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Why is it there in the first place?

by gvand 8y ago

geofft 8y ago
https://dbus.freedesktop.org/doc/dbus-uuidgen.1.html https://dbus.freedesktop.org/doc/dbus-uuidgen.1.html has some explanation, in particular: > The important properties of the machine UUID are that 1) it remains unchanged until the next reboot and 2) it is different for any two running instances of the OS kernel. That is, if two processes see the same UUID, they should also see the same shared memory, UNIX domain sockets, local X displays, localhost.localdomain resolution, process IDs, and so forth. Because it's possible to forward things like D-Bus, the X11 $DISPLAY, etc. over the network, two processes might be aware of each other over such a connection but not be running on the same machine and therefore be unable to share resources. The machine ID lets them check for that, so you can properly handle things like "I'm going to send a message to the screensaver in my display to not activate, I don't care if it's the same machine" vs. "I'm going to send a message to the terminal in my display to open a new tab, but only if it's actually on the same machine, otherwise I should start a new terminal". (These days I think that definition should be updated to "container" instead of "kernel": if you're running separate logical machines inside the same kernel with separate PIDs etc., they should have separate machine IDs.) systemd and (IIRC) cloud-init use it to run once-per-machine tasks on machines that could come from images: if you want to prep a number of machines in advance, do the install, then change the machine ID. At boot time, startup scripts will say "Oh, this machine ID has not been initialized yet" and do things, and then not do them on the next boot.
- LukeShu 8y ago
  > These days I think that definition should be updated to "container" instead of "kernel": if you're running separate logical machines inside the same kernel with separate PIDs etc., they should have separate machine IDs. Indeed. If that weren't the case, and the "same kernel" were enough, then things could just use /proc/sys/kernel/random/boot_id.
JdeBP 8y ago
See http://jdebp.uk./Softwares/nosh/guide/commands/machine-id.xml#HISTORY http://jdebp.uk./Softwares/nosh/guide/commands/machine-id.xm...

Why is it there in the first place?

by gvand 8y ago

Why is it there in the first place?

https://dbus.freedesktop.org/doc/dbus-uuidgen.1.html https://dbus.freedesktop.org/doc/dbus-uuidgen.1.html has some explanation, in particular: > The important properties of the machine UUID are that 1) it remains unchanged until the next reboot and 2) it is different for any two running instances of the OS kernel. That is, if two processes see the same UUID, they should also see the same shared memory, UNIX domain sockets, local X displays, localhost.localdomain resolution, process IDs, and so forth. Because it's possible to forward things like D-Bus, the X11 $DISPLAY, etc. over the network, two processes might be aware of each other over such a connection but not be running on the same machine and therefore be unable to share resources. The machine ID lets them check for that, so you can properly handle things like "I'm going to send a message to the screensaver in my display to not activate, I don't care if it's the same machine" vs. "I'm going to send a message to the terminal in my display to open a new tab, but only if it's actually on the same machine, otherwise I should start a new terminal". (These days I think that definition should be updated to "container" instead of "kernel": if you're running separate logical machines inside the same kernel with separate PIDs etc., they should have separate machine IDs.) systemd and (IIRC) cloud-init use it to run once-per-machine tasks on machines that could come from images: if you want to prep a number of machines in advance, do the install, then change the machine ID. At boot time, startup scripts will say "Oh, this machine ID has not been initialized yet" and do things, and then not do them on the next boot.

LukeShu 8y ago
> These days I think that definition should be updated to "container" instead of "kernel": if you're running separate logical machines inside the same kernel with separate PIDs etc., they should have separate machine IDs. Indeed. If that weren't the case, and the "same kernel" were enough, then things could just use /proc/sys/kernel/random/boot_id.

JdeBP 8y ago

See http://jdebp.uk./Softwares/nosh/guide/commands/machine-id.xml#HISTORY http://jdebp.uk./Softwares/nosh/guide/commands/machine-id.xm...