A FreeBSD machine can boot over the network and operate without a local disk, using file systems mounted from an NFS server. No system modification is necessary, beyond standard configuration files. Such a system is relatively easy to set up because all the necessary elements are readily available:
The Intel® Preboot eXecution Environment (PXE) can be used to load the kernel over the network. It provides a form of smart boot ROM built into some networking cards or motherboards. See pxeboot(8) for more details.
A sample script
(/usr/share/examples/diskless/clone_root)
eases the creation and maintenance of the workstation's root
file system on the server. The script will probably require
a little customization.
Standard system startup files exist in /etc to detect and support a
diskless system startup.
Swapping, if needed, can be done either to an NFS file or to a local disk.
There are many ways to set up diskless workstations. Many elements are involved, and most can be customized to suit local taste. The following will describe variations on the setup of a complete system, emphasizing simplicity and compatibility with the standard FreeBSD startup scripts. The system described has the following characteristics:
The diskless workstations use a shared, read-only
/and/usr.The root file system is a copy of a standard FreeBSD root, with some configuration files overridden by ones specific to diskless operation or, possibly, to the workstation they belong to.
The parts of the root which have to be writable are overlaid with md(4) file systems. Any changes will be lost when the system reboots.
Caution:
As described, this system is insecure. It should live in a protected area of a network and be untrusted by other hosts.
Setting up diskless workstations is both relatively straightforward and prone to errors. These are sometimes difficult to diagnose for a number of reasons. For example:
Compile time options may determine different behaviors at runtime.
Error messages are often cryptic or totally absent.
In this context, having some knowledge of the background mechanisms involved is useful to solve the problems that may arise.
Several operations need to be performed for a successful bootstrap:
The machine needs to obtain initial parameters such as its IP address, executable filename, server name, and root path. This is done using the DHCP or BOOTP protocols. DHCP is a compatible extension of BOOTP, and uses the same port numbers and basic packet format. It is possible to configure a system to use only BOOTP and bootpd(8) is included in the base FreeBSD system.
DHCP has a number of advantages over BOOTP such as nicer configuration files and support for PXE. This section describes mainly a DHCP configuration, with equivalent examples using bootpd(8) when possible. The sample configuration uses ISC DHCP which is available in the Ports Collection.
The machine needs to transfer one or several programs to local memory. Either TFTP or NFS are used. The choice between TFTP and NFS is a compile time option in several places. A common source of error is to specify filenames for the wrong protocol. TFTP typically transfers all files from a single directory on the server and expects filenames relative to this directory. NFS needs absolute file paths.
The possible intermediate bootstrap programs and the kernel need to be initialized and executed. PXE loads pxeboot(8), which is a modified version of the FreeBSD third stage loader, loader(8). The third stage loader will obtain most parameters necessary to system startup and leave them in the kernel environment before transferring control. It is possible to use a
GENERICkernel in this case.Finally, the machine needs to access its file systems using NFS.
Refer to diskless(8) for more information.
The ISC DHCP server can answer both BOOTP and DHCP requests.
ISC DHCP is not part of
the base system. Install the net/isc-dhcp42-server port or
package.
Once ISC DHCP is installed,
edit its configuration file,
/usr/local/etc/dhcpd.conf. Here
follows a commented example for PXE host
corbieres:
option subnet-mask 255.255.255.0;
option broadcast-address 192.168.4.255;
host corbieres {
hardware ethernet 00:02:b3:27:62:df;
fixed-address corbieres.example.com;
next-server 192.168.4.4; 
filename "pxeboot"; 
option root-path "192.168.4.4:/data/misc/diskless"; 
}
}
| This option tells dhcpd
to send the value in the |
| The |
| The |
| The |
By default, pxeboot(8) loads the kernel via
NFS. It can be compiled to use
TFTP instead by specifying the
LOADER_TFTP_SUPPORT option in
/etc/make.conf. See the comments in
/usr/share/examples/etc/make.conf for
instructions.
There are two other make.conf
options which may be useful for setting up a serial console
diskless machine:
BOOT_PXELDR_PROBE_KEYBOARD, and
BOOT_PXELDR_ALWAYS_SERIAL.
To use PXE when the machine starts,
select the Boot from network option in
the BIOS setup or type a function key
during system initialization.
If PXE is configured to use TFTP, enable tftpd(8) on the file server:
Create a directory from which tftpd(8) will serve the files, such as
/tftpboot.Add this line to
/etc/inetd.conf:tftp dgram udp wait root /usr/libexec/tftpd tftpd -l -s /tftpbootNote:
Some PXE versions require the TCP version of TFTP. In this case, add a second line, replacing
dgram udpwithstream tcp.Tell inetd(8) to reread its configuration file. Add
inetd_enable="YES"to/etc/rc.confin order for this command to execute correctly:#service inetd restart
Place tftpboot
anywhere on the server. Make sure that the location is
set in both /etc/inetd.conf and
/usr/local/etc/dhcpd.conf.
Enable NFS and export the appropriate file system on the NFS server.
Add this line to
/etc/rc.conf:nfs_server_enable="YES"Export the file system where the diskless root directory is located by adding the following to
/etc/exports. Adjust the mount point and replacecorbiereswith the names of the diskless workstations:/data/misc-alldirs -romargaux corbieresTell mountd(8) to reread its configuration file. If NFS is enabled in
/etc/rc.conf, it is recommended to reboot instead.#service mountd restart
When using PXE, building a custom
kernel with the following options is not strictly necessary.
These options cause more DHCP requests
to be issued during kernel startup, with a small risk of
inconsistency between the new values and those retrieved
by pxeboot(8) in some special cases. The advantage
is that the host name will be set. Otherwise, set the
host name in a client-specific
/etc/rc.conf.
The custom kernel can also include
BOOTP_NFSV3,
BOOT_COMPAT and
BOOTP_WIRED_TO. Refer to
NOTES for descriptions of these
options.
These option names are historical and slightly misleading as they actually enable indifferent use of DHCP and BOOTP inside the kernel.
Build the custom kernel, using the instructions in
Chapter 9, Configuring the FreeBSD Kernel, and copy it to the place
specified in
/usr/local/etc/dhcpd.conf.
Create a root file system for the diskless
workstations in the location listed as
root-path in
/usr/local/etc/dhcpd.conf.
This method is quick and will install a complete
virgin system, not just the root file system, into
DESTDIR. Execute the following
script:
Once done, customize
/etc/rc.conf and
/etc/fstab placed into
DESTDIR according to the system's
requirements.
If needed, a swap file located on the server can be accessed via NFS.
The kernel does not support enabling NFS swap at boot time. Swap must be enabled by the startup scripts, by mounting a writable file system and creating and enabling a swap file. To create a swap file:
# dd if=/dev/zero of=/path/to/swapfile bs=1k count=1 oseek=100000To enable the swap file, add the following line to
/etc/rc.conf:
/path/to/swapfileIf the diskless workstation is configured to run
Xorg, adjust the
XDM configuration file as it
puts the error log on /usr by default.
When the server for the root file system is not running FreeBSD, create the root file system on a FreeBSD machine, then copy it to its destination, using tar(1) or cpio(1).
In this situation, there are sometimes problems with
the special files in /dev, due to differing
major/minor integer sizes. A solution to this problem
is to export a directory from the non-FreeBSD server, mount
this directory onto a FreeBSD machine, and use devfs(5)
to allocate device nodes transparently for the
user.