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Service of type LoadBalancer

Table of Contents

In Kubernetes, implementing Services of type LoadBalancer usually requires an external load balancer. On cloud platforms (including public clouds and platforms like NSX-T) that support load balancers, Services of type LoadBalancer can be implemented by the Kubernetes Cloud Provider, which configures the cloud load balancers for the Services. However, the load balancer support is not available on all platforms, or in some cases, it is complex or has extra cost to deploy external load balancers. This document describes two options for supporting Services of type LoadBalancer with Antrea, without an external load balancer:

  1. Using Antrea's built-in external IP management for Services of type LoadBalancer
  2. Leveraging MetalLB

Service external IP management by Antrea

Antrea supports external IP management for Services of type LoadBalancer since version 1.5, which can work together with Antrea Proxy or kube-proxy to implement Services of type LoadBalancer, without requiring an external load balancer. With the external IP management feature, Antrea can allocate an external IP for a Service of type LoadBalancer from an ExternalIPPool, and select a Node based on the ExternalIPPool's NodeSelector to host the external IP. Antrea configures the Service's external IP on the selected Node, and thus Service requests to the external IP will get to the Node, and they are then handled by Antrea Proxy or kube-proxy on the Node and distributed to the Service's Endpoints. Antrea also implements a Node failover mechanism for Service external IPs. When Antrea detects a Node hosting an external IP is down, it will move the external IP to another available Node of the ExternalIPPool.

Preparation

If you are using kube-proxy in IPVS mode, you need to make sure strictARP is enabled in the kube-proxy configuration. For more information about how to configure kube-proxy, please refer to the Interoperability with kube-proxy IPVS mode section.

If you are using kube-proxy iptables mode or Antrea Proxy with proxyAll, no extra configuration change is needed.

Configuration

Enable Service external IP management feature

At this moment, external IP management for Services is an alpha feature of Antrea. The ServiceExternalIP feature gate of antrea-agent and antrea-controller must be enabled for the feature to work. You can enable the ServiceExternalIP feature gate in the antrea-config ConfigMap in the Antrea deployment YAML:

apiVersion: v1
kind: ConfigMap
metadata:
  name: antrea-config
  namespace: kube-system
data:
  antrea-agent.conf: |
    featureGates:
      ServiceExternalIP: true
  antrea-controller.conf: |
    featureGates:
      ServiceExternalIP: true

The feature works with both Antrea Proxy and kube-proxy, including the following configurations:

  • Antrea Proxy without proxyAll enabled - this is antrea-agent's default configuration, in which kube-proxy serves the request traffic for Services of type LoadBalancer (while Antrea Proxy handles Service requests from Pods).
  • Antrea Proxy with proxyAll enabled - in this case, Antrea Proxy handles all Service traffic, including Services of type LoadBalancer.
  • Antrea Proxy disabled - kube-proxy handles all Service traffic, including Services of type LoadBalancer.

Create an ExternalIPPool custom resource

Service external IPs are allocated from an ExternalIPPool, which defines a pool of external IPs and the set of Nodes to which the external IPs can be assigned. To learn more information about ExternalIPPool, please refer to the Egress documentation. The example below defines an ExternalIPPool with IP range "10.10.0.2 - 10.10.0.10", and it selects the Nodes with label "network-role: ingress-node" to host the external IPs:

apiVersion: crd.antrea.io/v1beta1
kind: ExternalIPPool
metadata:
  name: service-external-ip-pool
spec:
  ipRanges:
  - start: 10.10.0.2
    end: 10.10.0.10
  nodeSelector:
    matchLabels:
      network-role: ingress-node

Create a Service of type LoadBalancer

For Antrea to manage the externalIP for a Service of type LoadBalancer, the Service should be annotated with service.antrea.io/external-ip-pool. For example:

apiVersion: v1
kind: Service
metadata:
  name: my-service
  annotations:
    service.antrea.io/external-ip-pool: "service-external-ip-pool"
spec:
  selector:
    app: MyApp
  ports:
    - protocol: TCP
      port: 80
      targetPort: 9376
  type: LoadBalancer

