Add new plugin: external - resolve k8s ingress and LB address with external names (#2379)

* Add new plugin: external

This plugin works in conjunction with the kubernetes plugin and exports
ingress and LB addresses as DNS records. It bypasses backend.go and
backend_lookup.go flow because it is not needed.

README, tests are implemented. The tests only exercise the unit tests,
this has not been tested in any ci.

Signed-off-by: Miek Gieben <miek@miek.nl>

* Rename to k8s_external

Signed-off-by: Miek Gieben <miek@miek.nl>

* go gen

Signed-off-by: Miek Gieben <miek@miek.nl>

plugin/k8s_external/setup.go

@@ -0,0 +1,86 @@
+package external
+
+import (
+	"strconv"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("k8s_external", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	e, err := parse(c)
+	if err != nil {
+		return plugin.Error("k8s_external", err)
+	}
+
+	// Do this in OnStartup, so all plugins have been initialized.
+	c.OnStartup(func() error {
+		m := dnsserver.GetConfig(c).Handler("kubernetes")
+		if m == nil {
+			return nil
+		}
+		if x, ok := m.(Externaler); ok {
+			e.externalFunc = x.External
+			e.externalAddrFunc = x.ExternalAddress
+		}
+		return nil
+	})
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		e.Next = next
+		return e
+	})
+
+	return nil
+}
+
+func parse(c *caddy.Controller) (*External, error) {
+	e := New()
+
+	for c.Next() { // external
+		zones := c.RemainingArgs()
+		e.Zones = zones
+		if len(zones) == 0 {
+			e.Zones = make([]string, len(c.ServerBlockKeys))
+			copy(e.Zones, c.ServerBlockKeys)
+		}
+		for i, str := range e.Zones {
+			e.Zones[i] = plugin.Host(str).Normalize()
+		}
+		for c.NextBlock() {
+			switch c.Val() {
+			case "ttl":
+				args := c.RemainingArgs()
+				if len(args) == 0 {
+					return nil, c.ArgErr()
+				}
+				t, err := strconv.Atoi(args[0])
+				if err != nil {
+					return nil, err
+				}
+				if t < 0 || t > 3600 {
+					return nil, c.Errf("ttl must be in range [0, 3600]: %d", t)
+				}
+				e.ttl = uint32(t)
+			case "apex":
+				args := c.RemainingArgs()
+				if len(args) == 0 {
+					return nil, c.ArgErr()
+				}
+				e.apex = args[0]
+			default:
+				return nil, c.Errf("unknown property '%s'", c.Val())
+			}
+		}
+	}
+	return e, nil
+}