Ubuntu
cloud-init package

  1. Bug #1551419
  2. Comment #6

Comment 6 for bug 1551419

Revision history for this message
Dan Watkins (oddbloke) wrote : Re: Fix UUID endianness patch breaks cloud-init on Azure

OK, so I've done a bit of digging.

As of SMBIOS 2.6 (released in 2008), the first three fields of the UUID are well-defined as being encoded in little-endian. Before that, they could be encoded in "network byte order" (i.e. big-endian) if people felt like it.

Presumably enough people did feel like it, as dmidecode specifically has different code paths for pre-2.6 and post-2.6 SMBIOS versions; it will decode pre-2.6 UUIDs as big-endian and post-2.6 UUIDs as little-endian. (The comment in the dmidecode source code at [0] explains this pretty well.)

And, you've guessed it, Azure reports a pre-2.6 version of SMBIOS. (To be precise, 2.3, which was released in _1999_; Azure is old skool cool). This explains the difference between the kernel-reported UUID (always little-endian) and the dmidecode-reported UUID (big-endian on Azure, but little-endian on most modern systems).

[0] https://github.com/mirror/dmidecode/blob/master/dmidecode.c#L415