Create null entry connection on first IPTables hook

...all connections should be tracked by ConnTrack, so create a no-op
connection entry on the first hook into IPTables (Prerouting or
Output) and let NAT targets modify the connection entry if they
need to instead of letting the NAT target create their own connection
entry.

This also prepares for "twice-NAT" where a packet may have both DNAT and
SNAT performed on it (which requires the ability to update ConnTrack
entries).

Updates #5696.

PiperOrigin-RevId: 401360377

5 files changed, 217 insertions, 273 deletions

diff --git a/pkg/tcpip/stack/conntrack.go b/pkg/tcpip/stack/conntrack.go
index 30545f634..16d295271 100644
--- a/pkg/tcpip/stack/conntrack.go
+++ b/pkg/tcpip/stack/conntrack.go
@@ -64,13 +64,21 @@ type tuple struct {
 	// tupleEntry is used to build an intrusive list of tuples.
 	tupleEntry
 
-	tupleID
-
 	// conn is the connection tracking entry this tuple belongs to.
 	conn *conn
 
 	// direction is the direction of the tuple.
 	direction direction
+
+	mu sync.RWMutex `state:"nosave"`
+	// +checklocks:mu
+	tupleID tupleID
+}
+
+func (t *tuple) id() tupleID {
+	t.mu.RLock()
+	defer t.mu.RUnlock()
+	return t.tupleID
 }
 
 // tupleID uniquely identifies a connection in one direction. It currently
@@ -103,17 +111,23 @@ func (ti tupleID) reply() tupleID {
 //
 // +stateify savable
 type conn struct {
+	ct *ConnTrack
+
 	// original is the tuple in original direction. It is immutable.
 	original tuple
 
-	// reply is the tuple in reply direction. It is immutable.
+	// reply is the tuple in reply direction.
 	reply tuple
 
-	// manip indicates if the packet should be manipulated. It is immutable.
-	// TODO(gvisor.dev/issue/5696): Support updating manipulation type.
-	manip manipType
-
 	mu sync.RWMutex `state:"nosave"`
+	// Indicates that the connection has been finalized and may handle replies.
+	//
+	// +checklocks:mu
+	finalized bool
+	// manip indicates if the packet should be manipulated.
+	//
+	// +checklocks:mu
+	manip manipType
 	// tcb is TCB control block. It is used to keep track of states
 	// of tcp connection.
 	//
@@ -128,17 +142,6 @@ type conn struct {
 	lastUsed time.Time `state:".(unixTime)"`
 }
 
-// newConn creates new connection.
-func newConn(orig, reply tupleID, manip manipType) *conn {
-	conn := conn{
-		manip:    manip,
-		lastUsed: time.Now(),
-	}
-	conn.original = tuple{conn: &conn, tupleID: orig}
-	conn.reply = tuple{conn: &conn, tupleID: reply, direction: dirReply}
-	return &conn
-}
-
 // timedOut returns whether the connection timed out based on its state.
 func (cn *conn) timedOut(now time.Time) bool {
 	const establishedTimeout = 5 * 24 * time.Hour
@@ -235,168 +238,180 @@ func getTransportHeader(pkt *PacketBuffer) (header.ChecksummableTransport, bool)
 	return nil, false
 }
 
-// packetToTupleID converts packet to a tuple ID. It fails when pkt lacks a valid
-// TCP header.
-//
-// Preconditions: pkt.NetworkHeader() is valid.
-func packetToTupleID(pkt *PacketBuffer) (tupleID, tcpip.Error) {
+func (ct *ConnTrack) init() {
+	ct.mu.Lock()
+	defer ct.mu.Unlock()
+	ct.buckets = make([]bucket, numBuckets)
+}
+
+func (ct *ConnTrack) getConnOrMaybeInsertNoop(pkt *PacketBuffer) *tuple {
 	netHeader := pkt.Network()
 	transportHeader, ok := getTransportHeader(pkt)
 	if !ok {
-		return tupleID{}, &tcpip.ErrUnknownProtocol{}
+		return nil
 	}
 
