Using ExternalDNS with 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 being created, echoserver.mycluster.example.org and
echoserver.example.org, which both alias the ALB that is associated with the
Ingress object.

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 ALBs

AWS supports both IPv4 and “dualstack” (both IPv4 and IPv6) interfaces for ALBs.
The AWS Load Balancer Controller uses the alb.ingress.kubernetes.io/ip-address-type
annotation (which defaults to ipv4) to determine this. If this annotation is
set to dualstack then ExternalDNS will create two alias records (one A record
and one AAAA record) for each hostname associated with the Ingress object.

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. ExternalDNS will create both an A echoserver.example.org record and
an AAAA record of the same name, that each are aliases for the same ALB.