Kubernetes Service Security… or, “Why is everything binding to 0.0.0.0”¶

How are ports exposed?¶

While using HostNetwork=True (Net=Host in Docker parlance), processes running inside of a container are using the network namespace of the host, meaning that network operations are not containerized and, as far as the TCP/IP stack is concerned, the process is running in the parent host. This means that any process need to be just as careful about what ports are accessible and how they are managing them as a process running outside of the container. Thus, they must be careful which interface they listen to, who is allowed to connect, etc.

In Kubernetes, containers default to HostNetwork=False and thus work inside of the Kubernetes network framework. They have no inbound ports accessible by default unless you have set them to be exposed.

The normal way of exposing ports is via a Kubernetes Service. A service has a DNS alias exposed via SkyDNS (e.g. you are able to use mariadb to access MariaDB) that points to the service IP address which is generally backed by a Kubernetes Virtual IP. Services can be either internal services or external services. Only services specifically marked as external services and configured with either a LoadBalancer or a Ingress controller will be accessible outside of the cluster.

Services can be exposed with a type of NodePort, which means that a port from a configurable range will be allocated for a service on each node on each port will be configured to proxy, which is intended for users to be able to configure their own external load balancers.

Thus, a server running inside of a container that doesn’t have any services exposed as NodePort can safely bind to 0.0.0.0 and rely on the underlying network layer ensuring that attackers are unable to probe for it.

Containers that need to run as HostNetwork=True are unable to be exposed as services but are still able to connect to other Kubernetes services.

What about other services running inside of the Kubernetes cluster?¶

By default, processes running on compute nodes within the cluster are part of the same unrestricted network fabric.

Certain processes, Nova Compute nodes, for example, are running user workloads out of the control of the cluster administrator and thus should not have unrestricted access to the cluster. There are two alternatives:

First, compute nodes can be provisioned outside of the Kubernetes cluster. This is necessary if you are using compute nodes with KVM or Ironic and often times the easiest approach.

Second, some of the CNI drivers (Calico being one example) can be configured with NetworkPolicy objects to block access from certain nodes, which can prevent compute nodes from seeing the internal services. However, as currently implemented, pods will still be accessible from the host on which they are running, it is also necessary to schedule any containers with HostNetworking=True on dedicated hosts.