This message occurs when a workload is listening on a localhost network interface, but the port is exposed in the Service. When this occurs, the port will not be accessible to other pods.

This check is primarily added to detect workloads on older Istio versions that may break when upgrading to Istio 1.10 or later. This behavior matches what would occur in a standard Kubernetes cluster without Istio, but older versions of Istio exposed these ports.

An example

Consider a Service, selecting a Pod running the command nc localhost 8080 -l:

apiVersion: v1
kind: Service
  name: netcat
  - port: 8080
    protocol: TCP
    app: netcat

Because the application is serving traffic on localhost, it is not accessible from other pods.

The above example shows using the simple nc tool. Some equivalent examples in other languages:

  • Go: net.Listen("tcp", "localhost:8080")
  • Node.js: http.createServer().listen(8080, "localhost");
  • Python: socket.socket().bind(("localhost", 8083))

How to resolve

If you did not intend to expose the application to other pods, you can remove the port from the Service.

If you do want to expose the application to other pods, there are two options:

  • Modify the application to bind to a network interface exposed to other pods. Typically, this means binding to or ::, such as nc 8080 -l.
  • Create a Sidecar configuration to customize the inbound networking configuration for the pod. For example, with the above application:
apiVersion: networking.istio.io/v1beta1
kind: Sidecar
  name: ratings
      app: netcat
  - port:
      number: 8080
      protocol: TCP
      name: tcp