A provisioning script must record the non-loopback IP addresses currently configured on the machine's interfaces, and it must not depend on name resolution or on /etc/hosts. Which hostname invocation is appropriate?

LPIC-1 Exam 102-500, objective 109. Networking fundamentals medium

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The options

Correct hostname -I

Correct. Uppercase -I lists all addresses configured on all interfaces, excluding loopback and link-local addresses, and it obtains them from the kernel without performing any name lookup.

Not correct hostname -i

Wrong for this task. Lowercase -i resolves the host's own name through the resolver, so it depends on /etc/hosts or DNS and commonly returns 127.0.1.1 on Debian-derived systems.

Not correct hostname -f

Wrong. -f prints the fully qualified domain name, which is a name obtained by resolving the host name, not a list of addresses.

Not correct hostname -s

Wrong. -s prints the short host name, that is the part before the first dot. It reports no addressing information at all.

Why

hostname's address options differ in where the answer comes from: -i asks the resolver what the host's own name maps to, whereas -I reads the addresses actually assigned to the interfaces. Because -i goes through /etc/hosts it inherits whatever the installer wrote there, which is why it so often reports a loopback address. For scripting, -I, or `ip -o addr show` when the interface and prefix length are needed, gives the ground truth.

Where this comes from

Cited
manual page hostname(1)

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