external-dns/docs/tutorials/aws-load-balancer-controller.md

AWS Load Balancer Controller

This tutorial describes how to use ExternalDNS with the aws-load-balancer-controller.

Setting up ExternalDNS and aws-load-balancer-controller

Follow the AWS tutorial to setup ExternalDNS for use in Kubernetes clusters running in AWS. Specify the source=ingress argument so that ExternalDNS will look for hostnames in Ingress objects. In addition, you may wish to limit which Ingress objects are used as an ExternalDNS source via the ingress-class argument, but this is not required.

For help setting up the AWS Load Balancer Controller, follow the Setup Guide.

Note that the AWS Load Balancer Controller uses the same tags for subnet auto-discovery as Kubernetes does with the AWS cloud provider.

In the examples that follow, it is assumed that you configured the ALB Ingress Controller with the ingress-class=alb argument (not to be confused with the same argument to ExternalDNS) so that the controller will only respect Ingress objects with the ingressClassName field set to "alb".

Deploy an example application

Create the following sample "echoserver" application to demonstrate how ExternalDNS works with ALB ingress objects.

apiVersion: apps/v1
kind: Deployment
metadata:
  name: echoserver
spec:
  replicas: 1
  selector:
    matchLabels:
      app: echoserver
  template:
    metadata:
      labels:
        app: echoserver
    spec:
      containers:
      - image: gcr.io/google_containers/echoserver:1.4
        imagePullPolicy: Always
        name: echoserver
        ports:
        - containerPort: 8080
---
apiVersion: v1
kind: Service
metadata:
  name: echoserver
spec:
  ports:
    - port: 80
      targetPort: 8080
      protocol: TCP
  type: NodePort
  selector:
    app: echoserver

Note that the Service object is of type NodePort. We don't need a Service of type LoadBalancer here, since we will be using an Ingress to create an ALB.

Ingress examples

Create the following Ingress to expose the echoserver application to the Internet.

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  annotations:
    alb.ingress.kubernetes.io/scheme: internet-facing
  name: echoserver
spec:
  ingressClassName: alb
  rules:
  - host: echoserver.mycluster.example.org
    http: &echoserver_root
      paths:
      - path: /
        backend:
          service:
            name: echoserver
            port:
              number: 80
        pathType: Prefix
  - host: echoserver.example.org
    http: *echoserver_root

The above should result in the creation of an (ipv4) ALB in AWS which will forward traffic to the echoserver application.

If the --source=ingress argument is specified, then ExternalDNS will create DNS records based on the hosts specified in ingress objects. The above example would result in two alias records (A and AAAA) being created for each of the domains: echoserver.mycluster.example.org and echoserver.example.org. All four records alias the ALB that is associated with the Ingress object. As the ALB is IPv4 only, the AAAA alias records have no effect.

If you would like ExternalDNS to not create AAAA records at all, you can add the following command line parameter: --exclude-record-types=AAAA. Please be aware, this will disable AAAA record creation even for dualstack enabled load balancers.

Note that the above example makes use of the YAML anchor feature to avoid having to repeat the http section for multiple hosts that use the exact same paths. If this Ingress object will only be fronting one backend Service, we might instead create the following:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  annotations:
    alb.ingress.kubernetes.io/scheme: internet-facing
    external-dns.alpha.kubernetes.io/hostname: echoserver.mycluster.example.org, echoserver.example.org
  name: echoserver
spec:
  ingressClassName: alb
  rules:
  - http:
      paths:
      - path: /
        backend:
          service:
            name: echoserver
            port:
              number: 80
        pathType: Prefix

In the above example we create a default path that works for any hostname, and make use of the external-dns.alpha.kubernetes.io/hostname annotation to create multiple aliases for the resulting ALB.

Dualstack Load Balancers

AWS supports both IPv4 and "dualstack" (both IPv4 and IPv6) interfaces for ALBs and NLBs. The AWS Load Balancer Controller uses the alb.ingress.kubernetes.io/ip-address-type annotation (which defaults to ipv4) to determine this. ExternalDNS creates both A and AAAA alias DNS records by default, regardless of this annotation. It's possible to create only A records with the following command line parameter: --exclude-record-types=AAAA

Example:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  annotations:
    alb.ingress.kubernetes.io/scheme: internet-facing
    alb.ingress.kubernetes.io/ip-address-type: dualstack
  name: echoserver
spec:
  ingressClassName: alb
  rules:
  - host: echoserver.example.org
    http:
      paths:
      - path: /
        backend:
          service:
            name: echoserver
            port:
              number: 80
        pathType: Prefix

The above Ingress object will result in the creation of an ALB with a dualstack interface.

