DISKLESS(8) FreeBSD System Manager's Manual DISKLESS(8)
NAME
diskless - 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 file systems 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
interaction between client and server:
1. The stage-1 bootstrap, typically PXE built into your Ethernet card,
loads a second-stage boot program.
2. The second-stage boot program, typically pxeboot(8), loads modules
and the kernel, and boots the kernel.
3. The kernel NFS mounts the root directory and continues from there.
Each of these phases are described in further detail below.
First, the stage-1 bootstrap loads the stage-2 boot program over the
network. The stage-1 bootstrap typically uses BOOTP or DHCP to obtain
the filename to load, then uses TFTP to load the file. This file is
typically called pxeboot, and should be copied from /boot/pxeboot into
the TFTP directory on the server, which is typically /tftpdir.
The stage-2 boot program then loads additional modules and the kernel.
These files may not exist on the DHCP or BOOTP server. You can use the
next-server option available in DHCP configurations to specify the server
holding the second stage boot files and kernel. The stage-2 program uses
NFS or TFTP to obtain these files. By default, NFS is used. If you are
using pxeboot(8), you can install a version that uses TFTP by setting
LOADER_TFTP_SUPPORT=YES in your make.conf(5), then recompiling and
reinstalling pxeboot(8) via the command listed below. It is often
necessary to use TFTP here so you can place a custom kernel in /tftpdir/.
If you use NFS and do not have a custom root file system for the diskless
client, the stage-2 boot will load your server's kernel as the kernel for
the diskless machine, which may not be what you want to have happen.
cd /usr/src/stand
make clean; make; make install
cp /boot/pxeboot /tftpdir/
In phase 3, the kernel acquires IP networking configuration in one of two
ways, and then proceeds to mount the root file system and start
operation. If the phase 2 loader supports passing network configuration
to the kernel using the kernel environment, then the kernel will
configure the network interface using that information. Otherwise, it
must use DHCP or BOOTP to acquire configuration information. The boot
scripts recognize a diskless startup and perform the actions found in
/etc/rc.d/resolv, /etc/rc.d/tmp, /etc/rc.d/var, and /etc/rc.initdiskless.
CONFIGURATION
In order to run a diskless client, you need the following:
• An NFS server which exports a root and /usr partitions with
appropriate permissions. The diskless scripts work with read-only
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 following lines:
<ROOT> -ro -maproot=0 -alldirs <list of diskless clients>
/usr -ro -alldirs <list of diskless clients>
where <ROOT> is the mount point on the server of the root partition.
The script /usr/share/examples/diskless/clone_root can be used to
create a shared read-only root partition, but in many cases you may
decide to export (again as read-only) 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).
One difference that you should pay particular attention to is the
value of local_startup in /etc/defaults/rc.conf. A typical value for
a diskless boot is mountcritremote, however your needs may be
different.
The scripts provide four overriding directories situated in
/conf/base, /conf/default, /conf/<broadcast-ip>, and
/conf/<machine-ip>. You should always create /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 creating 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
Alternatively, if the server contains several independent roots, the
file might contain:
10.0.0.1:/usr/diskless/4.7-RELEASE/etc
This would work, but if you copied /usr/diskless/4.7-RELEASE to
/usr/diskless/4.8-RELEASE and upgraded the installation, you would
need to modify the diskless_remount files to reflect that move. To
avoid that, paths in diskless_remount files beginning with / have the
actual path of the client's root prepended to them so the file could
instead contain:
/etc
The diskless scripts create memory file systems to hold the
overridden directories. Only a 5MB 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 sectors, 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 provide
overrides 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/<BROADCASTIP>/etc directory, where <BROADCASTIP>
represents 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 then /etc, you simply create the directory you wish
to replace or override in /conf/{base,default,<broadcast>,<ip>}/* and
work it in the same way that you work /etc.
Since you normally clone the server's /etc using the
/conf/base/etc/diskless_remount, you might wish to remove unneeded
files from the memory file system. For example, if the server has a
firewall but you do not, you might wish to remove /etc/ipfw.conf.
You can do this by creating a /conf/base/<DIRECTORY>.remove file.
For example, /conf/base/etc.remove, which contains a list of relative
paths that the boot scripts should remove from the memory file
systems.
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
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
configuration in case a number of diskless clients share the same
configuration files.
• The kernel for the diskless clients, which will be loaded using NFS
or TFTP, must include support for the NFS client:
options NFSCL
options NFS_ROOT
If you are using a boot mechanism that does not pass network
configuration to the kernel using the kernel environment, you will
also need to include the following options:
options BOOTP
options BOOTP_NFSROOT
options BOOTP_COMPAT
Note: the PXE environment does not require these options.
The diskless booting environment relies on memory-backed file systems
to support temporary local storage in the event that the root file
system is mounted read-only; as such, it is necessary to add the
following to the device section of the kernel configuration:
device md
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.
SEE ALSO
ethers(5), exports(5), make.conf(5), bootpd(8), mountd(8), nfsd(8),
pxeboot(8), reboot(8), tftpd(8)
ports/net/etherboot
HISTORY
The diskless environment first appeared in FreeBSD 2.2.5.
BUGS
This manpage is probably incomplete.
FreeBSD sometimes requires to write onto the root partition, so the
startup scripts mount MFS file systems on some locations (e.g. /etc and
/var), while trying to preserve the original content. The process might
not handle all cases.
FreeBSD 13.1-RELEASE-p6 May 3, 2020 FreeBSD 13.1-RELEASE-p6
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