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Why the state of lo interface is reported as unknown with cat /sys/class/net/lo/operstate even though ifconfig says it's running?

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The lo interface does not support operstate (as some other network driver).

The value unknown is documentated as:

IF_OPER_UNKNOWN (0): Interface is in unknown state, neither driver nor userspace has set operational state. Interface must be considered for user data as setting operational state has not been implemented in every driver.

See here for a kernel patch that fixes this.

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