Frontend Network Load Balancer (NLB)

The AWS Load Balancer Controller supports fronting ALBs with an NLB for improved performance and static IP addresses. When this feature is enabled, the controller creates both an ALB and an NLB, resulting in two hostnames in the Ingress status.

Known Issue with Internal ALBs

When using an internal ALB (alb.ingress.kubernetes.io/scheme: internal) with frontend NLB, ExternalDNS may create DNS records pointing to the ALB instead of the NLB due to alphabetical ordering:

• Internal ALB hostname: internal-k8s-myapp-alb.us-east-1.elb.amazonaws.com
• NLB hostname: k8s-myapp-nlb-123456789.elb.us-east-1.amazonaws.com

When multiple targets exist, Route53 selects the first one alphabetically, which incorrectly selects the internal ALB. See issue #5661 for details.

Workarounds

There are several approaches to ensure DNS records point to the correct (NLB) target:

Option 1: Combine load balancer naming with target annotation (Recommended)

Use alb.ingress.kubernetes.io/load-balancer-name to create predictable hostnames, then explicitly reference the NLB using external-dns.alpha.kubernetes.io/target:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  annotations:
    alb.ingress.kubernetes.io/scheme: internal
    alb.ingress.kubernetes.io/enable-frontend-nlb: "true"
    alb.ingress.kubernetes.io/frontend-nlb-scheme: internal
    alb.ingress.kubernetes.io/load-balancer-name: myapp-alb
    external-dns.alpha.kubernetes.io/target: k8s-myapp-nlb.elb.us-east-1.amazonaws.com
  name: echoserver
spec:
  ingressClassName: alb
  rules:
  - host: echoserver.example.org
    http:
      paths:
      - path: /
        backend:
          service:
            name: echoserver
            port:
              number: 80
        pathType: Prefix

Benefits:

• Predictable, consistent load balancer naming across environments
• Explicit control over which target ExternalDNS uses
• Works reliably with internal ALBs
• No need to lookup auto-generated NLB names

NLB hostname pattern: When you set load-balancer-name: myapp-alb, the NLB hostname becomes k8s-myapp-nlb.elb.<region>.amazonaws.com (note the -nlb suffix).

ALB internal hostname pattern: When you set load-balancer-name: myapp-alb, the ALB hostname becomes internal-myapp-nlb.<region>.elb.amazonaws.com (note the internal- suffix).

Option 2: Use the target annotation only

If you cannot control the load balancer name, explicitly specify the NLB hostname:

apiVersion: networking.k8s.io/v1
kind: Ingress
metadata:
  annotations:
    alb.ingress.kubernetes.io/scheme: internal
    alb.ingress.kubernetes.io/enable-frontend-nlb: "true"
    alb.ingress.kubernetes.io/frontend-nlb-scheme: internal
    external-dns.alpha.kubernetes.io/target: k8s-myapp-nlb-123456789.elb.us-east-1.amazonaws.com
  name: echoserver
spec:
  ingressClassName: alb
  rules:
  - host: echoserver.example.org
    http:
      paths:
      - path: /
        backend:
          service:
            name: echoserver
            port:
              number: 80
        pathType: Prefix

Note: You'll need to lookup the auto-generated NLB hostname after the controller creates it.

Option 3: Use a DNSEndpoint resource

Create a DNSEndpoint custom resource to explicitly define the DNS record:

apiVersion: externaldns.k8s.io/v1alpha1
kind: DNSEndpoint
metadata:
  name: echoserver-dns
spec:
  endpoints:
  - dnsName: echoserver.example.org
    recordType: CNAME
    targets:
    - k8s-myapp-nlb-123456789.elb.us-east-1.amazonaws.com

This approach is useful when you want to manage DNS records independently of the Ingress resource.