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docs: Update info with grub.cfg netboot selection order 

Add documentation to the GRUB manual that specifies the order netboot
clients use to select a GRUB configuration file.

Also explain that the feature is enabled by default but can be disabled
by setting the "feature_net_search_cfg" environment variable to "n" in
an embedded configuration file.

Signed-off-by: Robert Marshall <rmarshall@redhat.com>
Signed-off-by: Javier Martinez Canillas <javierm@redhat.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
This commit is contained in:
Robert Marshall 2020-01-22 12:01:56 +01:00 committed by Daniel Kiper
parent cb2f15c544
commit 9f03dc5d7b
1 changed files with 51 additions and 0 deletions
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docs/grub.texi
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@ -2495,6 +2495,57 @@ grub-mknetdir --net-directory=/srv/tftp --subdir=/boot/grub -d /usr/lib/grub/i38
Then follow instructions printed out by grub-mknetdir on configuring your DHCP
server.
The grub.cfg file is placed in the same directory as the path output by
grub-mknetdir hereafter referred to as FWPATH. GRUB will search for its
configuration files in order using the following rules where the appended
value corresponds to a value on the client machine.
@example
@group
@samp{(FWPATH)}/grub.cfg-@samp{(UUID OF MACHINE)}
@samp{(FWPATH)}/grub.cfg-@samp{(MAC ADDRESS OF NIC)}
@samp{(FWPATH)}/grub.cfg-@samp{(IPv4 OR IPv6 ADDRESS)}
@samp{(FWPATH)}/grub.cfg
@end group
@end example
The UUID is the Client Machine Identifier Option Definition as specified in
RFC 4578. The client will only attempt to loouk up a UUID config file if it
was provided by the DHCP server.
The client will only attempt to look up an IPv6 address config once, however,
it will try the IPv4 multiple times. The concrete example below shows what
would happen under the IPv4 case.
@example
@group
UUID: 7726a678-7fc0-4853-a4f6-c85ac36a120a
MAC: 52:54:00:ec:33:81
IPV4: 10.0.0.130 (0A000082)
@end group
@end example
@example
@group
@samp{(FWPATH)}/grub.cfg-7726a678-7fc0-4853-a4f6-c85ac36a120a
@samp{(FWPATH)}/grub.cfg-52-54-00-ec-33-81
@samp{(FWPATH)}/grub.cfg-0A000082
@samp{(FWPATH)}/grub.cfg-0A00008
@samp{(FWPATH)}/grub.cfg-0A0000
@samp{(FWPATH)}/grub.cfg-0A000
@samp{(FWPATH)}/grub.cfg-0A00
@samp{(FWPATH)}/grub.cfg-0A0
@samp{(FWPATH)}/grub.cfg-0A
@samp{(FWPATH)}/grub.cfg-0
@samp{(FWPATH)}/grub.cfg
@end group
@end example
This feature is enabled by default but it can be disabled by setting the
@samp{feature_net_search_cfg} to @samp{n}. Since this happens before the
configuration file is read by GRUB, this option has to be disabled in an
embedded configuration file (@pxref{Embedded configuration}).
After GRUB has started, files on the TFTP server will be accessible via the
@samp{(tftp)} device.