freebsd-skq/share/man/man8/diskless.8

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.\" Copyright (c) 1994 Gordon W. Ross, Theo de Raadt
.\" Updated by Luigi Rizzo, Robert Watson
.\" All rights reserved.
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.\" notice, this list of conditions and the following disclaimer in the
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.\" derived from this software without specific prior written permission.
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1999-08-28 00:22:10 +00:00
.\" $FreeBSD$
.\"
.Dd May 3, 2020
.Dt DISKLESS 8
.Os
.Sh NAME
.Nm diskless
.Nd booting a system over the network
.Sh DESCRIPTION
The ability to boot a machine over the network is useful for
.Em diskless
or
.Em dataless
machines, or as a temporary measure while repairing or
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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.
.Sh OPERATION
When booting a system over the network, there are three
phases of interaction between client and server:
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.Bl -enum
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.It
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The stage-1 bootstrap, typically PXE built into your Ethernet
card, loads a second-stage boot program.
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.It
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The second-stage boot program, typically
.Xr pxeboot 8 ,
loads modules and
the kernel, and boots the kernel.
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.It
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The kernel
.Tn NFS
mounts the root directory and continues from there.
.El
.Pp
Each of these phases are described in further detail below.
.Pp
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First, the stage-1 bootstrap loads the stage-2 boot program over
the network.
The stage-1 bootstrap typically uses
.Tn BOOTP
or
.Tn DHCP
to obtain the filename to load, then uses
.Tn TFTP
to load the file.
This file is typically called
.Pa pxeboot ,
and should be copied from
.Pa /boot/pxeboot
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into the
.Tn TFTP
directory on the server, which is typically
.Pa /tftpdir .
.Pp
The stage-2 boot program then loads additional modules and the kernel.
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These files may not exist on the
.Tn DHCP
or
.Tn BOOTP
server.
You can use the
.Ic next-server
option available in
.Tn DHCP
configurations to specify the server holding
the second stage boot files and kernel.
The stage-2 program uses
.Tn NFS
or
.Tn TFTP
to obtain these files.
By default,
.Tn NFS
is used.
If you are using
.Xr pxeboot 8 ,
you can install a version that uses
.Tn TFTP
by setting
.Li LOADER_TFTP_SUPPORT=YES
in your
.Xr make.conf 5 ,
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then recompiling and reinstalling
.Xr pxeboot 8
via the command listed below.
It is often necessary to use
.Tn TFTP
here so you can place a custom kernel
in
.Pa /tftpdir/ .
If you use
.Tn NFS
and do not have a custom root file system for the
.Nm
client, the stage-2 boot will load your server's kernel as the kernel for
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the
.Nm
machine, which may not be what you want to have happen.
.Bd -literal -offset indent
cd /usr/src/stand
make clean; make; make install
cp /boot/pxeboot /tftpdir/
.Ed
.Pp
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
operation.
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
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.Tn DHCP
or
.Tn BOOTP
to acquire
configuration information.
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The boot
scripts recognize a
.Nm
startup and perform
the actions found in
.Pa /etc/rc.d/resolv ,
.Pa /etc/rc.d/tmp ,
.Pa /etc/rc.d/var ,
and
.Pa /etc/rc.initdiskless .
.Sh CONFIGURATION
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In order to run a
.Nm
client, you need the following:
.Bl -bullet
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.It
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An
.Tn NFS
server which exports a root and
.Pa /usr
partitions with appropriate permissions.
The
.Nm
scripts work with read-only partitions, as long as root is exported with
.Fl maproot Ns =0
so that some system files can be accessed.
As an example,
.Pa /etc/exports
can contain the following lines:
.Bd -literal -offset indent
<ROOT> -ro -maproot=0 -alldirs <list of diskless clients>
/usr -ro -alldirs <list of diskless clients>
.Ed
.Pp
where
.Aq ROOT
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is the mount point on the server of the root partition.
The script
.Pa /usr/share/examples/diskless/clone_root
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can be used to create a shared read-only root partition,
but in many cases you may decide to export
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(again as read-only) the root directory used by
the server itself.
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.It
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A
.Tn BOOTP
or
.Tn DHCP
server.
.Xr bootpd 8
can be enabled by
uncommenting the
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.Dq Li bootps
line in
.Pa /etc/inetd.conf .
A sample
.Pa /etc/bootptab
can be the following:
.Bd -literal -offset indent
.default:\\
hn:ht=1:vm=rfc1048:\\
:sm=255.255.255.0:\\
:sa=<SERVER>:\\
:gw=<GATEWAY>:\\
:rp="<SERVER>:<ROOT>":
<CLIENT>:ha=0123456789ab:tc=.default
.Ed
.Pp
where
.Aq SERVER ,
.Aq GATEWAY
and
.Aq ROOT
have the obvious meanings.
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.It
A properly initialized root partition.
The script
.Pa /usr/share/examples/diskless/clone_root
can help in creating it, using the server's root partition
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as a reference.
If you are just starting out, you should
simply use the server's own root directory,
.Pa / ,
and not try to clone it.
.Pp
You often do not want to use the same
.Pa rc.conf
or
.Pa rc.local
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files for the
.Nm
boot as you do on the server.
The
.Nm
boot
scripts provide a mechanism through which you can override various files
in
.Pa /etc
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(as well as other subdirectories of root).
.Pp
One difference that you should pay particular attention to is
the value of
.Va local_startup
in
.Pa /etc/defaults/rc.conf .
A typical value for a
.Nm
boot is
.Va mountcritremote ,
however your needs may be different.
.Pp
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The scripts provide four
overriding directories situated in
.Pa /conf/base ,
.Pa /conf/default ,
.Pa /conf/<broadcast-ip> ,
and
.Pa /conf/<machine-ip> .