-	return tupleID{
+	tid := tupleID{
 		srcAddr:    netHeader.SourceAddress(),
 		srcPort:    transportHeader.SourcePort(),
 		dstAddr:    netHeader.DestinationAddress(),
 		dstPort:    transportHeader.DestinationPort(),
 		transProto: pkt.TransportProtocolNumber,
 		netProto:   pkt.NetworkProtocolNumber,
-	}, nil
-}
+	}
 
-func (ct *ConnTrack) init() {
-	ct.mu.Lock()
-	defer ct.mu.Unlock()
-	ct.buckets = make([]bucket, numBuckets)
-}
+	bktID := ct.bucket(tid)
+
+	ct.mu.RLock()
+	bkt := &ct.buckets[bktID]
+	ct.mu.RUnlock()
+
+	now := time.Now()
+	if t := bkt.connForTID(tid, now); t != nil {
+		return t
+	}
 
-// connFor gets the conn for pkt if it exists, or returns nil
-// if it does not. It returns an error when pkt does not contain a valid TCP
-// header.
-// TODO(gvisor.dev/issue/6168): Support UDP.
-func (ct *ConnTrack) connFor(pkt *PacketBuffer) (*conn, direction) {
-	tid, err := packetToTupleID(pkt)
-	if err != nil {
-		return nil, dirOriginal
+	bkt.mu.Lock()
+	defer bkt.mu.Unlock()
+
+	// Make sure a connection wasn't added between when we last checked the
+	// bucket and acquired the bucket's write lock.
+	if t := bkt.connForTIDRLocked(tid, now); t != nil {
+		return t
+	}
+
+	// This is the first packet we're seeing for the connection. Create an entry
+	// for this new connection.
+	conn := &conn{
+		ct:       ct,
+		original: tuple{tupleID: tid, direction: dirOriginal},
+		reply:    tuple{tupleID: tid.reply(), direction: dirReply},
+		manip:    manipNone,
+		lastUsed: now,
 	}
-	return ct.connForTID(tid)
+	conn.original.conn = conn
+	conn.reply.conn = conn
+
+	// For now, we only map an entry for the packet's original tuple as NAT may be
+	// performed on this connection. Until the packet goes through all the hooks
+	// and its final address/port is known, we cannot know what the response
+	// packet's addresses/ports will look like.
+	//
+	// This is okay because the destination cannot send its response until it
+	// receives the packet; the packet will only be received once all the hooks
+	// have been performed.
+	//
+	// See (*conn).finalize.
+	bkt.tuples.PushFront(&conn.original)
+	return &conn.original
 }
 
-func (ct *ConnTrack) connForTID(tid tupleID) (*conn, direction) {
+func (ct *ConnTrack) connForTID(tid tupleID) *tuple {
 	bktID := ct.bucket(tid)
-	now := time.Now()
 
 	ct.mu.RLock()
 	bkt := &ct.buckets[bktID]
 	ct.mu.RUnlock()
 
+	return bkt.connForTID(tid, time.Now())
+}
+
+func (bkt *bucket) connForTID(tid tupleID, now time.Time) *tuple {
 	bkt.mu.RLock()
 	defer bkt.mu.RUnlock()
+	return bkt.connForTIDRLocked(tid, now)
+}
+
+// +checklocks:bkt.mu
+func (bkt *bucket) connForTIDRLocked(tid tupleID, now time.Time) *tuple {
 	for other := bkt.tuples.Front(); other != nil; other = other.Next() {
-		if tid == other.tupleID && !other.conn.timedOut(now) {
-			return other.conn, other.direction
+		if tid == other.id() && !other.conn.timedOut(now) {
+			return other
 		}
 	}
-
-	return nil, dirOriginal
+	return nil
 }
 
-func (ct *ConnTrack) insertRedirectConn(pkt *PacketBuffer, hook Hook, port uint16, address tcpip.Address) *conn {
-	tid, err := packetToTupleID(pkt)
-	if err != nil {
-		return nil
-	}
-	if hook != Prerouting && hook != Output {
-		return nil
-	}
+func (ct *ConnTrack) finalize(cn *conn) {
+	tid := cn.reply.id()
+	id := ct.bucket(tid)
 
