iptables: don't NAT existing connections

Fixes a NAT bug that manifested as:
- A SYN was sent from gVisor to another host, unaffected by iptables.
- The corresponding SYN/ACK was NATted by a PREROUTING REDIRECT rule
  despite being part of the existing connection.
- The socket that sent the SYN never received the SYN/ACK and thus a
  connection could not be established.

We handle this (as Linux does) by tracking all connections, inserting a
no-op conntrack rule for new connections with no rules of their own.

Needed for istio support (#170).

2 files changed, 59 insertions, 0 deletions

diff --git a/test/iptables/iptables_test.go b/test/iptables/iptables_test.go
index f5ac79370..f303030aa 100644
--- a/test/iptables/iptables_test.go
+++ b/test/iptables/iptables_test.go
@@ -263,6 +263,13 @@ func TestNATPreRedirectTCPPort(t *testing.T) {
 	singleTest(t, NATPreRedirectTCPPort{})
 }
 
+func TestNATPreRedirectTCPOutgoing(t *testing.T) {
+	singleTest(t, NATPreRedirectTCPOutgoing{})
+}
+
+func TestNATOutRedirectTCPIncoming(t *testing.T) {
+	singleTest(t, NATOutRedirectTCPIncoming{})
+}
 func TestNATOutRedirectUDPPort(t *testing.T) {
 	singleTest(t, NATOutRedirectUDPPort{})
 }
diff --git a/test/iptables/nat.go b/test/iptables/nat.go
index 8562b0820..149dec2bb 100644
--- a/test/iptables/nat.go
+++ b/test/iptables/nat.go
@@ -28,6 +28,8 @@ const (
 func init() {
 	RegisterTestCase(NATPreRedirectUDPPort{})
 	RegisterTestCase(NATPreRedirectTCPPort{})
+	RegisterTestCase(NATPreRedirectTCPOutgoing{})
+	RegisterTestCase(NATOutRedirectTCPIncoming{})
 	RegisterTestCase(NATOutRedirectUDPPort{})
 	RegisterTestCase(NATOutRedirectTCPPort{})
 	RegisterTestCase(NATDropUDP{})
@@ -91,6 +93,56 @@ func (NATPreRedirectTCPPort) LocalAction(ip net.IP) error {
 	return connectTCP(ip, dropPort, sendloopDuration)
 }
 
+// NATPreRedirectTCPOutgoing verifies that outgoing TCP connections aren't
+// affected by PREROUTING connection tracking.
+type NATPreRedirectTCPOutgoing struct{}
+
+// Name implements TestCase.Name.
+func (NATPreRedirectTCPOutgoing) Name() string {
+	return "NATPreRedirectTCPOutgoing"
+}
+
+// ContainerAction implements TestCase.ContainerAction.
+func (NATPreRedirectTCPOutgoing) ContainerAction(ip net.IP) error {
+	// Redirect all incoming TCP traffic to a closed port.
+	if err := natTable("-A", "PREROUTING", "-p", "tcp", "-j", "REDIRECT", "--to-ports", fmt.Sprintf("%d", dropPort)); err != nil {
+		return err
+	}
+
+	// Establish a connection to the host process.
+	return connectTCP(ip, acceptPort, sendloopDuration)
+}
+
+// LocalAction implements TestCase.LocalAction.
+func (NATPreRedirectTCPOutgoing) LocalAction(ip net.IP) error {
+	return listenTCP(acceptPort, sendloopDuration)
+}
+
+// NATOutRedirectTCPIncoming verifies that incoming TCP connections aren't
+// affected by OUTPUT connection tracking.
+type NATOutRedirectTCPIncoming struct{}
+
+// Name implements TestCase.Name.
+func (NATOutRedirectTCPIncoming) Name() string {
+	return "NATOutRedirectTCPIncoming"
+}
+
+// ContainerAction implements TestCase.ContainerAction.
+func (NATOutRedirectTCPIncoming) ContainerAction(ip net.IP) error {
+	// Redirect all outgoing TCP traffic to a closed port.
+	if err := natTable("-A", "OUTPUT", "-p", "tcp", "-j", "REDIRECT", "--to-ports", fmt.Sprintf("%d", dropPort)); err != nil {
+		return err
+	}
+
+	// Establish a connection to the host process.
+	return listenTCP(acceptPort, sendloopDuration)
+}
+
+// LocalAction implements TestCase.LocalAction.
+func (NATOutRedirectTCPIncoming) LocalAction(ip net.IP) error {
+	return connectTCP(ip, acceptPort, sendloopDuration)
+}
+
 // NATOutRedirectUDPPort tests that packets are redirected to different port.
 type NATOutRedirectUDPPort struct{}