Network Overview

This page provides an overview of the main aspects of Google Kubernetes Engine networking. This information is useful to those who are just getting started with Kubernetes, as well as experienced cluster operators or application developers who need more knowledge about Kubernetes networking in order to better design applications or configure Kubernetes workloads.

Kubernetes allows you to declaratively define how your applications are deployed, how applications communicate with each other and with the Kubernetes control plane, and how clients can reach your applications. This page also provides information about how GKE configures Google Cloud Platform services, where it is relevant to networking.

When you use Kubernetes to orchestrate your applications, it’s important to change how you think about the network design of your applications and their hosts. With Kubernetes, you think about how Pods, Services, and external clients communicate, rather than thinking about how your hosts or VMs are connected.

Kubernetes, along with GCP, dynamically configures IP filtering rules, routing tables, and firewall rules on each node, depending on the declarative model of your Kubernetes deployments and your cluster configuration on GCP. Do not make manual changes on the nodes, because those changes are overridden by GKE, and your cluster may not function correctly. The only reason to access a node directly is to debug problems with your configuration.

Prerequisites

This page uses terminology related to the Transport, Internet, and Application layers of the Internet protocol suite, including HTTP, and DNS, but you don’t need to be an expert on these topics.

In addition, you may find this content easier to understand if you have a basic understanding of Linux network management concepts and utilities such as iptables rules and routing.

The Kubernetes networking model relies heavily on IP addresses. Services, Pods, containers, and nodes communicate using IP addresses and ports. Kubernetes provides different types of load balancing to direct requests to the correct Pods. All of these mechanisms are described in more detail later in this topic. Keep the following terms in mind as you read:

  • ClusterIP: The IP address assigned to a Service. In other documents, it may be called the "Cluster IP". This address is stable for the lifetime of the Service, as discussed in the Services section in this topic.
  • Pod IP: The IP address assigned to a given Pod. This is ephemeral, as discussed in the Pods section in this topic.
  • Node IP: The IP address assigned to a given node.

Networking Inside the Cluster

This section discusses networking within a Kubernetes cluster, as it relates to IP allocation, Pods, and Services.

IP Allocation

Kubernetes uses various IP ranges to assign IP address to nodes, Pods, and Services.

  • Each node has an IP address assigned from the cluster’s Virtual Private Cloud (VPC) network. This node IP provides connectivity from control components like kube-proxy and the kubelet to the Kubernetes API server.
  • Each Pod has an IP address assigned from a range of 256 IP addresses, a /24 CIDR block. You can optionally specify this range when you create the cluster.
  • Each Service has an IP address, called the ClusterIP, assigned from the cluster’s VPC network. You can optionally customize the VPC network when you create the cluster.

For more information, visit Creating VPC-native clusters using Alias IPs.

Pods

In Kubernetes, a Pod is the most basic deployable unit within a Kubernetes cluster. A Pod runs one or more containers. Zero or more Pods run on a node. Each node in the cluster is part of a node pool. In GKE, these nodes are virtual machines, each running as an instance in Compute Engine.

Pods can also attach to external storage volumes and other custom resources. This diagram shows a single node running two Pods, each attached to two volumes.

Diagram showing a single node running two Pods, as described in paragraph above

When Kubernetes schedules a Pod to run on a node, it creates a network namespace for the Pod in the node's Linux kernel. This network namespace connects the node's physical network interface, such as eth0, with the Pod using a virtual network interface, so that packets can flow to and from the Pod. The associated virtual network interface in the node's root network namespace connects to a Linux bridge that allows communication among Pods on the same node. A Pod can also send packets outside of the node using the same virtual interface.

Kubernetes assigns an IP address (the Pod IP) to the virtual network interface in the Pod's network namespace from a range of addresses reserved for Pods on the node. This address range is a subset of the IP address range assigned to the cluster for Pods, which you can configure when you create a cluster.

A container running in a Pod uses the Pod's network namespace. From the container's point of view, the Pod appears to be a physical machine with one network interface. All containers in the Pod see this same network interface. Each container's localhost is connected, through the Pod, to the node's physical network interface, such as eth0.

By default, each Pod has unfiltered access to all the other Pods running on all nodes of the cluster, but you can limit access among Pods. Kubernetes regularly tears down and recreates Pods. This happens when a node pool is upgraded, when changing the Pod's declarative configuration or changing a container's image, or when a node becomes unavailable. Therefore, a Pod's IP address is an implementation detail, and you should not rely on them. Kubernetes provides stable IP addresses using Services.

Services

In Kubernetes, you can assign arbitrary key-value pairs called labels to any Kubernetes resource. Kubernetes uses labels to group multiple related Pods into a logical unit called a Service. A Service has a stable IP address and ports, and provides load balancing among the set of Pods whose labels match all the labels you define in the label selector when you create the Service.

