- Overview
- External Load Balancer Providers
- Configuration file
- Using kubectl
- Finding your IP address
- Loss of client source IP for external traffic
- Annotation to modify the LoadBalancer behavior for preservation of Source IP
Overview
When creating a service, you have the option of automatically creating a cloud network load balancer. This provides an externally-accessible IP address that sends traffic to the correct port on your cluster nodes provided your cluster runs in a supported environment and is configured with the correct cloud load balancer provider package.
External Load Balancer Providers
It is important to note that the datapath for this functionality is provided by a load balancer external to the Kubernetes cluster.
When the service type is set to LoadBalancer, Kubernetes provides functionality equivalent to type=ClusterIP to pods within the cluster and extends it by programming the (external to Kubernetes) load balancer with entries for the Kubernetes VMs. The Kubernetes service controller automates the creation of the external load balancer, health checks (if needed), firewall rules (if needed) and retrieves the external IP allocated by the cloud provider and populates it in the service object.
Configuration file
To create an external load balancer, add the following line to your service configuration file:
"type": "LoadBalancer"
Your configuration file might look like:
{
"kind": "Service",
"apiVersion": "v1",
"metadata": {
"name": "example-service"
},
"spec": {
"ports": [{
"port": 8765,
"targetPort": 9376
}],
"selector": {
"app": "example"
},
"type": "LoadBalancer"
}
}
Using kubectl
You can alternatively create the service with the kubectl expose command and
its --type=LoadBalancer flag:
$ kubectl expose rc example --port=8765 --target-port=9376 \
--name=example-service --type=LoadBalancer
This command creates a new service using the same selectors as the referenced
resource (in the case of the example above, a replication controller named
example.)
For more information, including optional flags, refer to the
kubectl expose reference.
Finding your IP address
You can find the IP address created for your service by getting the service
information through kubectl:
$ kubectl describe services example-service
Name: example-service
Selector: app=example
Type: LoadBalancer
IP: 10.67.252.103
LoadBalancer Ingress: 123.45.678.9
Port: <unnamed> 80/TCP
NodePort: <unnamed> 32445/TCP
Endpoints: 10.64.0.4:80,10.64.1.5:80,10.64.2.4:80
Session Affinity: None
No events.
The IP address is listed next to LoadBalancer Ingress.
Loss of client source IP for external traffic
Due to the implementation of this feature, the source IP for sessions as seen in the target container will not be the original source IP of the client. This is the default behavior as of Kubernetes v1.4. However, starting in v1.4, an optional alpha feature has been added that will preserve the client Source IP for GCE/GKE environments. This feature will be phased in for other cloud providers in subsequent releases.
Annotation to modify the LoadBalancer behavior for preservation of Source IP
In 1.4, an Alpha feature has been added that changes the behavior of the external LoadBalancer feature.
This feature can be activated by adding the alpha annotation below to the metadata section of the Service Configuration file.
{
"kind": "Service",
"apiVersion": "v1",
"metadata": {
"name": "example-service",
"annotations": {
"service.alpha.kubernetes.io/external-traffic": "OnlyLocal"
}
},
"spec": {
"ports": [{
"port": 8765,
"targetPort": 9376
}],
"selector": {
"app": "example"
},
"type": "LoadBalancer"
}
}
Alpha Feature Gate for the ‘service.alpha.kubernetes.io/external-traffic’ annotation
Alpha features are not enabled by default, they must be enabled using the release gate command line flags for kube-controller-manager and kube-proxy. See https://github.com/kubernetes/kubernetes/blob/master/docs/proposals/runtimeconfig.md for more details on feature gate flags.
If this feature is not enabled in your cluster, this annotation in your service configuration will be rejected.
Implementation across different cloudproviders/environments
Note that this feature is not currently implemented for all cloudproviders/environments. This feature does not work for nodePorts yet, so environments/cloud providers with proxy-style load-balancers cannot use it yet.
Caveats and Limitations when preserving source IPs
GCE/AWS load balancers do not provide weights for their target pools. This was not an issue with the old LB kube-proxy rules which would correctly balance across all endpoints.
With the new functionality, the external traffic will not be equally load balanced across pods, but rather equally balanced at the node level (because GCE/AWS and other external LB implementations do not have the ability for specifying the weight per node, they balance equally across all target nodes, disregarding the number of pods on each node).
We can, however, state that for NumServicePods « NumNodes or NumServicePods » NumNodes, a fairly close-to-equal distribution will be seen, even without weights.
Once the external load balancers provide weights, this functionality can be added to the LB programming path. Future Work: No support for weights is provided for the 1.4 release, but may be added at a future date
Internal pod to pod traffic should behave similar to ClusterIP services, with equal probability across all pods.