dns: fallback only once, i.e. prefer the working server next time

internal/dns/dns.go

@@ -39,7 +39,6 @@ type Server struct {
 
 	client    *dns.Client
 	domain    string
-	upstream  []string
 	sometimes *rate.Limiter
 	prom      struct {
 		registry  *prometheus.Registry
@@ -53,6 +52,9 @@ type Server struct {
 	hostsByName  map[string]string
 	hostsByIP    map[string]string
 	subnames     map[string]map[string]net.IP // hostname → subname → ip
+
+	upstreamMu sync.RWMutex
+	upstream   []string
 }
 
 func NewServer(addr, domain string) *Server {
@@ -320,6 +322,14 @@ func (s *Server) handleInternal(w dns.ResponseWriter, r *dns.Msg) {
 	w.WriteMsg(m)
 }
 
+func (s *Server) upstreams() []string {
+	s.upstreamMu.RLock()
+	defer s.upstreamMu.RUnlock()
+	result := make([]string, len(s.upstream))
+	copy(result, s.upstream)
+	return result
+}
+
 func (s *Server) handleRequest(w dns.ResponseWriter, r *dns.Msg) {
 	if len(r.Question) == 1 { // TODO: answer all questions we can answer
 		q := r.Question[0]
@@ -333,7 +343,7 @@ func (s *Server) handleRequest(w dns.ResponseWriter, r *dns.Msg) {
 	s.prom.questions.Observe(float64(len(r.Question)))
 	s.prom.upstream.WithLabelValues("DNS").Inc()
 
-	for _, u := range s.upstream {
+	for idx, u := range s.upstreams() {
 		in, _, err := s.client.Exchange(r, u)
 		if err != nil {
 			if s.sometimes.Allow() {
@@ -342,7 +352,13 @@ func (s *Server) handleRequest(w dns.ResponseWriter, r *dns.Msg) {
 			continue // fall back to next-slower upstream
 		}
 		w.WriteMsg(in)
-		break
+		if idx > 0 {
+			// re-order this upstream to the front of s.upstream.
+			s.upstreamMu.Lock()
+			s.upstream = append(append([]string{u}, s.upstream[:idx]...), s.upstream[idx+1:]...)
+			s.upstreamMu.Unlock()
+		}
+		return
 	}
 	// DNS has no reply for resolving errors
 }

internal/dns/dns_test.go

@@ -24,6 +24,7 @@ import (
 	"net/url"
 	"os"
 	"strings"
+	"sync/atomic"
 	"testing"
 	"time"
 
@@ -77,27 +78,58 @@ func TestResolveFallback(t *testing.T) {
 	s := NewServer("localhost:0", "lan")
 	s.upstream = []string{
 		"266.266.266.266:53",
-	}
-	{
-		pc, err := net.ListenPacket("udp", "localhost:0")
-		if err != nil {
-			t.Fatal(err)
-		}
-		go dns.ActivateAndServe(nil, pc, dns.HandlerFunc(func(w dns.ResponseWriter, r *dns.Msg) {
+		dnsServerAddr(t, dns.HandlerFunc(func(w dns.ResponseWriter, r *dns.Msg) {
 			rr, _ := dns.NewRR(r.Question[0].Name + " 3600 IN A 127.0.0.1")
 			m := new(dns.Msg)
 			m.SetReply(r)
 			m.Answer = append(m.Answer, rr)
 			w.WriteMsg(m)
-		}))
-		s.upstream = append(s.upstream, pc.LocalAddr().String())
+		})),
 	}
-
 	if err := resolveTestTarget(s, "google.ch.", net.ParseIP("127.0.0.1")); err != nil {
 		t.Fatal(err)
 	}
 }
 
+func dnsServerAddr(t *testing.T, h dns.Handler) string {
+	t.Helper()
+
+	pc, err := net.ListenPacket("udp", "localhost:0")
+	if err != nil {
+		t.Fatal(err)
+	}
+	go dns.ActivateAndServe(nil, pc, h)
+	return pc.LocalAddr().String()
+}
+
+func TestResolveFallbackOnce(t *testing.T) {
+	s := NewServer("localhost:0", "lan")
+	var slowHits uint32
+	s.upstream = []string{
+		dnsServerAddr(t, dns.HandlerFunc(func(w dns.ResponseWriter, r *dns.Msg) {
+			atomic.AddUint32(&slowHits, 1)
+			// trigger fallback by sending no reply
+		})),
+		dnsServerAddr(t, dns.HandlerFunc(func(w dns.ResponseWriter, r *dns.Msg) {
+			rr, _ := dns.NewRR(r.Question[0].Name + " 3600 IN A 127.0.0.1")
+			m := new(dns.Msg)
+			m.SetReply(r)
+			m.Answer = append(m.Answer, rr)
+			w.WriteMsg(m)
+		})),
+		"266.266.266.266:53",
+	}
+
+	for i := 0; i < 2; i++ {
+		if err := resolveTestTarget(s, "google.ch.", net.ParseIP("127.0.0.1")); err != nil {
+			t.Fatal(err)
+		}
+	}
+	if got, want := atomic.LoadUint32(&slowHits), uint32(1); got != want {
+		t.Errorf("slow upstream server hits = %d, wanted %d", got, want)
+	}
+}
+
 func TestDHCP(t *testing.T) {
 	r := &recorder{}
 	s := NewServer("localhost:0", "lan")