The following diagram shows two separate Services, each of which is comprised of multiple Pods. Each of the Pods in the diagram has the label app=demo, but their other labels differ. Service "frontend” matches all Pods with both app=demo and component=frontend, while Service “users” matches all Pods with app=demo and component=users. The Client Pod does not match either Service selector exactly, so it is not a part of either Service. However, the Client Pod can communicate with either of the Services because it runs in the same cluster.

Diagram of two separate Services, as described in the previous paragraph

Kubernetes assigns a stable, reliable IP address to each newly-created Service (the ClusterIP) from the cluster’s pool of available Service IP addresses. Kubernetes also assigns a hostname to the ClusterIP, by adding a DNS entry. The ClusterIP and hostname are unique within the cluster and do not change throughout the lifecycle of the Service. Kubernetes only releases the ClusterIP and hostname if the Service is deleted from the cluster’s configuration. You can reach a healthy Pod running your application using either the ClusterIP or the hostname of the Service.

At first glance, a Service may seem to be a single point of failure for your applications. However, Kubernetes spreads traffic as evenly as possible across the full set of Pods, running on many nodes, so a cluster can withstand an outage affecting one or more (but not all) nodes.

Kubernetes manages connectivity among Pods and Services using the kube-proxy component, which runs on each node. kube-proxy, which is not an in-line proxy, but an egress-based load-balancing controller, watches the Kubernetes API server and continually maps the ClusterIP to healthy Pods by adding and removing destination NAT (DNAT) rules to the node's iptables subsystem. When a container running in a Pod sends a packet to a Service’s ClusterIP, the node selects a Pod at random and routes traffic to that Pod.

When you configure a Service, you can optionally remap its listening port by defining values for port and targetPort.

  • The port is where clients reach the application.
  • The targetPort is the port where the application is actually listening for traffic within the Pod.

kube-proxy manages this port remapping by adding and removing iptables rules on the node.

This diagram illustrates the flow of packets from a client Pod to a server Pod on a different node. The client connects to the Service at 172.16.12.100:80. The Kubernetes API server maintains a list of Pods running the application. The kube-proxy process on each node uses this list to create an iptables rule to direct traffic to an appropriate Pod (such as 10.255.255.202:8080). The client Pod does not need to be aware of the topology of the cluster or any details about individual Pods or containers within them.

Diagram illustrating a client connecting to a Service and being routed to a Pod, described in the previous paragraph

Networking Outside the Cluster

This section explains how traffic from outside the cluster reaches applications running within a Kubernetes cluster. This information is important when designing your cluster’s applications and workloads.

You've already read about how Kubernetes uses Services to provide stable IP addresses for applications running within Pods. By default, Pods do not expose an external IP address, because kube-proxy manages all traffic on each node. Pods and their containers can communicate freely, but connections outside the cluster cannot access the Service. For instance, in the previous illustration, clients outside the cluster cannot access the frontend Service via its ClusterIP.

GKE provides three different types of Load Balancers to control access and to spread incoming traffic across your cluster as evenly as possible. You can configure one Service to use multiple types of Load Balancers simultaneously.

  • External Load Balancers manage traffic coming from outside the cluster and outside your GCP Virtual Private Cloud (VPC) network. They use forwarding rules associated with the GCP network to route traffic to a Kubernetes node.
  • Internal Load Balancers manage traffic coming from within the same VPC network. Like external load balancers, they use forwarding rules associated with the GCP network to route traffic to a Kubernetes node.
  • HTTP(S) Load Balancers are specialized external load balancers used for HTTP(S) traffic. They use an Ingress resource rather than a forwarding rule to route traffic to a Kubernetes node.

When traffic reaches a Kubernetes node, it is handled the same way, regardless of the type of load balancer. The load balancer is not aware of which nodes in the cluster are running Pods for its Service. Instead, it balances traffic across all nodes in the cluster, even those not running a relevant Pod. On a regional cluster, the load is spread across all nodes in all zones for the cluster’s region. When traffic is routed to a node, the node routes the traffic to a Pod, which may be running on the same node or a different node. If the node is running a relevant Pod, it handles the request. Otherwise, the node forwards the request to a Pod running on a different node, using the iptables rules that kube-proxy manages on the node.

In the following diagram, the network load balancer directs traffic to the middle node, which is not running an appropriate Pod. That traffic is redirected to the first node instead.

