atunnel: restrict the actor egress listener to IPv4

cmd/ateom-gvisor/main.go

@@ -65,8 +65,10 @@ var (
 	podUID = pflag.String("pod-uid", "", "The UID of the current pod")
 
 	// TODO(liorlieberman) have a sub package for all atunnel releated things like that
-	atunnelListenAddress        = pflag.String("atunnel-listen-address", "0.0.0.0:443", "Address for actor ingress HTTPS")
-	atunnelConnectListenAddress = pflag.String("atunnel-connect-listen-address", "0.0.0.0:444", "Address for actor ingress mTLS CONNECT")
+	//
+	// Ingress is dual-stack; egress is pinned to IPv4 by atunnel.ListenEgressIPv4.
+	atunnelListenAddress        = pflag.String("atunnel-listen-address", ":443", "Address for actor ingress HTTPS")
+	atunnelConnectListenAddress = pflag.String("atunnel-connect-listen-address", ":444", "Address for actor ingress mTLS CONNECT")
 	workerCredentialBundle      = pflag.String("atunnel-credential-bundle", "/run/podidentity.podcert.ate.dev/credential-bundle.pem", "Worker Pod credential bundle used by atunnel for inbound serving and outbound mTLS")
 	podIdentityTrustBundle      = pflag.String("atunnel-trust-bundle", "/run/podidentity.podcert.ate.dev/trust-bundle.pem", "Pod identity trust bundle used for router clients and the node-local atelet")
 	atunnelClientIdentity       = pflag.String("atunnel-client-identity", "spiffe://cluster.local/ns/ate-system/sa/atenet-router", "SPIFFE identity allowed to call actor ingress HTTPS")
@@ -277,7 +279,7 @@ func runAtunnel(ctx context.Context, upstream *url.URL) (*atunnel.Server, *atunn
 	if err != nil {
 		return nil, nil, 0, fmt.Errorf("while configuring atunnel egress: %w", err)
 	}
-	egressListener, err := net.Listen("tcp", *atunnelEgressListenAddress)
+	egressListener, err := atunnel.ListenEgressIPv4(*atunnelEgressListenAddress)
 	if err != nil {
 		return nil, nil, 0, fmt.Errorf("while opening atunnel egress listener: %w", err)
 	}

cmd/ateom-microvm/main.go

@@ -71,8 +71,9 @@ var (
 	otlpRelaySocket = flag.String("otlp-relay-socket", ateompath.AteletOTLPSocketPath(),
 		"Unix socket of atelet's OTLP relay to export telemetry through, keeping it off the pod network. Empty, or absent at startup, exports directly to OTEL_EXPORTER_OTLP_ENDPOINT instead.")
 
-	atunnelListenAddress        = flag.String("atunnel-listen-address", "0.0.0.0:443", "Address for actor ingress HTTPS")
-	atunnelConnectListenAddress = flag.String("atunnel-connect-listen-address", "0.0.0.0:444", "Address for actor ingress mTLS CONNECT")
+	// Ingress is dual-stack; egress is pinned to IPv4 by atunnel.ListenEgressIPv4.
+	atunnelListenAddress        = flag.String("atunnel-listen-address", ":443", "Address for actor ingress HTTPS")
+	atunnelConnectListenAddress = flag.String("atunnel-connect-listen-address", ":444", "Address for actor ingress mTLS CONNECT")
 	workerCredentialBundle      = flag.String("atunnel-credential-bundle", "/run/podidentity.podcert.ate.dev/credential-bundle.pem", "Worker Pod credential bundle used by atunnel for inbound serving and outbound mTLS")
 	podIdentityTrustBundle      = flag.String("atunnel-trust-bundle", "/run/podidentity.podcert.ate.dev/trust-bundle.pem", "Pod identity trust bundle used for router clients and the node-local atelet")
 	atunnelClientIdentity       = flag.String("atunnel-client-identity", "spiffe://cluster.local/ns/ate-system/sa/atenet-router", "SPIFFE identity allowed to call actor ingress HTTPS")
@@ -236,7 +237,7 @@ func do(ctx context.Context) error {
 	if err != nil {
 		return fmt.Errorf("while configuring atunnel egress: %w", err)
 	}
-	egressListener, err := net.Listen("tcp", *atunnelEgressListenAddress)
+	egressListener, err := atunnel.ListenEgressIPv4(*atunnelEgressListenAddress)
 	if err != nil {
 		return fmt.Errorf("while opening atunnel egress listener: %w", err)
 	}

internal/ateomnet/net.go

@@ -229,8 +229,10 @@ func InstallActorNftablesRules(egressPort uint16) error {
 	// rules in an ateom-owned table makes cleanup simple and avoids mutating
 	// Kubernetes or CNI-managed chains directly.
 	//
-	// TODO: Add IPv6 veth addressing, forwarding, and nftables rules once actor
-	// networking supports dual-stack pods. The current actor network is IPv4-only.
+	// TODO(#945): Add IPv6 veth addressing, forwarding, and nftables rules once
+	// actor networking supports dual-stack pods. The current actor network is
+	// IPv4-only, which is why atunnel opens the listener these rules redirect to
+	// with ListenEgressIPv4.
 	//
 	// The rules do three things:
 	//

internal/atunnel/egress.go

@@ -42,6 +42,22 @@ type actorCertificateSource interface {
 // connection originally targeted.
 type OriginalDestination func(net.Conn) (string, error)
 
