Ingress controllers usually use the standard k8s services. In fact metal-lb allows workloads (like the nginx ingress controller) in the cluster to use services of type LoadBalancer, which is the default configuration. This results in an actual IP being made available to your ingress controller.

To get nginx ingress to use the external clients IP, you can configure the ingress controllers traffic policy. Using the helm chart, I used these values:

controller:
          service:
            # this has a bunch of downsides, but allows source-ip based access white/deny listing.
            externalTrafficPolicy: Local

For the ingress IP, I configured metal-lb to receive traffic on a static IP (using IP4AddressPool and L2Advertisement CRDs from metal-lb), which is then used for the port forwarding. I’ve never tested it because I only have a single worker node, but I expect the metal-lb controller will continue receiving traffic to that same static IP if a node goes down.