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How to netboot using ELILO
--------------------------
Copyright (C) 2002-2003 Hewlett-Packard Co.
Contributed by Stephane Eranian <eranian@hpl.hp.com>
Last updated: 03/08/11
EFI has full support for the PXE and DHCP protocol. As such
it is relatively easy to boot a machine from the network using EFI.
The elilo loader has full support for both PXE and DHCP, therefore
it is possible to download the elilo config file, the Linux kernel image
and the initial ramdisk from a remote server. There are many ways
netbooting can be configured but in this document we focus
only on two very common cases:
- netboot but use local root filesystem.
- booting a diskless machine, i.e., use a NFS root filesystem.
1/ How to get EFI to netboot?
You do not need any additional software to get EFI to start a netboot session.
Any EFI machine can be configured to start a PXE/DHCP session IF it has a network
adapter that has support for the UNDI/PXE protocol. Most modern cards do have such
support.
To enable netbooting, you need to go into the EFI boot manager maintenance menu
and 'Add a boot option'. On the screen you see the list of devices to boot from.
At least one of them should be of the form:
Load File [Acpi(PNP0A03,0)/Pci(5|0)/Mac(00D0B7A6FC25)]
which represent Ethernet card (Mac address). If you don't have such option, it means that
you either do not have a network adapter in your machine or it does not have the
UNDI/PXE support in its option ROM.
You need to select this option and give it a logical name such as 'netboot', for instance.
Next, you leave the maintenance menu and go back to the main menu. You now have a new
boot menu option. If you select 'netboot' then EFI will start the PXE/DCHP discovery
request and look for a server to get an IP address.
On the server side, you can use a standard DHCP server, such as the one shipped on
Redhat7.2 (dhcpd) or a PXE server (not yet available for Linux, probably available for Windows).
In this document we show both options. You also need a TFTP server somewhere on the network,
it will be used to download the actual files.
2/ Netbooting using DHCP
There is nothing specific to EFI that needs to be set in the /etc/dhcpd.conf file.
Clearly the filename option must contains the path to the elilo.efi binary.
Elilo will auto-detect whether it was downloaded via PXE or DHCP and it will adapt
the kind of requests it makes to download the other files that it needs, such as
its config file.
A simple dhcpd.conf file which uses fixed IP addresses could be as follows:
subnet 192.168.2.0 netmask 255.255.255.0 {
host test_machine {
hardware ethernet 00:D0:B7:A6:FC:25;
fixed-address 192.168.2.10;
filename "elilo.efi";
option domain-name "mydomain.com";
option host-name "test_machine";
option routers 192.168.2.1;
option subnet-mask 255.255.255.0;
}
}
For the tftp server, you need to make sure that it is ACTIVATED by inetd or xinetd depending
on your distribution. In most distributions, it is disabled by default for security reasons.
On distributions using xinetd, you need to check /etc/xinet.d/tftp. For inetd you need to
check /etc/inetd.conf. It is typical to have the root directory for tftp be /tftpboot but it
can be anything. In this document we will use /tftpboot as the root directory. The files
that we need are as follows:
- elilo.efi
- the elilo config file
- the kernel image
- the initial ramdisk (optional)
a/ Location of the files in the tftp directory tree
For elilo version 3.3b or higher, it is possible to place the files listed above
in any subdirectory below the tftp root. Of course the dhcpd.conf file must
point to the location of elilo.efi and provide the path from the tftp root
directory.
Elilo will look for its config file, the kernel image, the initial ramdisk (optional)
only from the directory it was loaded from. This is useful when the same tftp server
is used to services many different kind of clients.
Here is a simple example, suppose the dhcpd.conf file contains the following definition:
subnet 192.168.2.0 netmask 255.255.255.0 {
host test_machine {
hardware ethernet 00:D0:B7:A6:FC:25;
fixed-address 192.168.2.10;
filename "/rx6000/elilo.efi";
option domain-name "mydomain.com";
option host-name "test_machine";
option routers 192.168.2.1;
option subnet-mask 255.255.255.0;
}
}
Elilo will be downloaded from /tftpboot/rx6000. Then elilo will look
for all the other files it needs in /tftpboot/rx6000. This rule is
applied to all files, including the all the variation of the config
file.
b/ Getting the config file
With DHCP, elilo will first try to download its configuration file. It will try
several file names and they are as follows:
1) AABBCCDD.conf
where AABBCCDD is the hexadecimal representation of the IP address assigned to
the machine by DHCP. The hexadecimal string (AABBCCDD) MUST use upper case
characters.
This filename is an opportunity to specify a machine specific configuration file.
2) AA[BB[CC]][-ia32|ia64].conf
As of version 3.5, elilo will also look for IPv4 class A,B,C
subnet-specific versions of the config file. This is useful when you
want to have a common config file for all machines connected to a
particular subnet.
