The Intel® Preboot eXecution Environment (PXE) allows booting the operating system over the network. PXE support is usually provided in the BIOS of modern motherboards, where it can be enabled in the BIOS settings which enable booting from the network. A fully functioning PXE setup also requires properly configured DHCP and TFTP servers.
When the host computer boots, it receives information over
DHCP about where to obtain the initial boot
loader via TFTP. After the host computer receives this
information, it downloads the boot loader via
TFTP, and then executes the boot loader.
This is documented in section 2.2.1 of the Preboot
Execution Environment (PXE) Specification. In FreeBSD,
the boot loader retrieved during the PXE
process is /boot/pxeboot. After
/boot/pxeboot executes, the FreeBSD kernel is
loaded, and the rest of the FreeBSD bootup sequence proceeds.
Refer to Chapter 13, The FreeBSD Booting Process for more information about the
FreeBSD booting process.
Choose a directory which will have a FreeBSD installation which will be NFS mountable. For example, a directory such as
/b/tftpboot/FreeBSD/installcan be used.#export NFSROOTDIR=/b/tftpboot/FreeBSD/install#mkdir -p ${NFSROOTDIR}Enable the NFS server by following the instructions in Section 30.3.2, “Configuring NFS”.
Export the directory via NFS by adding the following to
/etc/exports:/b -ro -alldirsRestart the NFS server:
#service nfsd restartEnable inetd(8) by following the steps outlined in Section 30.2.2, “Settings”.
Add the following line to
/etc/inetd.conf:tftp dgram udp wait root /usr/libexec/tftpd tftpd -l -s /b/tftpbootRestart inetd:
#service inetd restartRebuild the FreeBSD kernel and userland:
#cd /usr/src#make buildworld#make buildkernelInstall FreeBSD into the directory mounted over NFS:
#make installworld DESTDIR=${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 localhosttftp>get FreeBSD/install/boot/pxebootReceived 264951 bytes in 0.1 secondsEdit
${NFSROOTDIR}/etc/fstaband 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 0Replace
myhost.example.comwith the hostname or IP address of your 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 chroot(8) environment.
#chroot ${NFSROOTDIR}#passwdThis will set the root password for client machines which are PXE booting.
Enable ssh root logins for client machines which are PXE booting by editing
${NFSROOTDIR}/etc/ssh/sshd_configand enabling thePermitRootLoginoption. This is documented in sshd_config(5).Perform other customizations of the chroot(8) environment in ${NFSROOTDIR}. These customizations could include things like adding packages with pkg_add(1), editing the password file with vipw(8), or editing amd.conf(5) maps for automounting. For example:
#chroot ${NFSROOTDIR}#pkg_add -r bash
If you boot from an NFS root volume,
/etc/rc detects that you booted over NFS
and runs the /etc/rc.initdiskless script.
Read the comments in this script to understand what is going
on. We need to
make /etc
and /var
memory backed file systems because
these directories need to be 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 varWhen 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.
PXE requires a TFTP server and a DHCP server to be set up. The DHCP server does not necessarily need to be the same machine as the TFTP server, but it needs to be accessible in your network.
Install the DHCP server by following the instructions documented at Section 30.5.7, “Installing and Configuring a DHCP Server”. Make sure that
/etc/rc.confand/usr/local/etc/dhcpd.confare correctly configured.In
/usr/local/etc/dhcpd.conf, configure thenext-server,filename, andoption root-pathsettings, to specify your TFTP server IP address, the path to/boot/pxebootin TFTP, and the path to the NFS root file system. Here is a sampledhcpd.confsetup:subnet 192.168.0.0 netmask 255.255.255.0 { range 192.168.0.2 192.168.0.3 ; option subnet-mask 255.255.255.0 ; option routers 192.168.0.1 ; option broadcast-address 192.168.0.255 ; option domain-name-server 192.168.35.35, 192.168.35.36 ; option domain-name "example.com"; # IP address of TFTP server next-server 192.168.0.1 ; # path of boot loader obtained # via tftp filename "FreeBSD/install/boot/pxeboot" ; # pxeboot boot loader will try to NFS mount this directory for root FS option root-path "192.168.0.1:/b/tftpboot/FreeBSD/install/" ; }
When the client machine boots up, enter the BIOS configuration menu. Configure the BIOS to boot from the network. If all your previous configuration steps are correct, then everything should "just work".
Use the
net/wiresharkport to debug the network traffic involved during the PXE booting process, which is illustrated in the diagram below. In Section 32.8.3, “Setting up the DHCP Server”, an example configuration is shown where the DHCP, TFTP, and NFS servers are actually on the same machine. However, these severs can be on separate machines.Figure 32.1. PXE Booting Process with NFS Root Mount
Client broadcasts DHCPDISCOVER.
DHCP server responds with IP address,
next-server,filename, androot-path.
Client sends TFTP request to
next-serverasking to retrievefilename.
TFTP server responds and sends
filenameto client.
Client executes
filenamewhich is pxeboot(8). pxeboot(8) loads the kernel. When the kernel executes, the root filesystem specified byroot-pathis mounted over NFS.Make sure that the
pxebootfile can be retrieved by TFTP. On your TFTP server, look in/var/log/xferlogto ensure that thepxebootfile is being retrieved from the correct location. To test the configuration fromdhcpd.confabove:#tftp 192.168.0.1tftp>get FreeBSD/install/boot/pxebootReceived 264951 bytes in 0.1 secondsRead tftpd(8) and tftp(1). The
BUGSsections in these pages document some limitations with TFTP.Make sure that the root file system can be mounted via NFS. To test configuration from
dhcpd.confabove:#mount -t nfs 192.168.0.1:/b/tftpboot/FreeBSD/install /mntRead the code in
src/sys/boot/i386/libi386/pxe.cto understand how thepxebootloader sets variables likeboot.nfsroot.serverandboot.nfsroot.path. These variables are then used in the NFS diskless root mount code insrc/sys/nfsclient/nfs_diskless.c.Read pxeboot(8) and loader(8).