diff --git a/src/lib/net/dialgoogle_test.go b/src/lib/net/dialgoogle_test.go
index 690ba572d76..1e0c0aaf065 100644
--- a/src/lib/net/dialgoogle_test.go
+++ b/src/lib/net/dialgoogle_test.go
@@ -10,6 +10,7 @@ import (
 	"io";
 	"net";
 	"os";
+	"syscall";
 	"testing";
 )
 
@@ -81,7 +82,17 @@ func TestDialGoogle(t *testing.T) {
 		doDialTCP(t, "tcp", addr);
 		if addr[0] != '[' {
 			doDial(t, "tcp4", addr);
-			doDialTCP(t, "tcp4", addr)
+			doDialTCP(t, "tcp4", addr);
+
+			if !preferIPv4 {
+				// make sure preferIPv4 flag works.
+				preferIPv4 = true;
+				syscall.SocketDisableIPv6 = true;
+				doDial(t, "tcp4", addr);
+				doDialTCP(t, "tcp4", addr);
+				syscall.SocketDisableIPv6 = false;
+				preferIPv4 = false;
+			}
 		}
 		doDial(t, "tcp6", addr);
 		doDialTCP(t, "tcp6", addr)
diff --git a/src/lib/net/net.go b/src/lib/net/net.go
index 63074470bc6..b4bb48b24f2 100644
--- a/src/lib/net/net.go
+++ b/src/lib/net/net.go
@@ -345,7 +345,7 @@ func internetSocket(net, laddr, raddr string, proto int64, mode string)
 	default:
 		// Otherwise, guess.
 		// If the addresses are IPv4 and we prefer IPv4, use 4; else 6.
-		if preferIPv4 && lip.To4() != nil && rip.To4() != nil {
+		if preferIPv4 && (lip == nil || lip.To4() != nil) && (rip == nil || rip.To4() != nil) {
 			vers = 4
 		} else {
 			vers = 6
diff --git a/src/lib/syscall/socket_darwin.go b/src/lib/syscall/socket_darwin.go
index dc76b9bead3..ba640e95682 100644
--- a/src/lib/syscall/socket_darwin.go
+++ b/src/lib/syscall/socket_darwin.go
@@ -12,12 +12,19 @@ import (
 	"unsafe";
 )
 
+// For testing: clients can set this flag to force
+// creation of IPv6 sockets to return EAFNOSUPPORT.
+var SocketDisableIPv6 bool
+
 func SockaddrToSockaddrInet4(s *Sockaddr) *SockaddrInet4;
 func SockaddrToSockaddrInet6(s *Sockaddr) *SockaddrInet6;
 func SockaddrInet4ToSockaddr(s *SockaddrInet4) *Sockaddr;
 func SockaddrInet6ToSockaddr(s *SockaddrInet6) *Sockaddr;
 
 func Socket(domain, proto, typ int64) (ret int64, err int64) {
+	if domain == AF_INET6 && SocketDisableIPv6 {
+		return -1, EAFNOSUPPORT
+	}
 	r1, r2, e := Syscall(SYS_SOCKET, domain, proto, typ);
 	return r1, e
 }
diff --git a/src/lib/syscall/socket_linux.go b/src/lib/syscall/socket_linux.go
index 39b9aa60f55..8dc75dd3a4d 100644
--- a/src/lib/syscall/socket_linux.go
+++ b/src/lib/syscall/socket_linux.go
@@ -12,6 +12,10 @@ import (
 	"unsafe";
 )
 
+// For testing: clients can set this flag to force
+// creation of IPv6 sockets to return EAFNOSUPPORT.
+var SocketDisableIPv6 bool
+
 func SockaddrToSockaddrInet4(s *Sockaddr) *SockaddrInet4;
 func SockaddrToSockaddrInet6(s *Sockaddr) *SockaddrInet6;
 func SockaddrInet4ToSockaddr(s *SockaddrInet4) *Sockaddr;
@@ -30,6 +34,9 @@ func saLen(s *Sockaddr) int64 {
 }
 
 func Socket(domain, proto, typ int64) (ret int64, err int64) {
+	if domain == AF_INET6 && SocketDisableIPv6 {
+		return -1, EAFNOSUPPORT
+	}
 	r1, r2, e := Syscall(SYS_SOCKET, domain, proto, typ);
 	return r1, e
 }