+// ListenEgressIPv4 opens the transparent egress listener on address. The family
+// is in the name because it cannot be left to the address: Go treats every
+// unspecified address as a wildcard, so net.Listen("tcp", "0.0.0.0:15001") binds
+// "::" dual-stack and accepts IPv6 too. TCPOriginalDestination reads only the
+// IPv4 SOL_IP/SO_ORIGINAL_DST, so such a connection is accepted and then fails.
+//
+// TODO(#945): use "tcp" once actor networking is dual-stack; the IPv6
+// original-destination lookup is #686.
+func ListenEgressIPv4(address string) (net.Listener, error) {
+	listener, err := net.Listen("tcp4", address)
+	if err != nil {
+		return nil, fmt.Errorf("atunnel: egress listener is IPv4-only: %w", err)
+	}
+	return listener, nil
+}
+
 // Egress proxies actor TCP connections through an egress CONNECT dialer. It is
 // long-lived across actor activations, but only carries traffic while an actor
 // is assigned to its worker.

internal/atunnel/egress_test.go

@@ -22,6 +22,7 @@ import (
 	"io"
 	"net"
 	"net/http"
+	"strconv"
 	"strings"
 	"sync/atomic"
 	"testing"
@@ -31,6 +32,53 @@ import (
 	"google.golang.org/grpc/status"
 )
 
+// TestListenEgressIPv4Only pins the egress listener to IPv4, because
+// TCPOriginalDestination can only recover an IPv4 original destination. The
+// wildcard address does not carry that constraint. Every listener here uses the
+// same address, "0.0.0.0:0", so the network string is the only variable.
+func TestListenEgressIPv4Only(t *testing.T) {
+	// The negative control comes first because it decides whether the rest means
+	// anything: "tcp" and "tcp4" differ only on a host that binds "tcp"
+	// dual-stack, and only there can this test observe the bug it guards.
+	control, err := net.Listen("tcp", "0.0.0.0:0")
+	if err != nil {
+		t.Fatalf("net.Listen(%q, \"0.0.0.0:0\") = %v", "tcp", err)
+	}
+	t.Cleanup(func() { control.Close() })
+	if controlAddr := control.Addr().(*net.TCPAddr); controlAddr.IP.To4() != nil {
+		t.Skipf("net.Listen(%q, \"0.0.0.0:0\") bound IPv4 %v, so this host cannot "+
+			"distinguish it from %q", "tcp", controlAddr, "tcp4")
+	}
+
+	listener, err := ListenEgressIPv4("0.0.0.0:0")
+	if err != nil {
+		t.Fatalf("ListenEgressIPv4() = %v", err)
+	}
+	t.Cleanup(func() { listener.Close() })
+
+	addr, ok := listener.Addr().(*net.TCPAddr)
+	if !ok {
+		t.Fatalf("ListenEgressIPv4() bound %T, want *net.TCPAddr", listener.Addr())
+	}
+	if addr.IP.To4() == nil {
+		t.Fatalf("ListenEgressIPv4() bound %v, want an IPv4 address; net.Listen(%q, ...) "+
+			"bound %v on the same address, and an IPv6 client would be accepted and then "+
+			"fail TCPOriginalDestination", addr, "tcp", control.Addr())
+	}
+
+	// What the pin is for: there is no IPv6 socket, so an IPv6 client is refused
+	// at connect rather than accepted and then failed.
+	t.Run("an IPv6 client is refused", func(t *testing.T) {
+		port := strconv.Itoa(addr.Port)
+		conn, err := net.DialTimeout("tcp6", net.JoinHostPort("::1", port), 5*time.Second)
+		if err == nil {
+			conn.Close()
+			t.Errorf("net.Dial(%q, \"[::1]:%s\") succeeded, want refused; the egress listener "+
+				"is accepting IPv6 it cannot resolve an original destination for", "tcp6", port)
+		}
+	})
+}
+
 func TestEgressActivationFailsClosed(t *testing.T) {
 	egress, err := NewEgress(func(net.Conn) (string, error) { return "", nil })
 	if err != nil {

internal/atunnel/original_dst_linux.go

@@ -27,9 +27,9 @@ import (
 )
 
 // TCPOriginalDestination reads the IPv4 destination preserved by a Linux
-// REDIRECT rule. Actor networking is currently IPv4-only.
-// TODO(liorlieberman) add the IPv6 IP6T_SO_ORIGINAL_DST variant
-// when actor veth setup gains dual-stack support.
+// REDIRECT rule. Actor networking is currently IPv4-only; see ListenEgressIPv4.
+// TODO(#686): add the IPv6 IP6T_SO_ORIGINAL_DST variant when actor veth setup
+// gains dual-stack support.
 func TCPOriginalDestination(conn net.Conn) (string, error) {
 	tcpConn, ok := conn.(*net.TCPConn)
 	if !ok {