For example, if your IP address is 10.0.0.1 (0A000001 in hex), elilo
will look first for 0A000001.conf, then 0A0000.conf, then 0A00.conf,
and finally 0A.conf.
Elilo will also try architecture-specific versions of subnet-specific
config files first (So for example, on an Itanium system,
"0A0000-ia64.conf" will be tried before "0A0000.conf")
3) elilo-ia32.config or elilo-ia64.conf
Depending on the machine (client side) architecture elilo will try the IA-32 or
IA-64 file.
This filename is an opportunity to specify a architecture specific configuration file.
This distinction between the architectures is useful when the same TFTP server services
the two types of clients : IA32- and IA-64 machines.
4) elilo.conf
All files use the same format. Elilo will stop at the first match. In case no file is found,
it will try to download a default kernel file name (vmlinux).
c/ Getting the kernel
The second request from elilo is typically the kernel image. The filename is based on what
is in the elilo config file. The path name depends on how the TFTP server is configured.
For security reasons it is customary to have the server do a change root in /tftpboot.
Hence filenames are relative to /tftpboot and therefore you don't need to specify it.
For instance if elilo.conf contains:
image=vmlinuz.249
label=linux-up
root=/dev/sdb2
and the user selects linux-up, then elilo will request a filename of 'vmlinux.249'
which must therefore be in /tftpboot. Check the configuration of your TFTP server for
more on this.
d/ Getting the initial ramdisk
This step is optional. It follows exactly the same naming rules explained for the kernel image.
The initial ramdisk file must therefore be somewhere under /tftpboot.
For instance if elilo.conf contains:
image=vmlinuz.249
label=linux-up
root=/dev/sdb2
initrd=ramdisk/initrd.249
and the user selects linux-up, then elilo will request a filename of 'ramdisk/initrd.249'
which must therefore be under /tftpboot.
e/ Getting IP address information
When elilo is netbooted, the network filesystem module initializes some elilo variables
with the information it received from the DHCP server. At a minimum, it received the
IP address.
The following information is stored in the elilo variables indicated below:
- assigned IP address -> %I
- assigned netmask -> %M
- assigned domainname -> %D
- assigned gateway -> %G
These variables can be used to dynamically adjust the command line arguments passed to the kernel.
See section 5/ below for an example.
3/ Netbooting using PXE
EFI has builtin support for PXE. In fact it first tries PXE and then default to DHCP
when it does not find a valid PXE server.
There is a PXE server package available from Linux/ia32 however this package does not
have the necessary extensions to talk to the EFI side, at least on IA-64 platforms.
There is no need for special options or compile time flags to get elilo to work
with PXE instead of standard DHCP. When netbooted, elilo will automatically detect
if it has been downloaded via PXE or DHCP and it will adujst how subsequent files
are requested.
You need a special version of the DHCPD server developed by the Internet Software Consortium
(http://www.isc.org) with a special patch to add the PXE extensions. Unfortunately as of
version 3.0xx, the patch has not yet made it into the official tree. It is supposed to show
up in version 3.1 of the dhcpd server.
In any case, the elilo package contains a simple example of how you can configure the
/etc/dhcpd.conf file for a PXE-aware DHCP server using the extensions provided in the
patch. You can look in examples/dhcpd-pxe.conf. The syntax is very different from
a standard dhcpd server.
The key elements to keep in mind are the PXE layers used by elilo to request the different
files:
Layer 0 : to get the name of the boot loader (elilo.efi)
Layer 1 : to get the name of the elilo config file
Layer 2 : to get the name of the kernel image
There is an IMPORTANT distinction between those layers. The first two (0,1) and requested
systematically whereas the last one is used only when the configuration file is not found, i.e.,
what is the default kernel to boot. The actual files are STILL downloaded via TFTP. Therefore
the TFTP server must also be configured (see previous section for more on this).
a/ Getting the config file
In this mode, elilo use the PXE layer 1 to get the config file to use. Therefore this must
be set on the server side. Elilo will use the following sequence when
looking for a config file:
- use the name provide by the PXE server Layer 1 or
- elilo-ia64.conf/elilo-ia32.conf or
- elilo.conf
Elilo stops at the first match. With PXE, elilo does not try to download a config file named after
the assigned IP address as it does for DHCP because there is enough flexibility in the PXE server
configuration to do this.
b/ Getting the kernel image
When there is no config file, elilo will use the kernel name returned by
PXE layer 2. If it is not specified there, then it default to 'vmlinux'.
c/ Getting the initial ramdisk
The filename for the ramdisk MUST come from the config file. Elilo does not use a PXE layer
to ask for a default name.
d/ Getting IP address information
When elilo is netbooted, the network filesystem module initializes some elilo variables
with the information it received from the DHCP server. At a minimum, it received the
IP address.
The following information is stored in the variables indicated below:
- assigned IP address -> %I
- assigned netmask -> %M
- assigned domainname -> %D
- assigned gateway -> %G
These variables can be used to dynamically adjust the command line arguments passed to the kernel.
