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Re: Obtaining the UUID of the system for a PXE boot

From: Holger Goetz
Subject: Re: Obtaining the UUID of the system for a PXE boot
Date: Thu, 18 Jul 2013 12:39:44 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:23.0) Gecko/20100101 Thunderbird/23.0

On 17.07.2013 16:31, Andrey Borzenkov wrote:
On Wed, Jul 17, 2013 at 3:19 PM, Holger Goetz<address@hidden>  wrote:
Hello all,

i've started to use grub.efi as boot loader for UEFI based systems and am
getting slowly familiar w/ grub2.Now i've come across a problem w/ assigning
individual boot configurations to target system identification.
The BIOS line of systems gets booted through pxelinux and get their PXE
configuration through files w/ names based on the individual UUID of the
The same should be possible for UEFI booted systems, but i couldn't find a
way to get hold of the system-UUID yet. The MAC addresse(s) is/are not
available and possibly have changed since the definition of the
configuration, so even the lately introduced net_default_mac variable
doesn't work here. Having the UUID as unique system identifyer across NICs
which can be changed would be of big help.

Any pointer/help would be greatly appreciated.

So, you need to fetch system GUID used by PXE, right? To be honest, I
do not even know how (if it is possible at all) to obtain it. The only
words about PXE GUID in EFI allow to switch between using GUID and MAC
for client identification, but that's all:

SendGUIDThis field is used to change the Client Hardware Address
(chaddr) field in the DHCP and Discovery packets. Set to TRUE to send
the SystemGuid (if one is available).

Do you know how to get this information?

Hi Andrey,

the system UUID is part of the SMBIOS structure which can be searched between 0xf000 and 0x100000 in memory. It has a signature string "_SM_". With in the SMBIOS table (memory block) the table type 1 identifys the UUID table and it contains a 16byte binary number which typically is returned in a form like 00112233-4455-6677-8899-AABBCCDDEEFF. See the SMBIOS specs here:

Also in the spec is some discussions around byte swappig parts of the uuid, for consistency probably a byte by byte representation as a string would be best.

One implementation can be found in eg. syslinux's core/dmi.c - including a rudimentary validation check. Other implementations under Linux (eg. dmidecode) often take /proc/efi/systab or /sys/firmware/efi/systab as starting points to the SMBIOS structure, but that's obviously not available at the time grub starts ;-)


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