Fix dns-01-003 (#1634)

* plugin/{cache,forward,proxy}: don't allow responses that are bogus

Responses that are not matching what we've been querying for should be
dropped. They are converted into FormErrs by forward and proxy; as a 2nd
backstop cache will also not cache these.

* plug

* add explicit test

plugin/cache/spoof_test.go

@@ -0,0 +1,66 @@
+package cache
+
+import (
+	"testing"
+
+	"github.com/coredns/coredns/plugin"
+	"github.com/coredns/coredns/plugin/pkg/dnstest"
+
+	"github.com/coredns/coredns/plugin/test"
+	"github.com/miekg/dns"
+	"golang.org/x/net/context"
+)
+
+func TestSpoof(t *testing.T) {
+	// Send query for example.org, get reply for example.net; should not be cached.
+	c := New()
+	c.Next = spoofHandler()
+
+	req := new(dns.Msg)
+	req.SetQuestion("example.org.", dns.TypeA)
+	rec := dnstest.NewRecorder(&test.ResponseWriter{})
+
+	c.ServeDNS(context.TODO(), rec, req)
+
+	qname := rec.Msg.Question[0].Name
+	if c.pcache.Len() != 0 {
+		t.Errorf("cached %s, while reply had %s", "example.org.", qname)
+	}
+
+	// qtype
+	c.Next = spoofHandlerType()
+	req.SetQuestion("example.org.", dns.TypeMX)
+
+	c.ServeDNS(context.TODO(), rec, req)
+
+	qtype := rec.Msg.Question[0].Qtype
+	if c.pcache.Len() != 0 {
+		t.Errorf("cached %s type %d, while reply had %d", "example.org.", dns.TypeMX, qtype)
+	}
+}
+
+// spoofHandler is a fake plugin implementation which returns a single A records for example.org. The qname in the
+// question section is set to example.NET (i.e. they *don't* match).
+func spoofHandler() plugin.Handler {
+	return plugin.HandlerFunc(func(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+		m := new(dns.Msg)
+		m.SetQuestion("example.net.", dns.TypeA)
+		m.Response = true
+		m.Answer = []dns.RR{test.A("example.org. IN A 127.0.0.53")}
+		w.WriteMsg(m)
+		return dns.RcodeSuccess, nil
+	})
+}
+
+// spoofHandlerType is a fake plugin implementation which returns a single MX records for example.org. The qtype in the
+// question section is set to A.
+func spoofHandlerType() plugin.Handler {
+	return plugin.HandlerFunc(func(ctx context.Context, w dns.ResponseWriter, r *dns.Msg) (int, error) {
+		m := new(dns.Msg)
+		m.SetQuestion("example.org.", dns.TypeA)
+		m.Response = true
+		m.Answer = []dns.RR{test.MX("example.org. IN MX 10 mail.example.org.")}
+		w.WriteMsg(m)
+		return dns.RcodeSuccess, nil
+	})
+}