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

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.