-	replyTID := tid.reply()
-	replyTID.srcAddr = address
-	replyTID.srcPort = port
+	ct.mu.RLock()
+	bkt := &ct.buckets[id]
+	ct.mu.RUnlock()
 
-	conn, _ := ct.connForTID(tid)
-	if conn != nil {
-		// The connection is already tracked.
-		// TODO(gvisor.dev/issue/5696): Support updating an existing connection.
-		return nil
+	bkt.mu.Lock()
+	defer bkt.mu.Unlock()
+
+	if t := bkt.connForTIDRLocked(tid, time.Now()); t != nil {
+		// Another connection for the reply already exists. We can't do much about
+		// this so we leave the connection cn represents in a state where it can
+		// send packets but its responses will be mapped to some other connection.
+		// This may be okay if the connection only expects to send packets without
+		// any responses.
+		return
 	}
-	conn = newConn(tid, replyTID, manipDestination)
-	ct.insertConn(conn)
-	return conn
+
+	bkt.tuples.PushFront(&cn.reply)
 }
 
-func (ct *ConnTrack) insertSNATConn(pkt *PacketBuffer, hook Hook, port uint16, address tcpip.Address) *conn {
-	tid, err := packetToTupleID(pkt)
-	if err != nil {
-		return nil
-	}
-	if hook != Input && hook != Postrouting {
-		return nil
+func (cn *conn) finalize() {
+	{
+		cn.mu.RLock()
+		finalized := cn.finalized
+		cn.mu.RUnlock()
+		if finalized {
+			return
+		}
 	}
 
-	replyTID := tid.reply()
-	replyTID.dstAddr = address
-	replyTID.dstPort = port
-
-	conn, _ := ct.connForTID(tid)
-	if conn != nil {
-		// The connection is already tracked.
-		// TODO(gvisor.dev/issue/5696): Support updating an existing connection.
-		return nil
+	cn.mu.Lock()
+	finalized := cn.finalized
+	cn.finalized = true
+	cn.mu.Unlock()
+	if finalized {
+		return
 	}
-	conn = newConn(tid, replyTID, manipSource)
-	ct.insertConn(conn)
-	return conn
+
+	cn.ct.finalize(cn)
 }
 
-// insertConn inserts conn into the appropriate table bucket.
-func (ct *ConnTrack) insertConn(conn *conn) {
-	tupleBktID := ct.bucket(conn.original.tupleID)
-	replyBktID := ct.bucket(conn.reply.tupleID)
+// performNAT setups up the connection for the specified NAT.
+//
+// Generally, only the first packet of a connection reaches this method; other
+// other packets will be manipulated without needing to modify the connection.
+func (cn *conn) performNAT(pkt *PacketBuffer, hook Hook, r *Route, port uint16, address tcpip.Address, dnat bool) {
+	cn.performNATIfNoop(port, address, dnat)
+	cn.handlePacket(pkt, hook, r)
+}
 
-	ct.mu.RLock()
-	defer ct.mu.RUnlock()
+func (cn *conn) performNATIfNoop(port uint16, address tcpip.Address, dnat bool) {
+	cn.mu.Lock()
+	defer cn.mu.Unlock()
 
-	tupleBkt := &ct.buckets[tupleBktID]
-	if tupleBktID == replyBktID {
-		// Both tuples are in the same bucket.
-		tupleBkt.mu.Lock()
-		defer tupleBkt.mu.Unlock()
-		insertConn(tupleBkt, tupleBkt, conn)
+	if cn.finalized {
 		return
 	}
 
-	// Lock the buckets in the correct order.
-	replyBkt := &ct.buckets[replyBktID]
-	if tupleBktID < replyBktID {
-		tupleBkt.mu.Lock()
-		defer tupleBkt.mu.Unlock()
-		replyBkt.mu.Lock()
-		defer replyBkt.mu.Unlock()
-	} else {
-		replyBkt.mu.Lock()
-		defer replyBkt.mu.Unlock()
-		tupleBkt.mu.Lock()
-		defer tupleBkt.mu.Unlock()
+	if cn.manip != manipNone {
+		return
 	}
-	insertConn(tupleBkt, replyBkt, conn)
-}
 
