hostid

Display the numeric identifier for the current host in hexadecimal

$ hostid

hostid

The hostid command in Linux is used to display the numeric identifier of the current host in hexadecimal format. This identifier is typically a 32-bit number.

Historically, this host ID was often derived from the system's primary IPv4 address, specifically the loopback address 127.0.0.1, which translates to 0x7f000001.

It's crucial to understand that hostid does not provide a unique identifier for a physical or virtual machine. Many systems, especially those where the host ID hasn't been explicitly configured, will report the default 0x7f000001. For unique machine identification, other mechanisms like MAC addresses, DMI information, or system UUIDs (e.g., from dmidecode) are more appropriate.

The value returned by hostid can be set using the sethostid(8) command, which modifies the host ID stored in a system file (e.g., /etc/hostid). Its primary use is often in specific network environments or legacy applications that rely on this particular identifier.

The hostid command typically returns 0x7f000001 (representing 127.0.0.1) unless explicitly set. This means it is not a unique identifier for a machine and should not be relied upon for such purposes. Many different systems will report the exact same host ID.

The value displayed by hostid is usually obtained from the gethostid() system call. This value can be persistently set using the sethostid(8) command, which typically writes the ID to a file like /etc/hostid. If this file doesn't exist or is empty, the system might default to 0x7f000001.

While not for unique identification, hostid might be used by licensing schemes tied to this specific ID or by legacy distributed systems that rely on it for host differentiation within a small, controlled environment.

The hostid concept and the gethostid() system call originate from older Unix systems, dating back to Berkeley Software Distribution (BSD). It was part of the standard Unix utilities for system identification. Its usage has remained largely consistent, though its utility as a unique machine identifier has diminished over time as more robust identification methods emerged. It remains primarily for backward compatibility or very specific legacy application requirements.

hostname(1), uname(1), ip(8), gethostid(3), sethostid(8)