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>

diff --git a/docs/grub.texi b/docs/grub.texi
index 1b25466c73583ce071e017a1c836c453cdb86d02..83979af381d80edb7325b323e80d9d27fbb4c217 100644 (file)
--- a/docs/grub.texi
+++ b/docs/grub.texi
@@ -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.