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NAME    [Toc]    [Back]

     diskless -- booting a system over the network

DESCRIPTION    [Toc]    [Back]

     The ability to boot a machine over the network is useful for diskless or
     dataless machines, or as a temporary measure while repairing or reinstalling
 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    [Toc]    [Back]

     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 /etc/make.conf, 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/sys/i386/boot
	   make clean; make; make install
	   cp /boot/pxeboot /tftpdir/

     In phase 3, the kernel again uses DHCP or BOOTP to acquire configuration
     information, and proceeds to mount the root file system and start operation.
  The boot scripts recognize a diskless startup and perform the
     actions found in /etc/rc.d/initdiskless and /etc/rc.d/diskless.  On older
     systems, the scripts are located in /etc/rc.diskless1 and

CONFIGURATION    [Toc]    [Back]

     In order to run a diskless client, you need the following:

     +o	 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.

     +o	 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:



	 where <SERVER>, <GATEWAY> and <ROOT> have the obvious meanings.

     +o	 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 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:

	 Alternativly, if the server contains several independent roots, the
	 file might contain:

	 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 begining with / have the
	 actual path of the client's root prepended to them so the file could
	 instead contain:


	 The diskless scripts create memory file systems to hold the overriden
	 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
	 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

	 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

	       <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 configuration
 in case a number of diskless clients share the same configuration

     +o	 The kernel for the diskless clients, which will be loaded using NFS
	 or TFTP, should be built with at least the following options:

	       options BOOTP
	       options BOOTP_NFSROOT
	       options BOOTP_COMPAT

	 In the devices section add:

	       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    [Toc]    [Back]

     Be warned that using unencrypted NFS to mount root and user partitions
     may expose information such as encryption keys.

BUGS    [Toc]    [Back]

     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.

SEE ALSO    [Toc]    [Back]

     ethers(5), exports(5), bootpd(8), mountd(8), nfsd(8), pxeboot(8),
     reboot(8), tftpd(8)


FreeBSD 5.2.1		       December 23, 2002		 FreeBSD 5.2.1
[ Back ]
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