While the Boot ID

/proc/sys/kernel/random/boot_id

is newly generated with every reboot – the Machine ID uniquely identifies the host.

It should be considered „confidential“, and must not be exposed in untrusted environments, in particular on the network.

The

/etc/machine-id

file contains the unique machine ID of the local system that is set during installation. The machine ID is a single newline-terminated, hexadecimal, 32-character, lowercase ID. When decoded from hexadecimal, this corresponds to a 16-byte/128-bit value.

The machine ID is usually generated from a random source during system installation and stays constant for all subsequent boots. Optionally, for stateless systems, it is generated during runtime at early boot if it is found to be empty.

The machine ID does not change based on local or network configuration or when hardware is replaced. Due to this and its greater length, it is a more useful replacement for the gethostid(3) call that POSIX specifies.

This machine ID adheres to the same format and logic as the D-Bus machine ID.

src: https://www.freedesktop.org/software/systemd/man/machine-id.html

http://0pointer.de/blog/projects/ids.html

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