Add middleware/erratic (#471)

This middleware allows playing with responses. Only one type is
implemented: it allows you to drop queries. I.e. withhold the response
from the client.

middleware/erratic/erratic.go

@@ -0,0 +1,74 @@
+// Package erratic implements a middleware that returns erratic answers (delayed, dropped).
+package erratic
+
+import (
+	"sync/atomic"
+
+	"github.com/miekg/coredns/request"
+
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+// Erratic is a middleware that returns erratic repsonses to each client.
+type Erratic struct {
+	amount uint64
+
+	q uint64 // counter of queries
+}
+
+// ServeDNS implements the middleware.Handler interface.
+func (e *Erratic) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+	state := request.Request{W: w, Req: r}
+
+	drop := false
+	if e.amount > 0 {
+		queryNr := atomic.LoadUint64(&e.q)
+
+		if queryNr%e.amount == 0 {
+			drop = true
+		}
+
+		atomic.AddUint64(&e.q, 1)
+	}
+
+	m := new(dns.Msg)
+	m.SetReply(r)
+	m.Compress = true
+	m.Authoritative = true
+
+	// small dance to copy rrA or rrAAAA into a non-pointer var that allows us to overwrite the ownername
+	// in a non-racy manor.
+	switch state.QType() {
+	case dns.TypeA:
+		rr := *(rrA.(*dns.A))
+		rr.Header().Name = state.QName()
+		m.Answer = append(m.Answer, &rr)
+	case dns.TypeAAAA:
+		rr := *(rrAAAA.(*dns.AAAA))
+		rr.Header().Name = state.QName()
+		m.Answer = append(m.Answer, &rr)
+	default:
+		if !drop {
+			// coredns will return error.
+			return dns.RcodeServerFailure, nil
+		}
+	}
+
+	if drop {
+		return 0, nil
+	}
+
+	state.SizeAndDo(m)
+	w.WriteMsg(m)
+
+	return 0, nil
+}
+
+// Name implements the Handler interface.
+func (e *Erratic) Name() string { return "erratic" }
+
+var (
+	rrA, _    = dns.NewRR(". IN 0 A 192.0.2.53")
+	rrAAAA, _ = dns.NewRR(". IN 0 AAAA 2001:DB8::53")
+)