You should always create
.Pa /conf/base/etc ,
which will entirely replace the server's
.Pa /etc
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on the
.Nm
machine.
You can clone the server's
.Pa /etc
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here or you can create a special file which tells the
.Nm
boot scripts
to remount the server's
.Pa /etc
onto
.Pa /conf/base/etc .
You do this by creating the file
.Pa /conf/base/etc/diskless_remount
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containing the mount point to use as a basis of the
.Nm
machine's
.Pa /etc .
For example, the file might contain:
.Pp
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.Dl 10.0.0.1:/etc
.Pp
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Alternatively, if the server contains several independent roots, the file
might contain:
.Pp
.Dl 10.0.0.1:/usr/diskless/4.7-RELEASE/etc
.Pp
This would work, but if you copied
.Pa /usr/diskless/4.7-RELEASE
to
.Pa /usr/diskless/4.8-RELEASE
and upgraded the installation, you would need to modify the
.Pa diskless_remount
files to reflect that move.
To avoid that, paths in
.Pa diskless_remount
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files beginning with
.Pa /
have the actual path of the client's root prepended to them so the file
could instead contain:
.Pp
.Dl /etc
.Pp
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The
.Nm
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scripts create memory file systems to hold the overridden
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directories.
Only a 5MB partition is created by default, which may not
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be sufficient for your purposes.
To override this, you can create the
file
.Pa /conf/base/etc/md_size
containing the size, in 512 byte sectors, of the memory disk to create
for that directory.
.Pp
You then typically provide file-by-file overrides in the
.Pa /conf/default/etc
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directory.
At a minimum, you must provide overrides for
.Pa /etc/fstab , /etc/rc.conf ,
and
.Pa /etc/rc.local
via
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.Pa /conf/default/etc/fstab , /conf/default/etc/rc.conf ,
and
.Pa /conf/default/etc/rc.local .
.Pp
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Overrides are hierarchical.
You can supply network-specific defaults
in the
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.Pa /conf/ Ns Ao Ar BROADCASTIP Ac Ns Pa /etc
directory, where
.Aq Ar BROADCASTIP
represents the broadcast IP address of
the
.Nm
system as given to it via
.Tn BOOTP .
The
.Pa diskless_remount
and
.Pa md_size
features work in any of these directories.
The configuration feature works on directories other then
.Pa /etc ,
you simply create the directory you wish to replace or override in
.Pa /conf/{base,default,<broadcast>,<ip>}/*
and work it in the same way that you work
.Pa /etc .
.Pp
Since you normally clone the server's
.Pa /etc
using the
.Pa /conf/base/etc/diskless_remount ,
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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
.Pa /etc/ipfw.conf .
You can do this by creating a
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.Pa /conf/base/ Ns Ao Ar DIRECTORY Ac Ns Pa .remove
file.
For example,
.Pa /conf/base/etc.remove ,
which contains a list of relative paths that the boot scripts should remove
from the memory file systems.
.Pp
As a minimum, you normally need to have the following in
.Pa /conf/default/etc/fstab
.Bd -literal -offset indent
<SERVER>:<ROOT> / nfs ro 0 0
<SERVER>:/usr /usr nfs ro 0 0
.Ed
.Pp
You also need to create a customized version of
.Pa /conf/default/etc/rc.conf
which should contain
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the startup options for the
.Nm
client, and
.Pa /conf/default/etc/rc.local
which could be empty but prevents the server's own
.Pa /etc/rc.local
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from leaking onto the
.Nm
system.
.Pp
In
.Pa rc.conf ,
most likely
you will not need to set
.Va hostname
and
.Va ifconfig_*
because these will be already set by the startup code.
Finally, it might be convenient to use a
.Ic case
statement using
.Li `hostname`
as the switch variable to do machine-specific configuration
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in case a number of
.Nm
clients share the same configuration
files.
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.It
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The kernel for the
.Nm
clients, which will be loaded using
.Tn NFS
or
.Tn TFTP ,
must include support for the NFS client:
.Pp
.D1 Cd "options NFSCL"
.D1 Cd "options NFS_ROOT"
.Pp
If you are using a boot mechanism that does not pass network configuration
to the kernel using the kernel environment, you will also need to include
the following options:
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.Pp
.D1 Cd "options BOOTP"
.D1 Cd "options BOOTP_NFSROOT"
.D1 Cd "options BOOTP_COMPAT"
.Pp
.Em Note :
the PXE environment does not require these options.
.Pp
The
.Nm
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 following
to the device section of the kernel configuration:
.Pp
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.D1 Cd "device md"
.Pp
If you use the firewall, remember to default to
.Dq open ,
or your kernel
will not be able to send/receive the
.Tn BOOTP
packets.
.El
.Sh SECURITY ISSUES
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Be warned that using unencrypted
.Tn NFS
to mount root and user
partitions may expose information such as
encryption keys.
.Sh HISTORY
The
.Nm
environment first appeared in
.Fx 2.2.5 .
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.Sh SEE ALSO
.Xr ethers 5 ,
.Xr exports 5 ,
.Xr make.conf 5 ,
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.Xr bootpd 8 ,
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.Xr mountd 8 ,
.Xr nfsd 8 ,
.Xr pxeboot 8 ,
.Xr reboot 8 ,
.Xr tftpd 8
.Pp
.Pa ports/net/etherboot
.Sh BUGS
This manpage is probably incomplete.
.Pp
.Fx
sometimes requires to write onto
the root partition, so the startup scripts mount MFS
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file systems on some locations (e.g.\&
.Pa /etc
and
.Pa /var ) ,
while
trying to preserve the original content.
The process might not handle all cases.