plugin/proxy: return client error (#1646)

Return the client error if there was one instead of the generic
"no healthy upstream or error"

plugin/forward/forward.go

@@ -65,6 +65,7 @@ func (f *Forward) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 
 	fails := 0
 	var span, child ot.Span
+	var upstreamErr error
 	span = ot.SpanFromContext(ctx)
 
 	for _, proxy := range f.list() {
@@ -93,6 +94,7 @@ func (f *Forward) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 		}
 
 		ret, err = truncated(ret, err)
+		upstreamErr = err
 
 		if err != nil {
 			// Kick off health check to see if *our* upstream is broken.
@@ -124,6 +126,10 @@ func (f *Forward) ServeDNS(ctx context.Context, w dns.ResponseWriter, r *dns.Msg
 		return 0, nil
 	}
 
+	if upstreamErr != nil {
+		return dns.RcodeServerFailure, upstreamErr
+	}
+
 	return dns.RcodeServerFailure, errNoHealthy
 }
 
@@ -155,7 +161,7 @@ func (f *Forward) list() []*Proxy { return f.p.List(f.proxies) }
 
 var (
 	errInvalidDomain = errors.New("invalid domain for forward")
-	errNoHealthy     = errors.New("no healthy proxies or upstream error")
+	errNoHealthy     = errors.New("no healthy proxies")
 	errNoForward     = errors.New("no forwarder defined")
 )
 

plugin/forward/lookup.go

@@ -19,6 +19,7 @@ func (f *Forward) Forward(state request.Request) (*dns.Msg, error) {
 	}
 
 	fails := 0
+	var upstreamErr error
 	for _, proxy := range f.list() {
 		if proxy.Down(f.maxfails) {
 			fails++
@@ -33,6 +34,7 @@ func (f *Forward) Forward(state request.Request) (*dns.Msg, error) {
 		ret, err := proxy.connect(context.Background(), state, f.forceTCP, true)
 
 		ret, err = truncated(ret, err)
+		upstreamErr = err
 
 		if err != nil {
 			if fails < len(f.proxies) {
@@ -48,6 +50,11 @@ func (f *Forward) Forward(state request.Request) (*dns.Msg, error) {
 
 		return ret, err
 	}
+
+	if upstreamErr != nil {
+		return nil, upstreamErr
+	}
+
 	return nil, errNoHealthy
 }