net: make go resolver aware of network parameter

Currently, the go resolver always send two DNS queries (A and AAAA) even
if tcp4/udp4/ip4 or tcp6/udp6/ip6 is used. This can cause unwanted
latencies when making IPv4-only or IPv6-only connections.

This change make go resolver aware of network parameter. Now, only one A
query is sent when tcp4/udp4/ip4 is used, and vice versa for
tcp6/udp6/ip6.

src/net/dnsclient_unix.go

@@ -532,7 +532,7 @@ func (r *Resolver) goLookupHostOrder(ctx context.Context, name string, order hos
 			return
 		}
 	}
-	ips, _, err := r.goLookupIPCNAMEOrder(ctx, name, order)
+	ips, _, err := r.goLookupIPCNAMEOrder(ctx, "ip", name, order)
 	if err != nil {
 		return
 	}
@@ -558,13 +558,13 @@ func goLookupIPFiles(name string) (addrs []IPAddr) {
 
 // goLookupIP is the native Go implementation of LookupIP.
 // The libc versions are in cgo_*.go.
-func (r *Resolver) goLookupIP(ctx context.Context, host string) (addrs []IPAddr, err error) {
+func (r *Resolver) goLookupIP(ctx context.Context, network, host string) (addrs []IPAddr, err error) {
 	order := systemConf().hostLookupOrder(r, host)
-	addrs, _, err = r.goLookupIPCNAMEOrder(ctx, host, order)
+	addrs, _, err = r.goLookupIPCNAMEOrder(ctx, network, host, order)
 	return
 }
 
-func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, name string, order hostLookupOrder) (addrs []IPAddr, cname dnsmessage.Name, err error) {
+func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, network, name string, order hostLookupOrder) (addrs []IPAddr, cname dnsmessage.Name, err error) {
 	if order == hostLookupFilesDNS || order == hostLookupFiles {
 		addrs = goLookupIPFiles(name)
 		if len(addrs) > 0 || order == hostLookupFiles {
@@ -585,7 +585,13 @@ func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, name string, order
 		error
 	}
 	lane := make(chan result, 1)
-	qtypes := [...]dnsmessage.Type{dnsmessage.TypeA, dnsmessage.TypeAAAA}
+	qtypes := []dnsmessage.Type{dnsmessage.TypeA, dnsmessage.TypeAAAA}
+	switch ipVersion(network) {
+	case '4':
+		qtypes = []dnsmessage.Type{dnsmessage.TypeA}
+	case '6':
+		qtypes = []dnsmessage.Type{dnsmessage.TypeAAAA}
+	}
 	var queryFn func(fqdn string, qtype dnsmessage.Type)
 	var responseFn func(fqdn string, qtype dnsmessage.Type) result
 	if conf.singleRequest {
@@ -730,7 +736,7 @@ func (r *Resolver) goLookupIPCNAMEOrder(ctx context.Context, name string, order
 // goLookupCNAME is the native Go (non-cgo) implementation of LookupCNAME.
 func (r *Resolver) goLookupCNAME(ctx context.Context, host string) (string, error) {
 	order := systemConf().hostLookupOrder(r, host)
-	_, cname, err := r.goLookupIPCNAMEOrder(ctx, host, order)
+	_, cname, err := r.goLookupIPCNAMEOrder(ctx, "ip", host, order)
 	return cname.String(), err
 }
 

src/net/dnsclient_unix_test.go

@@ -601,14 +601,14 @@ func TestGoLookupIPOrderFallbackToFile(t *testing.T) {
 		name := fmt.Sprintf("order %v", order)
 
 		// First ensure that we get an error when contacting a non-existent host.
-		_, _, err := r.goLookupIPCNAMEOrder(context.Background(), "notarealhost", order)
+		_, _, err := r.goLookupIPCNAMEOrder(context.Background(), "ip", "notarealhost", order)
 		if err == nil {
 			t.Errorf("%s: expected error while looking up name not in hosts file", name)
 			continue
 		}
 
 		// Now check that we get an address when the name appears in the hosts file.
-		addrs, _, err := r.goLookupIPCNAMEOrder(context.Background(), "thor", order) // entry is in "testdata/hosts"
+		addrs, _, err := r.goLookupIPCNAMEOrder(context.Background(), "ip", "thor", order) // entry is in "testdata/hosts"
 		if err != nil {
 			t.Errorf("%s: expected to successfully lookup host entry", name)
 			continue

src/net/lookup_unix.go

@@ -90,7 +90,7 @@ func (r *Resolver) lookupHost(ctx context.Context, host string) (addrs []string,
 
 func (r *Resolver) lookupIP(ctx context.Context, network, host string) (addrs []IPAddr, err error) {
 	if r.preferGo() {
-		return r.goLookupIP(ctx, host)
+		return r.goLookupIP(ctx, network, host)
 	}
 	order := systemConf().hostLookupOrder(r, host)
 	if order == hostLookupCgo {
@@ -100,7 +100,7 @@ func (r *Resolver) lookupIP(ctx context.Context, network, host string) (addrs []
 		// cgo not available (or netgo); fall back to Go's DNS resolver
 		order = hostLookupFilesDNS
 	}
-	ips, _, err := r.goLookupIPCNAMEOrder(ctx, host, order)
+	ips, _, err := r.goLookupIPCNAMEOrder(ctx, network, host, order)
 	return ips, err
 }