Linux maintains a list of known IP neighbors. You can look at the list using the iproute2 family of commands.

$ ip neigh show dev enp0s3 lladdr 51:53:00:17:34:09 REACHABLE

You can add neighbors, change their details or modify the state. But you cannot delete a neighbor. There is a command to delete ip neigh del but it just puts the neighbor in the NUD_FAILED state. The entry still persists. Linux will remove it only when it feels like it.

It looks like Linux won’t let you delete neighbors even if you’re using the netlink interface. This was surprising to me, since I usually thought of netlink as a form of direct control over the kernel.

This could have real consequences if you ever want to implement a userspace ARP app, but still want the kernel to perform basic network functions.