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diskless - booting a system over the network



      diskless - booting a system over the network


      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.


      When booting a system over the network, there are three phases of inter‐
      action 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 net‐
      work.  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 typi‐
      cally 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 rein‐
      stalling 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/boot/i386
            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 opera‐
      tion.  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.


      In order to run a diskless client, you need the following:
            An NFS server which exports a root and /usr partitions with appropri‐
          ate permissions.  The diskless scripts work with read-only parti‐
          tions, as long as root is exported with -maproot=0 so that some sys‐
          tem 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
          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 differ‐
          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
          Alternatively, 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 beginning 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 overrid‐
          den 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 over‐
          rides 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〉 repre‐
          sents 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 sys‐
          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 configura‐
          tion in case a number of diskless clients share the same configura‐
          tion files.
            The kernel for the diskless clients, which will be loaded using NFS
          or TFTP, must include support for the NFS client:
                options NFSCLIENT
                options NFS_ROOT
          If you are using a boot mechanism that does not pass network configu‐
          ration 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 fol‐
          lowing to the device section of the kernel configuration:
                device md
          If you use the firewall, remember to default to “open”, or your ker‐
          nel will not be able to send/receive the BOOTP packets.
      Be warned that using unencrypted NFS to mount root and user partitions
      may expose information such as encryption keys.
      ethers(5), exports(5), make.conf(5), bootpd(8), mountd(8), nfsd(8),
      pxeboot(8), reboot(8), tftpd(8)


      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.


Based on BSD UNIX
FreeBSD is an advanced operating system for x86 compatible (including Pentium and Athlon), amd64 compatible (including Opteron, Athlon64, and EM64T), UltraSPARC, IA-64, PC-98 and ARM architectures. It is derived from BSD, the version of UNIX developed at the University of California, Berkeley. It is developed and maintained by a large team of individuals. Additional platforms are in various stages of development.