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

[Holger Goetz]

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
> > system.
> > 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: http://www.dmtf.org/standards/smbios.

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 ;-)

Thanks,
Holger