See section 5/ below for an example of how to use the variable.
4/ Netbooting and using a local root filesystem
This is the simplest configuration where the boot loader, its config file, the kernel
and its optional initial ramdisk are downloaded from the network BUT the kernel uses
the local disk for its root filesystem.
For such configuration there is no special option necessary in the elilo config file.
You simply need to specify which partition is the root partition. A typical elilo.conf
would look as follows:
image=vmlinuz.249
label=linux-up
root=/dev/sdb2
initrd=ramdisk/initrd.249
5/ Netbooting a diskless machine
In this configuration we do not use the local machine's disks but instead rely on
a remote server to provide the root filesystem via NFS.
a/ Prerequisites
By default most kernels shipped by distributors do not have the support
compiled in for such configuration. This means that you need to recompile
your own kernel. For instance, vmlinuz-2.4.9 as shipped in Redhat7.2 on
both ia32 and ia64 platforms does not have the support builtin.
To get this configuration to work, you need to have a kernel compiled
such that it accepts a root filesystem over NFS (CONFIG_ROOT_NFS). This
necessitates that the network stack be configured with the, so called,
IP plug-and-play support (CONFIG_IP_PNP).
b/ On the server side
You need to have:
- a NFS file server to provide the root filesystem.
- a DHCP/PXE server to get the IP address and download the boot loader.
Note that both do not need to be on the same machine. There is no special
DHCP/PXE configuration option required to get this working. All you need
is a kernel compiled with the options mentioned in a/. You also need to
make sure that the permission on the NFS server are set appropriately
to allow root access from the client machine (no_root_squash), see
man 'exports' for more on this.
c/ The elilo configuration file
To boot successfully, the kernel needs to:
- get an IP address and related networking parameters
- contact the NFS server to get its root filesystem
The 2.4.x kernel series provides several options to get the IP address:
- it can do an internal DHCP request (CONFIG_IP_PNP_DHCP)
- it can do an internal RARP request (CONFIG_IP_PNP_RARP)
- it can do an internal BOOTP request (CONFIG_IP_PNP_BOOTP)
- it can get the IP address from the command line
The choice is up to you but it is a little bit stupid to go through a
DHCP/BOOTP/RARP phase again when this is already done by the EFI firmware.
So in this document, we describe how you can pass the information provided
by EFI on the command line of the kernel.
The syntax used to pass IP information on the command line is described in
the kernel source tree in Documentation/nfsroot.txt. The option is called
"ip=" and has the following syntax:
ip=<client-ip>:<server-ip>:<gw-ip>:<netmask>:<hostname>:<device>:<autoconf>
To designate the NFS server, you must use the "nfsroot=" option. It has the
following syntax:
nfsroot=[<server-ip>:]<root-dir>[,<nfs-options>]
Depending on how you want your system configured you can hardcode the
values of the parameters in the elilo configuration file. For instance:
image=/vmlinuz
label=nfsroot
description="kernel with NFS root"
append="root=/dev/nfs nfsroot=192.168.2.22:/ia64_rootfs ip=192.168.2.5::192.168.2.1:255.255.255.0:test_machine:eth0:on"
Note the root=/dev/nfs indicates that the root filesystem is over NFS.
This example works fine however, it is not very flexible because the IP
address, the gateway, netmask and hostname are fixed and do not used the
values EFI used to download the boot loader and the kernel.
Elilo provides a way to dynamically customize the parameters passed on the
command line using substitution variables. We describe those variables in
elilovar.txt. The basic idea is to allow the parameters to use the dynamic
information obtained by the DHCP/PXE phase.
The network support in elilo defines several variables which contained
network related information produced by the DHCP/PXE phase. The set of
variable is:
%I -> the IP address obtained by DHCP/PXE
%M -> the netmask obtained by DHCP/PXE
%G -> the gateway obtained by DHCP/PXE
%H -> the hostname obtained by DHCP/PXE
%D -> the domain name obtained by DHCP/PXE
So, the configuration file can then be customized as follows:
image=/vmlinuz
label=nfsroot
description="kernel with NFS root"
append="root=/dev/nfs nfsroot=192.168.2.22:/ia64_rootfs ip=%I::%G:%M:%H:eth0:on"
Not all parameters are necessary or even used by the kernel or the user level
configuration scripts. There is no variable to substitute the NFS server or
the mount point on that server.
In the case of a DHCP boot, this type of customization makes sense only for
the shared configuration file, elilo-ia64.conf/elilo-ia32.conf or elilo.conf.
The configuration file based on the IP address (such as C0A80205.conf in this
case) would provide another way of customizing parameters for a specific
client (IP address). The same thing holds if the name of the config file
returned by the PXE server is specific to a client.
6/ References
More information on the PXE protocol can be found at the following web site:
http://developer.intel.com/ial/wfm/
The source code for the standard and (possibly) PXE-enhanced DHCPD can be
downloaded from:
http://www.isc.org/
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