[Pkg-zfsonlinux-devel] Bug#595790: hostid: useless unless fixed

[Petter Reinholdtsen]

Control: reassign -1 libc6
Control: found -1 2.19-18
Control: The value from gethostid() should be more unique and not change when the host IP changes

Reassigning to glibc as that is the source of gethostid() where the
problem with the missing unique identifier originates.  Using the
version number in stable, but the issue have been around before that.

In my work as a system administrator for tens of thousand of machines, I
have often had the need to get some semi-unique identifier out of the
operating system.  On all other Unix like operating systems, hostid and
gethostid() will provide this, but not on Linux.  I find this rather sad,
and have had to spend time generating our own solution to the problem
because gethostid() is useless on Linux.

Because of this, and to spare future system administrators to share that
pain, I fully support the request from Martin Kraft to extend Debian to
make sure the gethostid() value return something sensible.

The described approach from FreeBSD, using /etc/hostid,
/sys/class/dmi/id/product_uuid or a random value (in that order) seem
like a sensible one.  It might make sense to use other sources too, but
the goal should be to pick a value that will stay the same until the hardware
is replaced, and pick a value that will stay the same as long as the operating
system isn't reinstalled if such hardware dependent value do not exist.

To avoid changing the ID on running systems I believe it should only be done
when libc6 is installed for the first time.  Those willing to change their
hostid at runtime should be provided a simple script to do so instead of doing
it automatically.  It will fix the issue for future installations.  I am not
sure how to sensibly fix it for existing installations without ending up with
a lot of machines with the same hostid as 7f0100 is a very common hostid on
Linux already, and everyone with a private IP address like those on 192.168.*
will have collisions.  But then again a 32 bit number can only provide
4.294.967.296 unique IDs and with the amount of Linux machines in the world
there are going to be collisions anyway.  We just should reduce the chance to
a more sensible number.

Something like this should work, I guess:

if [ ! -f /etc/hostid ]; then
   if [ -e /sys/class/dmi/id/product_uuid ]; then
       sethostidfromuuid $(cat /sys/class/dmi/id/product_uuid)
   else
   	dd if=/dev/urandom bs=1 count=4 of=/etc/hostid 2>/dev/null
   fi
fi

We need to figure out how to transform the UUID to a 32 bit integer, of course.

-- 
Happy hacking
Petter Reinholdtsen