HostID

Hostid is a 32-bit identifier used by some Unix-like operating systems to uniquely identify a host. It is most often employed by software licensing schemes and certain network services that bind behavior to a particular machine. The hostid value is traditionally stored as a small binary file at /etc/hostid and is made available to programs via the gethostid() system call. On many systems, a command named hostid can display the hexadecimal representation of this value.

Generation and management: At installation time, the hostid is typically derived from hardware attributes such as the system’s network hardware address, or it may be assigned explicitly by an administrator when writing /etc/hostid. If the /etc/hostid file is missing, some systems derive a value at boot from available hardware information; on other systems the hostid may be fixed and not easily changed.

Usage and limitations: Because the hostid is not guaranteed to be unique across machines, and because it can change if hardware is modified or if the file is edited, it should not be relied upon as a persistent universal identifier. Modern Linux distributions and software increasingly prefer more stable identifiers like machine identifiers (such as /etc/machine-id) for licensing and tracking. The hostid remains relevant for legacy software and environments that expect a 32-bit identifier in the traditional form.

See also: gethostid, /etc/hostid, /etc/machine-id.