Add debugging for failed lookups (#199)

This PR adds debug support for failed lookups. I.e. when a record
is outside the configured domain, we do a forward lookup. If this
fails the error is silently dropped. This PR adds it back as an error
in when debugging is enabled.

Fixes #197

middleware/etcd/proxy_lookup_test.go

@@ -0,0 +1,77 @@
+// +build etcd
+
+package etcd
+
+import (
+	"sort"
+	"testing"
+
+	"github.com/miekg/coredns/middleware"
+	"github.com/miekg/coredns/middleware/etcd/msg"
+	"github.com/miekg/coredns/middleware/proxy"
+	"github.com/miekg/coredns/middleware/test"
+
+	"github.com/miekg/dns"
+)
+
+func TestProxyLookupFailDebug(t *testing.T) {
+	for _, serv := range servicesProxy {
+		set(t, etc, serv.Key, 0, serv)
+		defer delete(t, etc, serv.Key)
+	}
+
+	prxy := etc.Proxy
+	etc.Proxy = proxy.New([]string{"127.0.0.0:154"})
+	etc.Debug = true
+
+	defer func() { etc.Debug = false }()
+	defer func() { etc.Proxy = prxy }()
+
+	for _, tc := range dnsTestCasesProxy {
+		m := tc.Msg()
+
+		rec := middleware.NewResponseRecorder(&test.ResponseWriter{})
+		_, err := etc.ServeDNS(ctxt, rec, m)
+		if err != nil {
+			t.Errorf("expected no error, got %v\n", err)
+			continue
+		}
+		resp := rec.Msg()
+
+		sort.Sort(test.RRSet(resp.Answer))
+		sort.Sort(test.RRSet(resp.Ns))
+		sort.Sort(test.RRSet(resp.Extra))
+
+		if !test.Header(t, tc, resp) {
+			t.Logf("%v\n", resp)
+			continue
+		}
+		if !test.Section(t, tc, test.Answer, resp.Answer) {
+			t.Logf("%v\n", resp)
+		}
+		if !test.Section(t, tc, test.Ns, resp.Ns) {
+			t.Logf("%v\n", resp)
+		}
+		if !test.Section(t, tc, test.Extra, resp.Extra) {
+			t.Logf("%v\n", resp)
+		}
+	}
+}
+
+// Note the key is encoded as DNS name, while in "reality" it is a etcd path.
+var servicesProxy = []*msg.Service{
+	{Host: "www.example.org", Key: "a.dom.skydns.test."},
+}
+
+var dnsTestCasesProxy = []test.Case{
+	{
+		Qname: "dom.skydns.test.", Qtype: dns.TypeSRV,
+		Answer: []dns.RR{
+			test.SRV("dom.skydns.test. 300 IN SRV 10 100 0 www.example.org."),
+		},
+		Extra: []dns.RR{
+			test.TXT(".	0	CH	TXT	\"www.example.org. IN A: unreachable backend\""),
+			test.TXT(".	0	CH	TXT	\"www.example.org. IN AAAA: unreachable backend\""),
+		},
+	},
+}