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Asked: 2026-06-08T17:05:00+00:00

These two flags are often set together. What is the difference between them?

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These two flags are often set together. What is the difference between them?

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  1. Editorial Team

    From Linux Device Drivers 3:

    IFF_UP
    This flag is read-only for the driver. The kernel turns it on when the interface is active and ready to transfer packets.

    …

    IFF_RUNNING

    This flag indicates that the interface is up and running. It is mostly
    present for BSD compatibility; the kernel makes little use of it. Most
    network drivers need not worry about IFF_RUNNING.

    Digging a bit deeper, it seems that there is one significant difference:

    IFF_RUNNING is supposed to reflect the operational status on a network interface, rather than its administrative one. To provide an example, an Ethernet interface may be brought UP by the administrator (e.g. ifconfig eth0 up), but it will not be considered operational (i.e. RUNNING as per RFC2863) if the cable is not plugged in.

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