-// TODO(https://gvisor.dev/issue/6590): annotate r/w locking requirements.
-// +checklocks:tupleBkt.mu
-// +checklocks:replyBkt.mu
-func insertConn(tupleBkt *bucket, replyBkt *bucket, conn *conn) {
-	// Now that we hold the locks, ensure the tuple hasn't been inserted by
-	// another thread.
-	// TODO(gvisor.dev/issue/5773): Should check conn.reply.tupleID, too?
-	alreadyInserted := false
-	for other := tupleBkt.tuples.Front(); other != nil; other = other.Next() {
-		if other.tupleID == conn.original.tupleID {
-			alreadyInserted = true
-			break
-		}
-	}
+	cn.reply.mu.Lock()
+	defer cn.reply.mu.Unlock()
 
-	if !alreadyInserted {
-		// Add the tuple to the map.
-		tupleBkt.tuples.PushFront(&conn.original)
-		replyBkt.tuples.PushFront(&conn.reply)
+	if dnat {
+		cn.reply.tupleID.srcAddr = address
+		cn.reply.tupleID.srcPort = port
+		cn.manip = manipDestination
+	} else {
+		cn.reply.tupleID.dstAddr = address
+		cn.reply.tupleID.dstPort = port
+		cn.manip = manipSource
 	}
 }
 
-func (cn *conn) handlePacket(pkt *PacketBuffer, hook Hook, dir direction, r *Route) {
+func (cn *conn) handlePacket(pkt *PacketBuffer, hook Hook, r *Route) {
 	if pkt.NatDone {
 		return
 	}
@@ -417,26 +432,35 @@ func (cn *conn) handlePacket(pkt *PacketBuffer, hook Hook, dir direction, r *Rou
 
 	updateSRCFields := false
 
+	dir := pkt.tuple.direction
+
+	cn.mu.Lock()
+	defer cn.mu.Unlock()
+
 	switch hook {
 	case Prerouting, Output:
 		if cn.manip == manipDestination && dir == dirOriginal {
-			newPort = cn.reply.srcPort
-			newAddr = cn.reply.srcAddr
+			id := cn.reply.id()
+			newPort = id.srcPort
+			newAddr = id.srcAddr
 			pkt.NatDone = true
 		} else if cn.manip == manipSource && dir == dirReply {
-			newPort = cn.original.srcPort
-			newAddr = cn.original.srcAddr
+			id := cn.original.id()
+			newPort = id.srcPort
+			newAddr = id.srcAddr
 			pkt.NatDone = true
 		}
 	case Input, Postrouting:
 		if cn.manip == manipSource && dir == dirOriginal {
-			newPort = cn.reply.dstPort
-			newAddr = cn.reply.dstAddr
+			id := cn.reply.id()
+			newPort = id.dstPort
+			newAddr = id.dstAddr
 			updateSRCFields = true
 			pkt.NatDone = true
 		} else if cn.manip == manipDestination && dir == dirReply {
-			newPort = cn.original.dstPort
-			newAddr = cn.original.dstAddr
+			id := cn.original.id()
+			newPort = id.dstPort
+			newAddr = id.dstAddr
 			updateSRCFields = true
 			pkt.NatDone = true
 		}
@@ -479,51 +503,12 @@ func (cn *conn) handlePacket(pkt *PacketBuffer, hook Hook, dir direction, r *Rou
 		newAddr,
 	)
 
-	// Update the state of tcb.
-	cn.mu.Lock()
-	defer cn.mu.Unlock()
-
 	// Mark the connection as having been used recently so it isn't reaped.
 	cn.lastUsed = time.Now()
 	// Update connection state.
 	cn.updateLocked(pkt, dir)
 }
 
