A host has several routes and two interfaces, and you need to know which route, gateway and source address the kernel would actually select for traffic to 198.51.100.25 without sending any packets to it. Which command answers exactly that?

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

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The options

Correct ip route get 198.51.100.25

Correct. The get subcommand asks the kernel to perform a real route lookup for that destination and prints the chosen route, the output device, the next hop where one applies and the preferred source address.

Not correct ip route show

Wrong for this purpose. It lists the entries of the main routing table and leaves you to apply longest-prefix matching by eye, which is exactly the error-prone step you are trying to avoid.

Not correct ip addr show

Wrong. This lists addresses assigned to interfaces. It says nothing about which of them the kernel would pick as the source for a given destination.

Not correct traceroute -n 198.51.100.25

Wrong for the constraint given. traceroute does reveal the first hop, but only by transmitting probes to the destination, and it does not report the local source address the kernel selected.

Why

`ip route get` runs the same lookup the kernel performs for a real packet, so it resolves longest-prefix matching, policy routing rules and source-address selection in one step. That makes it the fastest way to prove which interface traffic to a given host will leave by, particularly on multi-homed machines where a stale or overly specific route is the fault. `ip route show` merely dumps table entries; it performs no lookup.

Where this comes from

Cited
manual page ip-route(8)

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