Initial implementation of ForwardCRD plugin (#4512)

* Add forwardcrd plugin README.md

Co-authored-by: Aidan Obley <aobley@vmware.com>

Signed-off-by: Christian Ang <angc@vmware.com>

* Create forwardcrd plugin

- Place forwardcrd before forward plugin in plugin list. This will avoid
forward from preventing the forwardcrd plugin from handling any queries
in the case of having a default upstream forwarder in a server block (as
is the case in the default kubernetes Corefile).

Co-authored-by: Aidan Obley <aobley@vmware.com>

Signed-off-by: Christian Ang <angc@vmware.com>

* Add Forward CRD

Signed-off-by: Christian Ang <angc@vmware.com>

* Add NewWithConfig to forward plugin

- allows external packages to instanciate forward plugins

Co-authored-by: Aidan Obley <aobley@vmware.com>

Signed-off-by: Christian Ang <angc@vmware.com>

* ForwardCRD plugin handles requests for Forward CRs

- add a Kubernetes controller that can read Forward CRs
- instances of the forward plugin are created based on Forward CRs from
the Kubernetes controller
- DNS requests are handled by calling matching Forward plugin instances
based on zone name
- Defaults to the kube-system namespace to align with Corefile RBAC

Signed-off-by: Christian Ang <angc@vmware.com>

Use klog v2 in forwardcrd plugin

* Refactor forward setup to use NewWithConfig

Co-authored-by: Christian Ang <angc@vmware.com>

Signed-off-by: Edwin Xie <exie@vmware.com>

* Use ParseInt instead of Atoi

- to ensure that the bitsize is 32 for later casting to uint32

Signed-off-by: Christian Ang <angc@vmware.com>

* Add @christianang to CODEOWNERS for forwardcrd

Signed-off-by: Christian Ang <angc@vmware.com>

Co-authored-by: Edwin Xie <exie@vmware.com>

plugin/forwardcrd/setup.go

@@ -0,0 +1,147 @@
+package forwardcrd
+
+import (
+	"context"
+	"os"
+	"time"
+
+	"github.com/coredns/caddy"
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/dnstap"
+	clog "github.com/coredns/coredns/plugin/pkg/log"
+
+	"k8s.io/client-go/tools/clientcmd"
+	"k8s.io/klog/v2"
+)
+
+const pluginName = "forwardcrd"
+
+var log = clog.NewWithPlugin(pluginName)
+
+func init() {
+	plugin.Register(pluginName, setup)
+}
+
+func setup(c *caddy.Controller) error {
+	klog.SetOutput(os.Stdout)
+
+	k, err := parseForwardCRD(c)
+	if err != nil {
+		return plugin.Error(pluginName, err)
+	}
+
+	err = k.InitKubeCache(context.Background())
+	if err != nil {
+		return plugin.Error(pluginName, err)
+	}
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		k.Next = next
+		return k
+	})
+
+	c.OnStartup(func() error {
+		go k.APIConn.Run(1)
+
+		timeout := time.After(5 * time.Second)
+		ticker := time.NewTicker(100 * time.Millisecond)
+		for {
+			select {
+			case <-ticker.C:
+				if k.APIConn.HasSynced() {
+					return nil
+				}
+			case <-timeout:
+				return nil
+			}
+		}
+	})
+
+	c.OnStartup(func() error {
+		if taph := dnsserver.GetConfig(c).Handler("dnstap"); taph != nil {
+			if tapPlugin, ok := taph.(dnstap.Dnstap); ok {
+				k.APIConn.(*forwardCRDControl).tapPlugin = &tapPlugin
+			}
+		}
+		return nil
+	})
+
+	c.OnShutdown(func() error {
+		return k.APIConn.Stop()
+	})
+
+	return nil
+}
+
+func parseForwardCRD(c *caddy.Controller) (*ForwardCRD, error) {
+	var (
+		k   *ForwardCRD
+		err error
+		i   int
+	)
+
+	for c.Next() {
+		if i > 0 {
+			return nil, plugin.ErrOnce
+		}
+		i++
+		k, err = parseStanza(c)
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	return k, nil
+}
+
+func parseStanza(c *caddy.Controller) (*ForwardCRD, error) {
+	k := New()
+
+	args := c.RemainingArgs()
+	k.Zones = plugin.OriginsFromArgsOrServerBlock(args, c.ServerBlockKeys)
+
+	for c.NextBlock() {
+		switch c.Val() {
+		case "endpoint":
+			args := c.RemainingArgs()
+			if len(args) != 1 {
+				return nil, c.ArgErr()
+			}
+			k.APIServerEndpoint = args[0]
+		case "tls":
+			args := c.RemainingArgs()
+			if len(args) != 3 {
+				return nil, c.ArgErr()
+			}
+			k.APIClientCert, k.APIClientKey, k.APICertAuth = args[0], args[1], args[2]
+		case "kubeconfig":
+			args := c.RemainingArgs()
+			if len(args) != 1 && len(args) != 2 {
+				return nil, c.ArgErr()
+			}
+			overrides := &clientcmd.ConfigOverrides{}
+			if len(args) == 2 {
+				overrides.CurrentContext = args[1]
+			}
+			config := clientcmd.NewNonInteractiveDeferredLoadingClientConfig(
+				&clientcmd.ClientConfigLoadingRules{ExplicitPath: args[0]},
+				overrides,
+			)
+			k.ClientConfig = config
+		case "namespace":
+			args := c.RemainingArgs()
+			if len(args) == 0 {
+				k.Namespace = ""
+			} else if len(args) == 1 {
+				k.Namespace = args[0]
+			} else {
+				return nil, c.ArgErr()
+			}
+		default:
+			return nil, c.Errf("unknown property '%s'", c.Val())
+		}
+	}
+
+	return k, nil
+}