-// maybeInsertNoop tries to insert a no-op connection entry to keep connections
-// from getting clobbered when replies arrive. It only inserts if there isn't
-// already a connection for pkt.
-//
-// This should be called after traversing iptables rules only, to ensure that
-// pkt.NatDone is set correctly.
-func (ct *ConnTrack) maybeInsertNoop(pkt *PacketBuffer) {
-	// If there were a rule applying to this packet, it would be marked
-	// with NatDone.
-	if pkt.NatDone {
-		return
-	}
-
-	switch pkt.TransportProtocolNumber {
-	case header.TCPProtocolNumber, header.UDPProtocolNumber:
-	default:
-		// TODO(https://gvisor.dev/issue/5915): Track ICMP and other trackable
-		// connections.
-		return
-	}
-
-	// This is the first packet we're seeing for the TCP connection. Insert
-	// the noop entry (an identity mapping) so that the response doesn't
-	// get NATed, breaking the connection.
-	tid, err := packetToTupleID(pkt)
-	if err != nil {
-		return
-	}
-	conn := newConn(tid, tid.reply(), manipNone)
-	ct.insertConn(conn)
-	conn.mu.Lock()
-	defer conn.mu.Unlock()
-	conn.updateLocked(pkt, dirOriginal)
-}
-
 // bucket gets the conntrack bucket for a tupleID.
 func (ct *ConnTrack) bucket(id tupleID) int {
 	h := jenkins.Sum32(ct.seed)
@@ -617,7 +602,7 @@ func (ct *ConnTrack) reapTupleLocked(tuple *tuple, bktID int, bkt *bucket, now t
 
 	// To maintain lock order, we can only reap these tuples if the reply
 	// appears later in the table.
-	replyBktID := ct.bucket(tuple.reply())
+	replyBktID := ct.bucket(tuple.id().reply())
 	if bktID > replyBktID {
 		return true
 	}
@@ -658,14 +643,19 @@ func (ct *ConnTrack) originalDst(epID TransportEndpointID, netProto tcpip.Networ
 		transProto: transProto,
 		netProto:   netProto,
 	}
-	conn, _ := ct.connForTID(tid)
-	if conn == nil {
+	t := ct.connForTID(tid)
+	if t == nil {
 		// Not a tracked connection.
 		return "", 0, &tcpip.ErrNotConnected{}
-	} else if conn.manip != manipDestination {
+	}
+
+	t.conn.mu.RLock()
+	defer t.conn.mu.RUnlock()
+	if t.conn.manip != manipDestination {
 		// Unmanipulated destination.
 		return "", 0, &tcpip.ErrInvalidOptionValue{}
 	}
 
-	return conn.original.dstAddr, conn.original.dstPort, nil
+	id := t.conn.original.id()
+	return id.dstAddr, id.dstPort, nil
 }
diff --git a/pkg/tcpip/stack/iptables.go b/pkg/tcpip/stack/iptables.go
index 1021e484a..5808be685 100644
--- a/pkg/tcpip/stack/iptables.go
+++ b/pkg/tcpip/stack/iptables.go
@@ -277,8 +277,9 @@ func (it *IPTables) CheckPrerouting(pkt *PacketBuffer, addressEP AddressableEndp
 		return true
 	}
 
-	if conn, dir := it.connections.connFor(pkt); conn != nil {
-		conn.handlePacket(pkt, hook, dir, nil /* route */)
+	if t := it.connections.getConnOrMaybeInsertNoop(pkt); t != nil {
+		pkt.tuple = t
+		t.conn.handlePacket(pkt, hook, nil /* route */)
 	}
 
 	return it.check(hook, pkt, nil /* route */, addressEP, inNicName, "" /* outNicName */)
@@ -297,20 +298,16 @@ func (it *IPTables) CheckInput(pkt *PacketBuffer, inNicName string) bool {
 		return true
 	}
 
-	shouldTrack := true
-	if conn, dir := it.connections.connFor(pkt); conn != nil {
-		conn.handlePacket(pkt, hook, dir, nil /* route */)
-		shouldTrack = false
+	if t := pkt.tuple; t != nil {
+		t.conn.handlePacket(pkt, hook, nil /* route */)
 	}
 
