machineid

Machine-id, often written as machine-id, is a unique, persistent identifier assigned to a computer host. It is used by system software to identify the machine across reboots and sessions, enabling coordinated services, logging, and telemetry. In Linux and other Unix-like systems, the machine-id is typically a 32-character hexadecimal string stored in the file /etc/machine-id. Some distributions also maintain a copy of this identifier in /var/lib/dbus/machine-id to satisfy D-Bus expectations; in either case, the value represents a single host identity.

Generation and persistence: The machine-id is generated at install time or on first boot by a utility such as systemd-machine-id-setup. The identifier is intended to be stable for the lifetime of the installation. It is not tied to specific hardware and may be recreated if the installation image is re-imaged or restored, though doing so can affect services that rely on a stable host identity.

Cloning and virtualization: When creating clones of machines or virtual machines, the machine-id should be unique for each clone to avoid identity conflicts. Administrators can regenerate the machine-id with tools like systemd-machine-id-setup --init or by removing /etc/machine-id and running the initialization again. In containers, machine-id behavior depends on the image and runtime, with some environments generating a temporary or inherited identifier.

Privacy and security: Because the machine-id serves as a persistent host identifier, it can be used to correlate activity across logs and telemetry. Some deployments treat it as sensitive information and manage it accordingly.

See also: systemd, D-Bus, hostid.