New plugin: loop (#1989)

* New plugin: loop

Add a plugin that detects loops. It does this by sending an unique query
to our selves. If we see the query more than twice we stop the process.
If there isn't a loop, the plugin disables it self and becomes a noop
plugin.

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

plugin/loop/setup.go

@@ -0,0 +1,89 @@
+package loop
+
+import (
+	"math/rand"
+	"net"
+	"strconv"
+	"time"
+
+	"github.com/coredns/coredns/core/dnsserver"
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/pkg/dnsutil"
+
+	"github.com/mholt/caddy"
+)
+
+func init() {
+	caddy.RegisterPlugin("loop", caddy.Plugin{
+		ServerType: "dns",
+		Action:     setup,
+	})
+}
+
+func setup(c *caddy.Controller) error {
+	l, err := parse(c)
+	if err != nil {
+		return plugin.Error("loop", err)
+	}
+
+	dnsserver.GetConfig(c).AddPlugin(func(next plugin.Handler) plugin.Handler {
+		l.Next = next
+		return l
+	})
+
+	// Send query to ourselves and see if it end up with us again.
+	c.OnStartup(func() error {
+		// Another Go function, otherwise we block startup and can't send the packet.
+		go func() {
+			deadline := time.Now().Add(30 * time.Second)
+			conf := dnsserver.GetConfig(c)
+
+			for time.Now().Before(deadline) {
+				lh := conf.ListenHosts[0]
+				addr := net.JoinHostPort(lh, conf.Port)
+				if _, err := l.exchange(addr); err != nil {
+					time.Sleep(1 * time.Second)
+					continue
+				}
+
+				go func() {
+					time.Sleep(2 * time.Second)
+					l.setDisabled()
+				}()
+			}
+			l.setDisabled()
+		}()
+		return nil
+	})
+
+	return nil
+}
+
+func parse(c *caddy.Controller) (*Loop, error) {
+	i := 0
+	zone := "."
+	for c.Next() {
+		if i > 0 {
+			return nil, plugin.ErrOnce
+		}
+		i++
+		if c.NextArg() {
+			return nil, c.ArgErr()
+		}
+
+		if len(c.ServerBlockKeys) > 0 {
+			zone = plugin.Host(c.ServerBlockKeys[0]).Normalize()
+		}
+	}
+	return New(zone), nil
+}
+
+// qname returns a random name. <rand.Int()>.<rand.Int().<zone>.
+func qname(zone string) string {
+	l1 := strconv.Itoa(r.Int())
+	l2 := strconv.Itoa(r.Int())
+
+	return dnsutil.Join([]string{l1, l2, zone})
+}
+
+var r = rand.New(rand.NewSource(time.Now().UnixNano()))