Each of these phases are described in further detail below.
In phase 1, the stage-1 bootstrap code loads a boot program, which is
typically able to control the network card. The boot program can be
stored in the BIOS, in a BOOT ROM located on the network card (PXE,
etherboot, netboot), or come from a disk unit (e.g. etherboot or net‐
boot).
In phase 2, the boot program loads a kernel. Operation in this phase
depends on the design of the boot program. Typically, the boot program
uses the BOOTP or DHCP protocol to get the client's IP address and other
boot information, including but not limited to the IP addresses of the
NFS server, router and nameserver, and the name of the kernel to load.
Then the kernel is loaded, either directly using NFS (as it is the case
for etherboot and netboot), or through an intermediate loader called pxe‐
boot and loaded using TFTP or NFS.
In phase 3, the kernel again uses DHCP or BOOTP to acquire configuration
information, and proceeds to mount the root filesystem and start opera‐
tion. The boot scripts recognize a diskless startup and perform the
actions found in /etc/rc.d/initdiskless and /etc/rc.d/diskless.
CONFIGURATION
In order to run a diskless client, you need the following:
· An NFS server which exports a root and /usr partition with appropri‐
ate permissions. The diskless scripts work with readonly partitions,
as long as root is exported with -maproot=0 so that some system files
can be accessed. As an example, /etc/exports can contain the follow‐
ing lines:
<ROOT> -ro -maproot=0 -alldirs <list of diskless clients>
/usr -ro -alldirs <list of diskless clients>
where ⟨ROOT⟩ is the mountpoint on the server of the root partition.
The script /usr/share/examples/diskless/clone_root can be used to
create a shared readonly root partition, but in many cases you may
decide to export (again as readonly) the root directory used by the
server itself.
· a BOOTP or DHCP server. bootpd(8) can be enabled by uncommenting the
bootps line in /etc/inetd.conf. A sample /etc/bootptab can be the
following:
.default:\
hn:ht=1:vm=rfc1048:\
:sm=255.255.255.0:\
:sa=<SERVER>:\
:gw=<GATEWAY>:\
:rp="<SERVER>:<ROOT>":
<CLIENT>:ha=0123456789ab:tc=.default
where ⟨SERVER⟩, ⟨GATEWAY⟩ and ⟨ROOT⟩ have the obvious meanings.
· A properly initialized root partition. The script
/usr/share/examples/diskless/clone_root can help in creating it,
using the server's root partition as a reference. If you are just
starting out you should simply use the server's own root directory,
/, and not try to clone it.
You often do not want to use the same rc.conf or rc.local files for
the diskless boot as you do on the server. The diskless boot scripts
provide a mechanism through which you can override various files in
/etc (as well as other subdirectories of root). The scripts provide
four overriding directories situated in /conf/base, /conf/default,
/conf/<BROADCAST-IP>, and /conf/<MACHINE-IP>. You should always cre‐
ate /conf/base/etc, which will entirely replace the server's /etc on
the diskless machine. You can clone the server's /etc here or you
can create a special file which tells the diskless boot scripts to
remount the server's /etc onto /conf/base/etc. You do this by creat‐
ing the file /conf/base/etc/diskless_remount containing the mount
point to use as a basis of the diskless machine's /etc. For example,
the file might contain:
10.0.0.1:/etc
The diskless scripts create memory filesystems to hold the overridden
directories. Only a 2MB partition is created by default, which may
not be sufficient for your purposes. To override this you can create
the file /conf/base/etc/md_size containing the size, in 512 byte sec‐
tors, of the memory disk to create for that directory.
You then typically provide file-by-file overrides in the
/conf/default/etc directory. At a minimum you must provides over‐
rides for /etc/fstab, /etc/rc.conf, and /etc/rc.local via
/conf/default/etc/fstab, /conf/default/etc/rc.conf, and
/conf/default/etc/rc.local.
Overrides are hierarchical. You can supply network-specific defaults
in the /conf/<BROADCAST-IP>/etc directory, where <BROADCAST-IP> rep‐
resents the broadcast IP address of the diskless system as given to
it via BOOTP. The diskless_remount and md_size features work in any
of these directories. The configuration feature works on directories
other than /etc, you simply create the directory you wish to replace
or override in /conf/{base,default,<BROADCAST-IP>,<MACHINE-IP>}/* and
work it in the same way that you work /etc.
As a minimum, you normally need to have the following in
/conf/default/etc/fstab
<SERVER>:<ROOT> / nfs ro 0 0
<SERVER>:/usr /usr nfs ro 0 0
proc /proc procfs rw 0 0
You also need to create a customized version of
/conf/default/etc/rc.conf which should contain the startup options
for the diskless client, and /conf/default/etc/rc.local which could
be empty but prevents the server's own /etc/rc.local from leaking
onto the diskless system.
In rc.conf, most likely you will not need to set hostname and
ifconfig_* because these will be already set by the startup code.
Finally, it might be convenient to use a case statement using
`hostname` as the switch variable to do machine-specific configura‐
tion in case a number of diskless clients share the same configura‐
tion files.
· The kernel for the diskless clients, which will be loaded using NFS
or TFTP, should be built with at least the following options:
options MFS
options BOOTP
options BOOTP_NFSROOT
options BOOTP_COMPAT
If you use the firewall, remember to default to open or your kernel
will not be able to send/receive the bootp packets.
SECURITY ISSUES
Be warned that using unencrypted NFS to mount root and user partitions
may expose information such as encryption keys.
FILES
/usr/share/examples/diskless/clone_root script to clone root filesystem
/conf/base 1st override
/conf/default 2nd override
/conf/<BROADCAST-IP> 3rd override
/conf/<MACHINE-IP> 4th override
/conf/{base,default,<BROADCAST-IP>,<MACHINE-IP>}/<DIR>
override for /<DIR>
/conf/{base,default,<BROADCAST-IP>,<MACHINE-IP>}/<DIR>/md_size
size of memory disk for /<DIR>
/conf/{base,default,<BROADCAST-IP>,<MACHINE-IP>}/<DIR>/diskless_remount
path to mount on /<DIR>
/conf/{base,default,<BROADCAST-IP>,<MACHINE-IP>}/<DIR>.cpio.gz
archive to unpack to /<DIR>
/conf/{base,default,<BROADCAST-IP>,<MACHINE-IP>}/<DIR>.remove
files to remove from /<DIR>
SEE ALSObootptab(5), ethers(5), exports(5), rc.conf(5), bootpd(8), dhcpd(8)
(pkgsrc/net/isc-dhcpd4), mountd(8), nfsd(8), pxeboot(8), rc(8),
reboot(8), tftpd(8)BUGS
This manpage is probably incomplete.
DragonFly sometimes requires to write onto the root partition, so the
startup scripts mount MFS filesystems on some locations (e.g. /etc and
/var), while trying to preserve the original content. The process might
not handle all cases.
BSD February 19, 2008 BSD
