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/loop.go

@@ -0,0 +1,90 @@
+package loop
+
+import (
+	"context"
+	"sync"
+
+	"github.com/coredns/coredns/plugin"
+	clog "github.com/coredns/coredns/plugin/pkg/log"
+	"github.com/coredns/coredns/request"
+
+	"github.com/miekg/dns"
+)
+
+var log = clog.NewWithPlugin("loop")
+
+// Loop is a plugin that implements loop detection by sending a "random" query.
+type Loop struct {
+	Next plugin.Handler
+
+	zone  string
+	qname string
+
+	sync.RWMutex
+	i   int
+	off bool
+}
+
+// New returns a new initialized Loop.
+func New(zone string) *Loop { return &Loop{zone: zone, qname: qname(zone)} }
+
+// ServeDNS implements the plugin.Handler interface.
+func (l *Loop) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	if r.Question[0].Qtype != dns.TypeHINFO {
+		return plugin.NextOrFailure(l.Name(), l.Next, ctx, w, r)
+	}
+	if l.disabled() {
+		return plugin.NextOrFailure(l.Name(), l.Next, ctx, w, r)
+	}
+
+	state := request.Request{W: w, Req: r}
+
+	zone := plugin.Zones([]string{l.zone}).Matches(state.Name())
+	if zone == "" {
+		return plugin.NextOrFailure(l.Name(), l.Next, ctx, w, r)
+	}
+
+	if state.Name() == l.qname {
+		l.inc()
+	}
+
+	if l.seen() > 2 {
+		log.Fatalf("Seen \"HINFO IN %s\" more than twice, loop detected", l.qname)
+	}
+
+	return plugin.NextOrFailure(l.Name(), l.Next, ctx, w, r)
+}
+
+// Name implements the plugin.Handler interface.
+func (l *Loop) Name() string { return "loop" }
+
+func (l *Loop) exchange(addr string) (*dns.Msg, error) {
+	m := new(dns.Msg)
+	m.SetQuestion(l.qname, dns.TypeHINFO)
+
+	return dns.Exchange(m, addr)
+}
+
+func (l *Loop) seen() int {
+	l.RLock()
+	defer l.RUnlock()
+	return l.i
+}
+
+func (l *Loop) inc() {
+	l.Lock()
+	defer l.Unlock()
+	l.i++
+}
+
+func (l *Loop) setDisabled() {
+	l.Lock()
+	defer l.Unlock()
+	l.off = true
+}
+
+func (l *Loop) disabled() bool {
+	l.RLock()
+	defer l.RUnlock()
+	return l.off
+}