You can also request a particular IP from an ExternalIPPool by setting the loadBalancerIP field in the Service spec to that specific IP available in the ExternalIPPool, Antrea will allocate the IP from the ExternalIPPool for the Service. For example:

apiVersion: v1
kind: Service
metadata:
  name: my-service
  annotations:
    service.antrea.io/external-ip-pool: "service-external-ip-pool"
spec:
  selector:
    app: MyApp
  loadBalancerIP: "10.10.0.2"
  ports:
    - protocol: TCP
      port: 80
      targetPort: 9376
  type: LoadBalancer

By default, Antrea doesn't allocate a single IP to multiple Services. Before Antrea v2.1, if multiple Services requested the same IP, only one of them would get the IP assigned. Starting with Antrea v2.1, to share an IP between multiple Services, you can annotate the Services with service.antrea.io/allow-shared-load-balancer-ip: true when requesting a particular IP. Note that the IP will only be shared between Services having the annotation. If not all Services are annotated, the IP may either be allocated to one of the unannotated Services or shared between the annotated Services, depending on the order in which they are processed. The annotation only takes effect during the IP allocation phase. Once the IP has been allocated, removing this annotation from a Service will not result in the IP being reclaimed from it or other Services.

For example, the following two Services will share an IP:

apiVersion: v1
kind: Service
metadata:
  name: my-service-1
  annotations:
    service.antrea.io/external-ip-pool: "service-external-ip-pool"
    service.antrea.io/allow-shared-load-balancer-ip: "true"
spec:
  selector:
    app: MyApp1
  loadBalancerIP: "10.10.0.2"
  ports:
    - protocol: TCP
      port: 80
      targetPort: 80
  type: LoadBalancer
---
apiVersion: v1
kind: Service
metadata:
  name: my-service-2
  annotations:
    service.antrea.io/external-ip-pool: "service-external-ip-pool"
    service.antrea.io/allow-shared-load-balancer-ip: "true"
spec:
  selector:
    app: MyApp2
  loadBalancerIP: "10.10.0.2"
  ports:
    - protocol: TCP
      port: 8080
      targetPort: 8080
  type: LoadBalancer

Note that sharing a LoadBalancer IP between multiple Services only works under the following conditions:

  • The Services use different ports.
  • The Services use the Cluster external traffic policy. Sharing a LoadBalancer IP between Services using the Local external traffic policy can also work if they have identical Endpoints. However, in such cases, it may be preferable to consolidate the Services into a single Service.

Otherwise, the datapath may not work even though the IP is allocated to the Services successfully.

Validate Service external IP

Once Antrea allocates an external IP for a Service of type LoadBalancer, it will set the IP to the loadBalancer.ingress field in the Service resource status. For example:

apiVersion: v1
kind: Service
metadata:
  name: my-service
  annotations:
    service.antrea.io/external-ip-pool: "service-external-ip-pool"
spec:
  selector:
    app: MyApp
  ports:
    - protocol: TCP
      port: 80
      targetPort: 9376
  clusterIP: 10.96.0.11
  type: LoadBalancer
status:
  loadBalancer:
    ingress:
    - ip: 10.10.0.2
      hostname: node-1

You can validate that the Service can be accessed from the client using the <external IP>:<port> (10.10.0.2:80/TCP in the above example).

Limitations

As described above, the Service externalIP management by Antrea configures a Service's external IP to a Node, so that the Node can receive Service requests. However, this requires that the externalIP on the Node be reachable through the Node network. The simplest way to achieve this is to reserve a range of IPs from the Node network subnet, and define Service ExternalIPPools with the reserved IPs, when the Nodes are connected to a layer 2 subnet. Or, another possible way might be to manually configure Node network routing (e.g. by adding a static route entry to the underlay router) to route the Service traffic to the Node that hosts the Service's externalIP.

As of now, Antrea supports Service externalIP management only on Linux Nodes. Windows Nodes are not supported yet.

Using MetalLB with Antrea

MetalLB also implements external IP management for Services of type LoadBalancer, and it can be deployed to a Kubernetes cluster with Antrea. MetalLB supports two modes - layer 2 mode and BGP mode - to advertise an Service external IP to the Node network. The layer 2 mode is similar to what Antrea external IP management implements and has the same limitation that the external IPs must be allocated from the Node network subnet. The BGP mode leverages BGP to advertise external IPs to the Node network router. It does not have the layer 2 subnet limitation, but requires the Node network to support BGP.

