You cannot mark a neighbor STALE in old Linux

Reporting another ARP quirk with the Linux kernel.

On versions lower than 4.9.0, the kernel does not let you modify the stale of any neighbor from NUD_CONNECTED to NUD_STALE. I don’t know what reasoning led to this behavior or if this was a bug, but it works fine in later versions.

As with the last such weirdness, this really only matters if you want to implement a userspace ARP app but still want the kernel to handle standard networking tasks.

The workaround is just to mark the neighor NUD_FAILED and then immediately mark it NUD_STALE.

Another thing to note is that when a stale neighbor transitions to NUD_DELAY due to network activity, the kernel will not notify you about this even if you have registered for the RTM_GETNEIGH category via netlink.