Comment 3 for bug 1685445

Yes, and your comment about hard-probing reminds me of a subtle issue with aging things out: we don't do any hard-probing by default.

That is, discovered neighbour data might be out-of-date if active discovery isn't enabled. So you might bring up a device (let's say something that is fairly transparent on the network, like a firewall or IoT device) that doesn't do much ARP broadcasting. Such a device might keep to itself, and only broadcast ARP initially to find its gateway. Now, if active discovery enabled, every so often we'll confirm that the device is still using the address, and we can be reasonably confident when aging out entries that haven't been seen in awhile.

Unfortunately (and for good reason; this was hotly debated) active discovery is turned off by default on each subnet, and must be specifically enabled. That means you could easily have devices that were seen once, two weeks ago, and is still using its IP address.

So to solve this once-and-for-all, we would want a way to make it easier to have active discovery enabled; the active discovery interval could be coupled to the cleanup mechanism. So if the active discovery interval is set to 3 hours, and it's been some multiple of 3 hours since the device was last seen, we should be able to remove it right then and there.