Diagram showing traffic being routed from a node not running an appropriate Pod to one that is, as described in the previous paragraph

External Load Balancer

If your Service needs to be reachable from outside the cluster and outside your VPC network, you can configure your Service as a LoadBalancer, by setting the Service's type field to LoadBalancer when defining the Service. GKE then provisions a Network Load Balancer in front of the Service. The Network Load Balancer is aware of all nodes in your cluster and configures your VPC network’s firewall rules to allow connections to the Service from outside the VPC network, using the Service's external IP address. You can assign a static external IP address to the Service. Visit Configuring Domain Names with Static IP Addresses for more information.

Technical Details

When using the external load balancer, arriving packets are initially routed to a node using a forwarding rule associated with the GCP network. After a packet reaches the node, the node uses its iptables NAT table to choose a Pod. kube-proxy manages the iptables rules on the node.

Internal Load Balancer

For traffic that needs to reach your cluster from within the same VPC network, you can configure your Service to provision an Internal Load Balancer. The Internal Load Balancer chooses an IP address from your cluster’s VPC subnet instead of an external IP address. Applications or services within the VPC network can use this IP address to communicate with Services inside the cluster.

Technical Details

Internal load balancing functionality is provided by GCP. When the traffic reaches a given node, that node uses its iptables NAT table to choose a Pod for the request, even if the Pod is on a different node. kube-proxy manages the iptables rules on the node.

For more information about internal load balancers, visit the Internal Load Balancer documentation.

HTTP(S) Load Balancer

Many applications, such as RESTful web service APIs, communicate using HTTP(S). You can allow clients external to your VPC network to access this type of application using a Kubernetes Ingress resource. An Ingress resource allows you to map hostnames and URL paths to Services within the cluster. When using a HTTP(S) load balancer, you must configure the Service to use a NodePort, as well as a ClusterIP. When traffic accesses the Service on a node’s IP at the NodePort, GKE routes traffic to a healthy Pod for the Service. You can specify a NodePort or allow GKE to assign a random unused port.

When you create the Ingress resource, GKE provisions an HTTP(S) Load Balancer in the GCP project. The load balancer sends a request to a node's IP address at the NodePort. After the packet reaches the node, the node uses its iptables NAT table to choose a Pod. kube-proxy manages the iptables rules on the node.

This Ingress definition routes traffic for demo.example.com to a Service named frontend on port 80, and demo-backend.example.com to a Service named users on port 8080.

apiVersion: extensions/v1beta1
kind: Ingress
metadata:
  name: demo
spec:
  rules:
  - host: demo.example.com
    http:
      paths:
      - backend:
          serviceName: frontend
          servicePort: 80
  - host: demo-backend.example.com
    http:
      paths:
      - backend:
          serviceName: users
          servicePort: 8080

Visit Ingress on Google Cloud Platform for more information.

Technical Details

When you create an Ingress object, the GKE Ingress controller configures a GCP HTTP(S) load balancer according to the rules in the Ingress manifest and the associated Service manifests. The client sends a request to the HTTP(S) load balancer. The load balancer is an actual proxy; it chooses a node and forwards the request to that node’s NodeIP:NodePort combination. The node uses its iptables NAT table to choose a Pod. kube-proxy manages the iptables rules on the node.

Limiting Connectivity to Pods and Services

By default, all Pods running within the same cluster can communicate freely. However, you can limit connectivity within a cluster in different ways, depending on your needs.

Limiting access among Pods

You can limit access among Pods using a network policy. Network policy definitions allow you to restrict the ingress and egress of Pods based on an arbitrary combination of labels, IP ranges, and port numbers. By default, there is no network policy, so all traffic among Pods in the cluster is allowed. As soon as you create the first network policy in a namespace,all other traffic is denied.

Visit Network Policies for more details about how to specify the policy itself.

After creating a network policy, you must explicitly enable it for the cluster. Visit Configuring Network Policies for Applications for more information.

Limiting Access to an External Load Balancer

If your Service uses an External Load Balancer, traffic from any external IP address can access your Service by default. You can restrict which IP address ranges can access endpoints within your cluster, by configuring the loadBalancerSourceRanges option when configuring the Service. You can specify multiple ranges, and you can update the configuration of a running Service at any time. The kube-proxy instance running on each node configures that node’s iptables rules to deny all traffic that does not match the specified loadBalancerSourceRanges. No VPC firewall rule is created.

Limiting Access to an HTTP(S) Load Balancer

If your service uses the HTTP(S) Load Balancer, you can use a Cloud Armor security policy to limit which external IP addresses can access your Service and which responses to return when access is denied because of the security policy. You can configure Stackdriver Logging to log information about these interactions.

If a Cloud Armor security policy is not fine-grained enough, you can enable the Cloud Identity-Aware Proxy on your endpoints to implement user-based authentication and authorization for your application. Visit the detailed tutorial for configuring Cloud IAP for more information.

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