fix: don't allow dialing DNS addresses

See https://github.com/libp2p/go-libp2p/issues/841

tcp.go

@@ -70,10 +70,12 @@ func NewTCPTransport(upgrader *tptu.Upgrader) *TcpTransport {
 	return &TcpTransport{Upgrader: upgrader, ConnectTimeout: DefaultConnectTimeout}
 }
 
+var dialMatcher = mafmt.And(mafmt.IP, mafmt.Base(ma.P_TCP))
+
 // CanDial returns true if this transport believes it can dial the given
 // multiaddr.
 func (t *TcpTransport) CanDial(addr ma.Multiaddr) bool {
-	return mafmt.TCP.Matches(addr)
+	return dialMatcher.Matches(addr)
 }
 
 func (t *TcpTransport) maDial(ctx context.Context, raddr ma.Multiaddr) (manet.Conn, error) {

tcp_test.go

@@ -36,6 +36,27 @@ func TestTcpTransport(t *testing.T) {
 	envReuseportVal = true
 }
 
+func TestTcpTransportCantDialDNS(t *testing.T) {
+	for i := 0; i < 2; i++ {
+		dnsa, err := ma.NewMultiaddr("/dns4/example.com/tcp/1234")
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		tpt := NewTCPTransport(&tptu.Upgrader{
+			Secure: makeInsecureTransport(t),
+			Muxer:  new(mplex.Transport),
+		})
+
+		if tpt.CanDial(dnsa) {
+			t.Fatal("shouldn't be able to dial dns")
+		}
+
+		envReuseportVal = false
+	}
+	envReuseportVal = true
+}
+
 func TestTcpTransportCantListenUtp(t *testing.T) {
 	for i := 0; i < 2; i++ {
 		utpa, err := ma.NewMultiaddr("/ip4/127.0.0.1/udp/0/utp")