Query all resolvers in parallel

3 files changed, 52 insertions, 15 deletions

diff --git a/dns/dnsutil/dnsutil.go b/dns/dnsutil/dnsutil.go
index 6f791ef..13cfd0a 100644
--- a/dns/dnsutil/dnsutil.go
+++ b/dns/dnsutil/dnsutil.go
@@ -1,6 +1,8 @@
 package dnsutil
 
 import (
+	"errors"
+	"sync"
 	"time"
 
 	"github.com/miekg/dns"
@@ -14,6 +16,44 @@ var (
 	RcodeToString = dns.RcodeToString
 )
 
+// Resolver is the interface that wraps the Exchange method of a DNS client.
+type Resolver interface {
+	Exchange(*dns.Msg, string) (*dns.Msg, time.Duration, error)
+}
+
+// Exchange sends a DNS query to addr and returns the response. If more than one addr is given, all are queried and the
+// first successful response is returned.
+func Exchange(resolver Resolver, msg *dns.Msg, addr ...string) (*dns.Msg, error) {
+	done := make(chan bool)
+	c := make(chan *dns.Msg)
+	var wg sync.WaitGroup
+	wg.Add(len(addr))
+	err := errors.New("addr is empty")
+	for _, a := range addr {
+		go func(addr string) {
+			defer wg.Done()
+			r, _, err1 := resolver.Exchange(msg, addr)
+			if err1 != nil {
+				err = err1
+				return
+			}
+			c <- r
+		}(a)
+	}
+	go func() {
+		wg.Wait()
+		done <- true
+	}()
+	for {
+		select {
+		case <-done:
+			return nil, err
+		case rr := <-c:
+			return rr, nil
+		}
+	}
+}
+
 // Answers returns all values in the answer section of DNS message msg.
 func Answers(msg *dns.Msg) []string {
 	var answers []string
diff --git a/dns/proxy.go b/dns/proxy.go
index d4ded21..84f1b48 100644
--- a/dns/proxy.go
+++ b/dns/proxy.go
@@ -46,6 +46,7 @@ type Proxy struct {
 	logMode   int
 	server    *dns.Server
 	client    client
+	timeout   time.Duration
 }
 
 // ProxyOptions represents proxy configuration.
@@ -80,6 +81,7 @@ func NewProxy(cache *cache.Cache, logger logger, options ProxyOptions) (*Proxy,
 		resolvers: options.Resolvers,
 		logMode:   options.LogMode,
 		client:    c,
+		timeout:   options.Timeout,
 	}, nil
 }
 
@@ -173,21 +175,14 @@ func (p *Proxy) ServeDNS(w dns.ResponseWriter, r *dns.Msg) {
 		p.writeMsg(w, msg, false)
 		return
 	}
-	for i, resolver := range p.resolvers {
-		rr, _, err := p.client.Exchange(r, resolver)
-		if err != nil {
-			p.logger.Printf("resolver %s failed: %s", resolver, err)
-			if i == len(p.resolvers)-1 { // No more resolvers to try
-				break
-			} else {
-				continue
-			}
-		}
+	rr, err := dnsutil.Exchange(p.client, r, p.resolvers...)
+	if err == nil {
 		p.cache.Set(key, rr)
 		p.writeMsg(w, rr, false)
-		return
+	} else {
+		p.logger.Printf("resolver(s) failed: %s", err)
+		dns.HandleFailed(w, r)
 	}
-	dns.HandleFailed(w, r)
 }
 
 // ListenAndServe listens on the network address addr and uses the server to process requests.
diff --git a/dns/proxy_test.go b/dns/proxy_test.go
index c675f90..5df7611 100644
--- a/dns/proxy_test.go
+++ b/dns/proxy_test.go
@@ -68,7 +68,7 @@ func testProxy(t *testing.T) *Proxy {
 	if err != nil {
 		t.Fatal(err)
 	}
-	proxy, err := NewProxy(cache.New(0), log, ProxyOptions{})
+	proxy, err := NewProxy(cache.New(0), log, ProxyOptions{Timeout: 2 * time.Second})
 	if err != nil {
 		t.Fatal(err)
 	}
@@ -159,12 +159,14 @@ func TestProxy(t *testing.T) {
 
 func TestProxyWithResolvers(t *testing.T) {
 	p := testProxy(t)
-	p.resolvers = []string{"resolver1"}
 	client := make(testClient)
 	p.client = client
 	defer p.Close()
+	// No resolvers
+	assertFailure(t, p, TypeA, "host1")
 
 	// First and only resolver responds succesfully
+	p.resolvers = []string{"resolver1"}
 	reply := ReplyA("host1", net.ParseIP("192.0.2.1"))
 	m := dns.Msg{}
 	m.Id = dns.Id()
@@ -217,7 +219,7 @@ func TestProxyWithCache(t *testing.T) {
 
 func TestProxyWithLogging(t *testing.T) {
 	logger := &testLogger{}
-	p, err := NewProxy(cache.New(0), logger, ProxyOptions{})
+	p, err := NewProxy(cache.New(0), logger, ProxyOptions{Timeout: 2 * time.Second})
 	if err != nil {
 		t.Fatal(err)
 	}