-	if !it.check(hook, pkt, nil /* route */, nil /* addressEP */, inNicName, "" /* outNicName */) {
-		return false
+	ret := it.check(hook, pkt, nil /* route */, nil /* addressEP */, inNicName, "" /* outNicName */)
+	if t := pkt.tuple; t != nil {
+		t.conn.finalize()
 	}
-
-	// This is the last hook a packet will perform so if the packet's
-	// connection is not tracked, we may need to add a no-op entry.
-	it.maybeinsertNoopConn(pkt, hook, shouldTrack)
-	return true
+	pkt.tuple = nil
+	return ret
 }
 
 // CheckForward performs the forward hook on the packet.
@@ -323,7 +320,6 @@ func (it *IPTables) CheckForward(pkt *PacketBuffer, inNicName, outNicName string
 	if it.shouldSkip(pkt.NetworkProtocolNumber) {
 		return true
 	}
-
 	return it.check(Forward, pkt, nil /* route */, nil /* addressEP */, inNicName, outNicName)
 }
 
@@ -340,8 +336,9 @@ func (it *IPTables) CheckOutput(pkt *PacketBuffer, r *Route, outNicName string)
 		return true
 	}
 
-	if conn, dir := it.connections.connFor(pkt); conn != nil {
-		conn.handlePacket(pkt, hook, dir, r)
+	if t := it.connections.getConnOrMaybeInsertNoop(pkt); t != nil {
+		pkt.tuple = t
+		t.conn.handlePacket(pkt, hook, r)
 	}
 
 	return it.check(hook, pkt, r, nil /* addressEP */, "" /* inNicName */, outNicName)
@@ -360,20 +357,16 @@ func (it *IPTables) CheckPostrouting(pkt *PacketBuffer, r *Route, addressEP Addr
 		return true
 	}
 
-	shouldTrack := true
-	if conn, dir := it.connections.connFor(pkt); conn != nil {
-		conn.handlePacket(pkt, hook, dir, r)
-		shouldTrack = false
+	if t := pkt.tuple; t != nil {
+		t.conn.handlePacket(pkt, hook, r)
 	}
 
-	if !it.check(hook, pkt, r, addressEP, "" /* inNicName */, outNicName) {
-		return false
+	ret := it.check(hook, pkt, r, addressEP, "" /* inNicName */, outNicName)
+	if t := pkt.tuple; t != nil {
+		t.conn.finalize()
 	}
-
-	// This is the last hook a packet will perform so if the packet's
-	// connection is not tracked, we may need to add a no-op entry.
-	it.maybeinsertNoopConn(pkt, hook, shouldTrack)
-	return true
+	pkt.tuple = nil
+	return ret
 }
 
 func (it *IPTables) shouldSkip(netProto tcpip.NetworkProtocolNumber) bool {
@@ -426,7 +419,7 @@ func (it *IPTables) check(hook Hook, pkt *PacketBuffer, r *Route, addressEP Addr
 			// Any Return from a built-in chain means we have to
 			// call the underflow.
 			underflow := table.Rules[table.Underflows[hook]]
-			switch v, _ := underflow.Target.Action(pkt, &it.connections, hook, r, addressEP); v {
+			switch v, _ := underflow.Target.Action(pkt, hook, r, addressEP); v {
 			case RuleAccept:
 				continue
 			case RuleDrop:
@@ -445,22 +438,6 @@ func (it *IPTables) check(hook Hook, pkt *PacketBuffer, r *Route, addressEP Addr
 	return true
 }
 
-func (it *IPTables) maybeinsertNoopConn(pkt *PacketBuffer, hook Hook, shouldTrack bool) {
-	// If this connection should be tracked, try to add an entry for it. If
-	// traversing the nat table didn't end in adding an entry,
-	// maybeInsertNoop will add a no-op entry for the connection. This is
-	// needeed when establishing connections so that the SYN/ACK reply to an
-	// outgoing SYN is delivered to the correct endpoint rather than being
-	// redirected by a prerouting rule.
-	//
-	// From the iptables documentation: "If there is no rule, a `null'
-	// binding is created: this usually does not map the packet, but exists
-	// to ensure we don't map another stream over an existing one."
-	if shouldTrack {
-		it.connections.maybeInsertNoop(pkt)
-	}
-}
-
 // beforeSave is invoked by stateify.
 func (it *IPTables) beforeSave() {
 	// Ensure the reaper exits cleanly.
@@ -606,7 +583,7 @@ func (it *IPTables) checkRule(hook Hook, pkt *PacketBuffer, table Table, ruleIdx
 	}
 