MetalLB will automatically allocate external IPs for every Service of type LoadBalancer, and it sets the allocated IP to the loadBalancer.ingress field in the Service resource status. MetalLB also supports user specified loadBalancerIP in the Service spec. For more information, please refer to the MetalLB usage.

To learn more about MetalLB concepts and functionalities, you can read the MetalLB concepts.

Install MetalLB

You can run the following commands to install MetalLB using the YAML manifests:

kubectl apply -f https://raw.githubusercontent.com/metallb/metallb/v0.13.11/config/manifests/metallb-native.yaml

The commands will deploy MetalLB version 0.13.11 into Namespace metallb-system. You can also refer to this MetalLB installation guide for other ways of installing MetalLB.

As MetalLB will allocate external IPs for all Services of type LoadBalancer, once it is running, the Service external IP management feature of Antrea should not be enabled to avoid conflicts with MetalLB. You can deploy Antrea with the default configuration (in which the ServiceExternalIP feature gate of antrea-agent is set to false). MetalLB can work with both Antrea Proxy and kube-proxy configurations of antrea-agent.

Configure MetalLB with layer 2 mode

Similar to the case of Antrea Service external IP management, MetalLB layer 2 mode also requires kube-proxy's strictARP configuration to be enabled, when you are using kube-proxy IPVS. Please refer to the Interoperability with kube-proxy IPVS mode section for more information.

MetalLB is configured through Custom Resources (since v0.13). To configure MetalLB to work in the layer 2 mode, you need to create an L2Advertisement resource, as well as an IPAddressPool resource, which provides the IP ranges to allocate external IPs from. The IP ranges should be from the Node network subnet.

For example:

apiVersion: metallb.io/v1beta1
kind: IPAddressPool
metadata:
  name: first-pool
  namespace: metallb-system
spec:
  addresses:
  - 10.10.0.2-10.10.0.10
---
apiVersion: metallb.io/v1beta1
kind: L2Advertisement
metadata:
  name: example
  namespace: metallb-system

Configure MetalLB with BGP mode

The BGP mode of MetalLB requires more configuration parameters to establish BGP peering to the router. The example resources below configure MetalLB using AS number 64500 to connect to peer router 10.0.0.1 with AS number 64501:

apiVersion: metallb.io/v1beta2
kind: BGPPeer
metadata:
  name: sample
  namespace: metallb-system
spec:
  myASN: 64500
  peerASN: 64501
  peerAddress: 10.0.0.1
---
apiVersion: metallb.io/v1beta1
kind: IPAddressPool
metadata:
  name: first-pool
  namespace: metallb-system
spec:
  addresses:
  - 10.10.0.2-10.10.0.10
---
apiVersion: metallb.io/v1beta1
kind: BGPAdvertisement
metadata:
  name: example
  namespace: metallb-system

In addition to the basic layer 2 and BGP mode configurations described in this document, MetalLB supports a few more advanced BGP configurations and supports configuring multiple IP pools which can use different modes. For more information, please refer to the MetalLB configuration guide.

Interoperability with kube-proxy IPVS mode

Both Antrea Service external IP management and MetalLB layer 2 mode require kube-proxy's strictARP configuration to be enabled, to work with kube-proxy in IPVS mode. You can check the strictARP configuration in the kube-proxy ConfigMap:

$ kubectl describe configmap -n kube-system kube-proxy | grep strictARP
  strictARP: false

You can set strictARP to true by editing the kube-proxy ConfigMap:

kubectl edit configmap -n kube-system kube-proxy

Or, simply run the following command to set it:

$ kubectl get configmap kube-proxy -n kube-system -o yaml | \
 sed -e "s/strictARP: false/strictARP: true/" | \
 kubectl apply -f - -n kube-system

Last, to check the change is made:

$ kubectl describe configmap -n kube-system kube-proxy | grep strictARP
  strictARP: true

Issue with Antrea Egress

If you are using Antrea v1.7.0 or later, please ignore the issue. The previous implementation of Antrea Egress before v1.7.0 does not work with the strictARP configuration of kube-proxy. It means Antrea Egress cannot work together with Service external IP management or MetalLB layer 2 mode, when kube-proxy IPVS is used. This issue was fixed in Antrea v1.7.0.