On a Debian host using the traditional ifupdown scheme you have edited the static address of eth0 in /etc/network/interfaces. The interface is currently up with the old address. Which command sequence puts the new settings into effect?

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

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The options

Correct ifdown eth0 && ifup eth0

Correct. ifdown tears the interface down using the stored state and clears it, and ifup then reads the current stanza from /etc/network/interfaces and applies it.

Not correct ip link set eth0 down && ip link set eth0 up

Wrong. This toggles the link at kernel level without consulting /etc/network/interfaces, so the address is not reconfigured and ifupdown's record of the interface's state becomes inconsistent.

Not correct ifconfig eth0 down && ifconfig eth0 up

Wrong for the same reason as the iproute2 version, with the added drawback that net-tools is deprecated. Neither command parses the configuration file.

Not correct ifup -a

Wrong. -a brings up every interface marked auto, but it skips interfaces ifupdown already considers up, so eth0 is left alone with its old address.

Why

ifup and ifdown are wrappers that read the stanzas of /etc/network/interfaces and record which interfaces they have configured, historically under /run/network. Because ifup refuses to reconfigure an interface it believes is already up, changes are applied by cycling the interface with ifdown followed by ifup rather than by toggling the link directly. `ifquery eth0` prints the settings ifupdown would apply, which is useful for checking a stanza before committing to it.

Where this comes from

Cited
manual page ifup(8)

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