 	// All the matchers matched, so run the target.
-	return rule.Target.Action(pkt, &it.connections, hook, r, addressEP)
+	return rule.Target.Action(pkt, hook, r, addressEP)
 }
 
 // OriginalDst returns the original destination of redirected connections. It
diff --git a/pkg/tcpip/stack/iptables_targets.go b/pkg/tcpip/stack/iptables_targets.go
index 949c44c9b..8b74677d0 100644
--- a/pkg/tcpip/stack/iptables_targets.go
+++ b/pkg/tcpip/stack/iptables_targets.go
@@ -29,7 +29,7 @@ type AcceptTarget struct {
 }
 
 // Action implements Target.Action.
-func (*AcceptTarget) Action(*PacketBuffer, *ConnTrack, Hook, *Route, AddressableEndpoint) (RuleVerdict, int) {
+func (*AcceptTarget) Action(*PacketBuffer, Hook, *Route, AddressableEndpoint) (RuleVerdict, int) {
 	return RuleAccept, 0
 }
 
@@ -40,7 +40,7 @@ type DropTarget struct {
 }
 
 // Action implements Target.Action.
-func (*DropTarget) Action(*PacketBuffer, *ConnTrack, Hook, *Route, AddressableEndpoint) (RuleVerdict, int) {
+func (*DropTarget) Action(*PacketBuffer, Hook, *Route, AddressableEndpoint) (RuleVerdict, int) {
 	return RuleDrop, 0
 }
 
@@ -52,7 +52,7 @@ type ErrorTarget struct {
 }
 
 // Action implements Target.Action.
-func (*ErrorTarget) Action(*PacketBuffer, *ConnTrack, Hook, *Route, AddressableEndpoint) (RuleVerdict, int) {
+func (*ErrorTarget) Action(*PacketBuffer, Hook, *Route, AddressableEndpoint) (RuleVerdict, int) {
 	log.Debugf("ErrorTarget triggered.")
 	return RuleDrop, 0
 }
@@ -67,7 +67,7 @@ type UserChainTarget struct {
 }
 
 // Action implements Target.Action.
-func (*UserChainTarget) Action(*PacketBuffer, *ConnTrack, Hook, *Route, AddressableEndpoint) (RuleVerdict, int) {
+func (*UserChainTarget) Action(*PacketBuffer, Hook, *Route, AddressableEndpoint) (RuleVerdict, int) {
 	panic("UserChainTarget should never be called.")
 }
 
@@ -79,7 +79,7 @@ type ReturnTarget struct {
 }
 
 // Action implements Target.Action.
-func (*ReturnTarget) Action(*PacketBuffer, *ConnTrack, Hook, *Route, AddressableEndpoint) (RuleVerdict, int) {
+func (*ReturnTarget) Action(*PacketBuffer, Hook, *Route, AddressableEndpoint) (RuleVerdict, int) {
 	return RuleReturn, 0
 }
 
@@ -97,7 +97,7 @@ type RedirectTarget struct {
 }
 
 // Action implements Target.Action.
-func (rt *RedirectTarget) Action(pkt *PacketBuffer, ct *ConnTrack, hook Hook, r *Route, addressEP AddressableEndpoint) (RuleVerdict, int) {
+func (rt *RedirectTarget) Action(pkt *PacketBuffer, hook Hook, r *Route, addressEP AddressableEndpoint) (RuleVerdict, int) {
 	// Sanity check.
 	if rt.NetworkProtocol != pkt.NetworkProtocolNumber {
 		panic(fmt.Sprintf(
@@ -154,15 +154,8 @@ func (rt *RedirectTarget) Action(pkt *PacketBuffer, ct *ConnTrack, hook Hook, r
 
