Allow cidr based reverse zone config (#500)

* add cidrs opt

* remove state data from middleware object

middleware/kubernetes/handler.go

@@ -26,8 +26,13 @@ func (k Kubernetes) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.M
 	// otherwise delegate to the next in the pipeline.
 	zone := middleware.Zones(k.Zones).Matches(state.Name())
 	if zone == "" {
+		// If this is a PTR request, and a the request is in a defined
+		// pod/service cidr range, process the request in this middleware,
+		// otherwise pass to next middleware.
+		if state.Type() != "PTR" || !k.IsRequestInReverseRange(state) {
 			return middleware.NextOrFailure(k.Name(), k.Next, ctx, w, r)
 		}
+	}
 
 	var (
 		records, extra []dns.RR

middleware/kubernetes/kubernetes.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"log"
+	"net"
 	"strings"
 	"time"
 
@@ -41,6 +42,7 @@ type Kubernetes struct {
 	LabelSelector *unversionedapi.LabelSelector
 	Selector      *labels.Selector
 	PodMode       string
+	ReverseCidrs  []net.IPNet
 }
 
 const (
@@ -128,6 +130,16 @@ func (k *Kubernetes) Reverse(state request.Request, exact bool, opt middleware.O
 	return records, nil, nil
 }
 
+func (k *Kubernetes) IsRequestInReverseRange(state request.Request) bool {
+	ip := dnsutil.ExtractAddressFromReverse(state.Name())
+	for _, c := range k.ReverseCidrs {
+		if c.Contains(net.ParseIP(ip)) {
+			return true
+		}
+	}
+	return false
+}
+
 // Lookup implements the ServiceBackend interface.
 func (k *Kubernetes) Lookup(state request.Request, name string, typ uint16) (*dns.Msg, error) {
 	return k.Proxy.Lookup(state, name, typ)

middleware/kubernetes/setup.go

@@ -3,6 +3,7 @@ package kubernetes
 import (
 	"errors"
 	"fmt"
+	"net"
 	"strings"
 	"time"
 
@@ -84,6 +85,20 @@ func kubernetesParse(c *caddy.Controller) (*Kubernetes, error) {
 
 			for c.NextBlock() {
 				switch c.Val() {
+				case "cidrs":
+					args := c.RemainingArgs()
+					if len(args) > 0 {
+						for _, cidrStr := range args {
+							_, cidr, err := net.ParseCIDR(cidrStr)
+							if err != nil {
+								return nil, errors.New(c.Val() + " contains an invalid cidr: " + cidrStr)
+							}
+							k8s.ReverseCidrs = append(k8s.ReverseCidrs, *cidr)
+
+						}
+						continue
+					}
+					return nil, c.ArgErr()
 				case "pods":
 					args := c.RemainingArgs()
 					if len(args) == 1 {