fix: calculate cluster dns correctly for non /16 blocks

Signed-off-by: Jonah Back <jonah@jonahback.com>

controllers/providers/aws/eks.go

@@ -218,6 +218,10 @@ func (w *AwsWorker) GetDNSClusterIP(cluster *eks.Cluster) string {
 		return ""
 	}
 	serviceCidr := aws.StringValue(cluster.KubernetesNetworkConfig.ServiceIpv4Cidr)
-	// addresses assigned from either the 10.100.0.0/16 or 172.20.0.0/16 CIDR blocks
-	return strings.ReplaceAll(serviceCidr, "0/16", "10")
+	// addresses are by default assigned from either the 10.100.0.0/16 or 172.20.0.0/16 CIDR blocks.
+	// custom ranges could be blocks that are not /16 size.
+	ip := strings.Split(serviceCidr, "/")[0] // remove block size
+	blocks := strings.Split(ip, ".")
+	blocks[3] = "10" // replace last byte in IP address with "10" as is the convention
+	return strings.Join(blocks, ".")
 }

controllers/providers/aws/eks_test.go

@@ -0,0 +1,19 @@
+package aws
+
+import (
+	"github.com/aws/aws-sdk-go/service/eks"
+	"github.com/onsi/gomega"
+	"testing"
+)
+
+func TestClusterDns(t *testing.T) {
+	var (
+		g       = gomega.NewGomegaWithT(t)
+	)
+
+	awsWorker := AwsWorker{}
+	cidr := "172.16.0.0/12"
+	ip := awsWorker.GetDNSClusterIP(&eks.Cluster{KubernetesNetworkConfig: &eks.KubernetesNetworkConfigResponse{ServiceIpv4Cidr: &cidr}})
+	g.Expect(ip).To(gomega.Equal("172.16.0.10"))
+
+}