Diskless Operation with PXE

The Intel® Preboot eXecution Environment (PXE) allows an operating system to boot over the network. For example, a FreeBSD system can boot over the network and operate without a local disk, using file systems mounted from an NFS server. PXE support is usually available in the BIOS. 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.

In order to provide the files needed for an operating system to boot over the network, a PXE setup also requires properly configured DHCP, TFTP, and NFS servers, where:

When a computer PXE boots, it receives information over DHCP about where to obtain the initial boot loader file. After the host computer receives this information, it downloads the boot loader via TFTP and then executes the boot loader. In FreeBSD, the boot loader file is /boot/pxeboot. After /boot/pxeboot executes, the FreeBSD kernel is loaded and the rest of the FreeBSD bootup sequence proceeds, as described in Chapter 13, The FreeBSD Booting Process.

This section describes how to configure these services on a FreeBSD system so that other systems can PXE boot into FreeBSD. Refer to diskless(8) for more information.

• Start inetd(8):
  # service inetd start
• Rebuild the FreeBSD kernel and userland (refer to Section 24.6, “Rebuilding World” for more detailed instructions):
  # cd /usr/src
  # make buildworld
  # make buildkernel
• Install FreeBSD into the directory mounted over NFS:
  # make installworld DESTDIR=${NFSROOTDIR}
  # make installkernel DESTDIR=${NFSROOTDIR}
  # make distribution DESTDIR=${NFSROOTDIR}
• Test that the TFTP server works and can download the boot loader which will be obtained via PXE:
  # tftp localhost
  tftp> get FreeBSD/install/boot/pxeboot
  Received 264951 bytes in 0.1 seconds
• Edit ${NFSROOTDIR}/etc/fstab and create an entry to mount the root file system over NFS:
  # Device Mountpoint FSType Options Dump Pass
  myhost.example.com:/b/tftpboot/FreeBSD/install / nfs ro 0 0
  Replace myhost.example.com with the hostname or IP address of the NFS server. In this example, the root file system is mounted read-only in order to prevent NFS clients from potentially deleting the contents of the root file system.
• Set the root password in the PXE environment for client machines which are PXE booting :
  # chroot ${NFSROOTDIR}
  # passwd
• If needed, enable ssh(1) root logins for client machines which are PXE booting by editing ${NFSROOTDIR}/etc/ssh/sshd_config and enabling PermitRootLogin. This option is documented in sshd_config(5).
• Perform any other needed customizations of the PXE environment in ${NFSROOTDIR}. These customizations could include things like installing packages or editing the password file with vipw(8).

When booting from an NFS root volume, /etc/rc detects the NFS boot and runs /etc/rc.initdiskless. In this case, /etc and /var need to be memory backed file systems so that these directories are writable but the NFS root directory is read-only:
# chroot ${NFSROOTDIR}
# mkdir -p conf/base
# tar -c -v -f conf/base/etc.cpio.gz –format cpio –gzip etc
# tar -c -v -f conf/base/var.cpio.gz –format cpio –gzip var
When the system boots, memory file systems for /etc and /var will be created and mounted and the contents of the cpio.gz files will be copied into them.