Enable CoreDNS fallthrough for cluster.local DNS resolution

Pods inside a bink cluster cannot resolve bink-managed DNS names
like registry.cluster.local because CoreDNS's kubernetes plugin
handles the cluster.local zone but only resolves Kubernetes
services/pods/endpoints.

After kubeadm deploys CoreDNS, patch the Corefile to change
"fallthrough in-addr.arpa ip6.arpa" to just "fallthrough". This
removes the zone restriction so all unresolved cluster.local
queries (not just reverse DNS) fall through to the forward plugin,
which reaches the bink dnsmasq container via the node's
/etc/resolv.conf.

Fixes: #70

Assisted-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
Signed-off-by: Alice Frosi <afrosi@redhat.com>

Author: alicefr

internal/cluster/init.go

@@ -135,6 +135,14 @@ func (c *Cluster) Init(ctx context.Context, opts InitOptions) error {
 		return fmt.Errorf("failed to install CoreDNS: %w", err)
 	}
 
+	// Enable fallthrough in CoreDNS so that non-Kubernetes cluster.local names
+	// (e.g. registry.cluster.local) are forwarded to the bink DNS container.
+	c.logger.Info("")
+	c.logger.Info("=== Patching CoreDNS for cluster.local fallthrough ===")
+	if err := kubeClient.EnableCoreDNSFallthrough(ctx); err != nil {
+		return fmt.Errorf("failed to enable CoreDNS fallthrough: %w", err)
+	}
+
 	// Patch CoreDNS to run as root - CRI-O doesn't set ambient capabilities
 	// for non-root users, so NET_BIND_SERVICE doesn't take effect for UID 65532
 	c.logger.Info("")

internal/kube/resources.go

@@ -7,6 +7,7 @@ import (
 	"context"
 	"encoding/json"
 	"fmt"
+	"strings"
 	"time"
 
 	policyv1 "k8s.io/api/policy/v1"
@@ -127,6 +128,34 @@ func (c *Client) DeleteNode(ctx context.Context, nodeName string) error {
 	return nil
 }
 
+// EnableCoreDNSFallthrough patches the CoreDNS ConfigMap so that unresolved
+// cluster.local queries fall through to the forward plugin (which reaches the
+// bink dnsmasq container via the node's /etc/resolv.conf).
+func (c *Client) EnableCoreDNSFallthrough(ctx context.Context) error {
+	cm, err := c.clientset.CoreV1().ConfigMaps("kube-system").Get(ctx, "coredns", metav1.GetOptions{})
+	if err != nil {
+		return fmt.Errorf("getting coredns ConfigMap: %w", err)
+	}
+
+	corefile, ok := cm.Data["Corefile"]
+	if !ok {
+		return fmt.Errorf("Corefile key not found in coredns ConfigMap")
+	}
+
+	updated := strings.Replace(corefile, "fallthrough in-addr.arpa ip6.arpa", "fallthrough", 1)
+	if updated == corefile {
+		return nil
+	}
+
+	cm.Data["Corefile"] = updated
+	_, err = c.clientset.CoreV1().ConfigMaps("kube-system").Update(ctx, cm, metav1.UpdateOptions{})
+	if err != nil {
+		return fmt.Errorf("updating coredns ConfigMap: %w", err)
+	}
+
+	return nil
+}
+
 // LabelNode adds the given labels to a node, overwriting any that already exist.
 func (c *Client) LabelNode(ctx context.Context, nodeName string, labels map[string]string) error {
 	patch := map[string]interface{}{