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Re: Obtaining the MAC address of the boot NIC for a PXE boot

From: Andrey Borzenkov
Subject: Re: Obtaining the MAC address of the boot NIC for a PXE boot
Date: Wed, 1 May 2013 18:59:34 +0400

В Mon, 29 Apr 2013 05:55:32 -0700 (PDT)
Rigoberto Corujo <address@hidden> пишет:

> Hello everyone,
> With pxelinux, you can specify "IPAPPEND 2" in your boot loader configuration 
> file and the MAC address of the boot NIC automatically gets appended to your 
> kernel parameters as "BOOTIF=XX:XX:XX:XX:XX:XX". 
> I am now trying to become familiar with Grub 2, because if its EFI support.   
> I noticed that with Grub 2, some of the documentation, which may be outdated, 
> says that there is a "net_pxe_mac" variable which, as I understand it, should 
> contain the MAC address of the boot NIC.  However, I've built the latest Grub 
> 2 sources and, when I PXE boot and drop into a grub shell prompt, if type 
> "set", I don't see "net_pxe_mac" listed as one of the variables.  I know 
> there is a "net_ls_addr" command which prints out something like "efinet8 
> 3c:4a:92:b2:6a:e8", but I don't know how to use Grub scripting to 
> parse out the MAC address.  I also don't know if there will ever be a case 
> where "net_ls_addr" will print out multiple entries on a PXE boot, making it 
> difficult to determine which one corresponds to the boot NIC.
> Basically, I'm using the "ksdevice=bootif" kernel parameter when PXE booting 
> my Red Hat kernel.  With PXE Linux, "bootif" provides the MAC address of the 
> boot NIC.  I was hoping to do something like "ksdevice=${net_pxe_mac}", but 
> since "net_pxe_mac" does not appear to be getting set, I need to find some 
> other method.

As it stands currently, net_pxe_* variables are defined on PC BIOS
platform only. For UEFI (which you apparently have) GRUB2 defines
net_efinetNN_* variables where efinetNN is symbolic name for interface
that was used to boot GRUB2.

May be GRUB2 should also define net_pxe_* namespace for the case of
UEFI. There is no real way to find out which interface was used for
booting and even if there were, grub does not support nested
variables substitution or eval'ing (like ${net_${boot_if}_mac).

Probably, adding "eval" support is really the most simple. Could you
test the patch below. What it does, is

- it adds "eval" command (same as known from UNIX shells) which
  executes its argument

- it exports boot interface as "efi_boot_interface" variable

This should allow you to do

if $grub_platform = efi ; then
  eval "set net_pxe_mac=\$net_${efi_boot_interface}_mac" 

and then use $net_pxe_mac on both platforms.

If it works for you, I will clean it up; for consistency this is
probably needed for ieee1275 as well.

 grub-core/Makefile.core.def        |  5 ++++
 grub-core/commands/eval.c          | 51 ++++++++++++++++++++++++++++++++++++++
 grub-core/net/drivers/efi/efinet.c |  2 ++
 3 files changed, 58 insertions(+)

diff --git a/grub-core/Makefile.core.def b/grub-core/Makefile.core.def
index 7e19acb..8844f2f 100644
--- a/grub-core/Makefile.core.def
+++ b/grub-core/Makefile.core.def
@@ -699,6 +699,11 @@ module = {
 module = {
+  name = eval;
+  common = commands/eval.c;
+module = {
   name = extcmd;
   common = commands/extcmd.c;
   common = lib/arg.c;
diff --git a/grub-core/commands/eval.c b/grub-core/commands/eval.c
new file mode 100644
index 0000000..a21ff89
--- /dev/null
+++ b/grub-core/commands/eval.c
@@ -0,0 +1,51 @@
+ *  GRUB  --  GRand Unified Bootloader
+ *  Copyright (C) 2009  Free Software Foundation, Inc.
+ *
+ *  GRUB is free software: you can redistribute it and/or modify
+ *  it under the terms of the GNU General Public License as published by
+ *  the Free Software Foundation, either version 3 of the License, or
+ *  (at your option) any later version.
+ *
+ *  GRUB is distributed in the hope that it will be useful,
+ *  but WITHOUT ANY WARRANTY; without even the implied warranty of
+ *  GNU General Public License for more details.
+ *
+ *  You should have received a copy of the GNU General Public License
+ *  along with GRUB.  If not, see <>.
+ */
+#include <grub/dl.h>
+#include <grub/misc.h>
+#include <grub/extcmd.h>
+#include <grub/i18n.h>
+#include <grub/term.h>
+static grub_err_t
+grub_cmd_eval (grub_extcmd_context_t ctxt __attribute__((__unused__)),
+              int argc, char *argv[])
+  char *dummy[1] = { NULL };
+  if (argc != 1)
+    return  grub_error (GRUB_ERR_BAD_ARGUMENT, N_("one argument expected"));
+  return grub_script_execute_sourcecode (argv[0], 0, dummy);
+static grub_extcmd_t cmd;
+  cmd = grub_register_extcmd ("eval", grub_cmd_eval, 0,
+                             N_("[STRING]"), N_("Evaluate commands"),
+                              NULL);
+  grub_unregister_extcmd (cmd);
diff --git a/grub-core/net/drivers/efi/efinet.c 
index 2b344d6..f7d8fd5 100644
--- a/grub-core/net/drivers/efi/efinet.c
+++ b/grub-core/net/drivers/efi/efinet.c
@@ -23,6 +23,7 @@
 #include <grub/efi/api.h>
 #include <grub/efi/efi.h>
 #include <grub/i18n.h>
+#include <grub/env.h>
@@ -250,6 +251,7 @@ grub_efi_net_config_real (grub_efi_handle_t hnd, char 
                                    sizeof (pxe_mode->dhcp_ack),
                                    1, device, path);
+    grub_env_set ("efi_boot_interface", card->name);
tg: (93daf86..) e/uefi_pxe_vars (depends on: master)

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