plugin/k8s_extenral: Supports fallthrough option (#5959)

* Add fallthrough option to k8s_external plugin to allow transitioning control to the next plugin if the domain is not found
* Exit on start up if required plugin is not present.

Signed-off-by: vanceli <vanceli@tencent.com>

---------

Signed-off-by: vanceli <vanceli@tencent.com>
Co-authored-by: vanceli <vanceli@tencent.com>

plugin/k8s_external/README.md

@@ -10,8 +10,7 @@ This plugin allows an additional zone to resolve the external IP address(es) of
 service and headless services. This plugin is only useful if the *kubernetes* plugin is also loaded.
 
 The plugin uses an external zone to resolve in-cluster IP addresses. It only handles queries for A,
-AAAA, SRV, and PTR records; all others result in NODATA responses. To make it a proper DNS zone, it handles
+AAAA, SRV, and PTR records; To make it a proper DNS zone, it handles SOA and NS queries for the apex of the zone.
-SOA and NS queries for the apex of the zone.
 
 By default the apex of the zone will look like the following (assuming the zone used is `example.org`):
 
@@ -67,6 +66,14 @@ k8s_external [ZONE...] {
 
 * if there is a headless service with external IPs set, external IPs will be resolved
 
+If the queried domain does not exist, you can fall through to next plugin by adding the `fallthrough` option.
+
+~~~
+k8s_external [ZONE...] {
+    fallthrough [ZONE...]
+}
+~~~
+
 ## Examples
 
 Enable names under `example.org` to be resolved to in-cluster DNS addresses.
@@ -106,6 +113,18 @@ zone transfers.  Notifies are not supported.
      }
  ~~~
 
+With the `fallthrough` option, if the queried domain does not exist, it will be passed to the next plugin that matches the zone.
+
+~~~
+. {
+   kubernetes cluster.local
+   k8s_external example.org {
+     fallthrough
+   }
+   forward . 8.8.8.8
+}
+~~~
+
 # See Also
 
 For some background see [resolve external IP address](https://github.com/kubernetes/dns/issues/242).

plugin/k8s_external/external.go

@@ -16,6 +16,7 @@ import (
 
 	"github.com/coredns/coredns/plugin"
 	"github.com/coredns/coredns/plugin/etcd/msg"
+	"github.com/coredns/coredns/plugin/pkg/fall"
 	"github.com/coredns/coredns/plugin/pkg/upstream"
 	"github.com/coredns/coredns/request"
 
@@ -39,6 +40,7 @@ type Externaler interface {
 type External struct {
 	Next  plugin.Handler
 	Zones []string
+	Fall  fall.F
 
 	hostmaster string
 	apex       string
@@ -68,10 +70,6 @@ func (e *External) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Ms
 		return plugin.NextOrFailure(e.Name(), e.Next, ctx, w, r)
 	}
 
-	if e.externalFunc == nil {
-		return plugin.NextOrFailure(e.Name(), e.Next, ctx, w, r)
-	}
-
 	state.Zone = zone
 	for _, z := range e.Zones {
 		// TODO(miek): save this in the External struct.
@@ -93,6 +91,10 @@ func (e *External) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Ms
 	m.Authoritative = true
 
 	if len(svc) == 0 {
+		if e.Fall.Through(state.Name()) && rcode == dns.RcodeNameError {
+			return plugin.NextOrFailure(e.Name(), e.Next, ctx, w, r)
+		}
+
 		m.Rcode = rcode
 		m.Ns = []dns.RR{e.soa(state)}
 		w.WriteMsg(m)

plugin/k8s_external/setup.go

@@ -1,6 +1,7 @@
 package external
 
 import (
+	"errors"
 	"strconv"
 
 	"github.com/coredns/caddy"
@@ -9,7 +10,9 @@ import (
 	"github.com/coredns/coredns/plugin/pkg/upstream"
 )
 
-func init() { plugin.Register("k8s_external", setup) }
+const pluginName = "k8s_external"
+
+func init() { plugin.Register(pluginName, setup) }
 
 func setup(c *caddy.Controller) error {
 	e, err := parse(c)
@@ -21,14 +24,18 @@ func setup(c *caddy.Controller) error {
 	c.OnStartup(func() error {
 		m := dnsserver.GetConfig(c).Handler("kubernetes")
 		if m == nil {
-			return nil
+			return plugin.Error(pluginName, errors.New("kubernetes plugin not loaded"))
 		}
-		if x, ok := m.(Externaler); ok {
+
-			e.externalFunc = x.External
+		x, ok := m.(Externaler)
-			e.externalAddrFunc = x.ExternalAddress
+		if !ok {
-			e.externalServicesFunc = x.ExternalServices
+			return plugin.Error(pluginName, errors.New("kubernetes plugin does not implement the Externaler interface"))
-			e.externalSerialFunc = x.ExternalSerial
 		}
+
+		e.externalFunc = x.External
+		e.externalAddrFunc = x.ExternalAddress
+		e.externalServicesFunc = x.ExternalServices
+		e.externalSerialFunc = x.ExternalSerial
 		return nil
 	})
 
@@ -70,6 +77,8 @@ func parse(c *caddy.Controller) (*External, error) {
 				e.apex = args[0]
 			case "headless":
 				e.headless = true
+			case "fallthrough":
+				e.Fall.SetZonesFromArgs(c.RemainingArgs())
 			default:
 				return nil, c.Errf("unknown property '%s'", c.Val())
 			}

plugin/k8s_external/setup_test.go

@@ -4,24 +4,33 @@ import (
 	"testing"
 
 	"github.com/coredns/caddy"
+	"github.com/coredns/coredns/plugin/pkg/fall"
 )
 
 func TestSetup(t *testing.T) {
 	tests := []struct {
-		input            string
+		input               string
-		shouldErr        bool
+		shouldErr           bool
-		expectedZone     string
+		expectedZone        string
-		expectedApex     string
+		expectedApex        string
-		expectedHeadless bool
+		expectedHeadless    bool
+		expectedFallthrough fall.F
 	}{
-		{`k8s_external`, false, "", "dns", false},
+		{`k8s_external`, false, "", "dns", false, fall.Zero},
-		{`k8s_external example.org`, false, "example.org.", "dns", false},
+		{`k8s_external example.org`, false, "example.org.", "dns", false, fall.Zero},
 		{`k8s_external example.org {
 			apex testdns
-}`, false, "example.org.", "testdns", false},
+}`, false, "example.org.", "testdns", false, fall.Zero},
 		{`k8s_external example.org {
 	headless
-}`, false, "example.org.", "dns", true},
+}`, false, "example.org.", "dns", true, fall.Zero},
+		{`k8s_external example.org {
+	fallthrough
+}`, false, "example.org.", "dns", false, fall.Root},
+		{`k8s_external example.org {
+	fallthrough ip6.arpa inaddr.arpa foo.com
+}`, false, "example.org.", "dns", false,
+			fall.F{Zones: []string{"ip6.arpa.", "inaddr.arpa.", "foo.com."}}},
 	}
 
 	for i, test := range tests {
@@ -53,5 +62,10 @@ func TestSetup(t *testing.T) {
 				t.Errorf("Test %d, expected headless %q for input %s, got: %v", i, test.expectedApex, test.input, e.headless)
 			}
 		}
+		if !test.shouldErr {
+			if !e.Fall.Equal(test.expectedFallthrough) {
+				t.Errorf("Test %d, expected to be initialized with fallthrough %q for input %s, got: %v", i, test.expectedFallthrough, test.input, e.Fall)
+			}
+		}
 	}
 }