1 files changed, 48 insertions, 0 deletions
diff --git a/middleware/kubernetes/subzone.go b/middleware/kubernetes/subzone.go
new file mode 100644
index 000000000..48efba472
--- /dev/null
+++ b/middleware/kubernetes/subzone.go
@@ -0,0 +1,48 @@
+package kubernetes
+
+import (
+	"fmt"
+
+	"github.com/miekg/dns"
+)
+
+// NormalizeZoneList filters the zones argument to remove
+// array items that conflict with other items in zones.
+// For example, providing the following zones array:
+//    [ "a.b.c", "b.c", "a", "e.d.f", "a.b" ]
+// Returns:
+//    [ "a.b.c", "a", "e.d.f", "a.b" ]
+// Zones filted out:
+//    - "b.c" because "a.b.c" and "b.c" share the common top
+//      level "b.c". First listed zone wins if there is a conflict.
+//
+// Note: This may prove to be too restrictive in practice.
+//       Need to find counter-example use-cases.
+func NormalizeZoneList(zones []string) []string {
+	filteredZones := []string{}
+
+	for _, z := range zones {
+		zoneConflict, _ := subzoneConflict(filteredZones, z)
+		if zoneConflict {
+			fmt.Printf("[WARN] new zone '%v' from Corefile conflicts with existing zones: %v\n        Ignoring zone '%v'\n", z, filteredZones, z)
+		} else {
+			filteredZones = append(filteredZones, z)
+		}
+	}
+
+	return filteredZones
+}
+
+// subzoneConflict returns true if name is a child or parent zone of
+// any element in zones. If conflicts exist, return the conflicting zones.
+func subzoneConflict(zones []string, name string) (bool, []string) {
+	conflicts := []string{}
+
+	for _, z := range zones {
+		if dns.IsSubDomain(z, name) || dns.IsSubDomain(name, z) {
+			conflicts = append(conflicts, z)
+		}
+	}
+
+	return (len(conflicts) != 0), conflicts
+}