 		pkt.NatDone = true
 	case header.TCPProtocolNumber:
-		if ct == nil {
-			return RuleAccept, 0
-		}
-
-		// Set up conection for matching NAT rule. Only the first
-		// packet of the connection comes here. Other packets will be
-		// manipulated in connection tracking.
-		if conn := ct.insertRedirectConn(pkt, hook, rt.Port, address); conn != nil {
-			conn.handlePacket(pkt, hook, dirOriginal, r)
+		if t := pkt.tuple; t != nil {
+			t.conn.performNAT(pkt, hook, r, rt.Port, address, true /* dnat */)
 		}
 	default:
 		return RuleDrop, 0
@@ -181,7 +174,7 @@ type SNATTarget struct {
 	NetworkProtocol tcpip.NetworkProtocolNumber
 }
 
-func snatAction(pkt *PacketBuffer, ct *ConnTrack, hook Hook, r *Route, port uint16, address tcpip.Address) (RuleVerdict, int) {
+func snatAction(pkt *PacketBuffer, hook Hook, r *Route, port uint16, address tcpip.Address) (RuleVerdict, int) {
 	// Packet is already manipulated.
 	if pkt.NatDone {
 		return RuleAccept, 0
@@ -197,30 +190,21 @@ func snatAction(pkt *PacketBuffer, ct *ConnTrack, hook Hook, r *Route, port uint
 	if port == 0 {
 		switch protocol := pkt.TransportProtocolNumber; protocol {
 		case header.UDPProtocolNumber:
-			if port == 0 {
-				port = header.UDP(pkt.TransportHeader().View()).SourcePort()
-			}
+			port = header.UDP(pkt.TransportHeader().View()).SourcePort()
 		case header.TCPProtocolNumber:
-			if port == 0 {
-				port = header.TCP(pkt.TransportHeader().View()).SourcePort()
-			}
+			port = header.TCP(pkt.TransportHeader().View()).SourcePort()
 		}
 	}
 
-	// Set up conection for matching NAT rule. Only the first packet of the
-	// connection comes here. Other packets will be manipulated in connection
-	// tracking.
-	//
-	// Does nothing if the protocol does not support connection tracking.
-	if conn := ct.insertSNATConn(pkt, hook, port, address); conn != nil {
-		conn.handlePacket(pkt, hook, dirOriginal, r)
+	if t := pkt.tuple; t != nil {
+		t.conn.performNAT(pkt, hook, r, port, address, false /* dnat */)
 	}
 
 	return RuleAccept, 0
 }
 
 // Action implements Target.Action.
-func (st *SNATTarget) Action(pkt *PacketBuffer, ct *ConnTrack, hook Hook, r *Route, _ AddressableEndpoint) (RuleVerdict, int) {
+func (st *SNATTarget) Action(pkt *PacketBuffer, hook Hook, r *Route, _ AddressableEndpoint) (RuleVerdict, int) {
 	// Sanity check.
 	if st.NetworkProtocol != pkt.NetworkProtocolNumber {
 		panic(fmt.Sprintf(
@@ -236,7 +220,7 @@ func (st *SNATTarget) Action(pkt *PacketBuffer, ct *ConnTrack, hook Hook, r *Rou
 		panic(fmt.Sprintf("%s unrecognized", hook))
 	}
 
-	return snatAction(pkt, ct, hook, r, st.Port, st.Addr)
+	return snatAction(pkt, hook, r, st.Port, st.Addr)
 }
 
 // MasqueradeTarget modifies the source port/IP in the outgoing packets.
@@ -247,7 +231,7 @@ type MasqueradeTarget struct {
 }
 
 // Action implements Target.Action.
-func (mt *MasqueradeTarget) Action(pkt *PacketBuffer, ct *ConnTrack, hook Hook, r *Route, addressEP AddressableEndpoint) (RuleVerdict, int) {
+func (mt *MasqueradeTarget) Action(pkt *PacketBuffer, hook Hook, r *Route, addressEP AddressableEndpoint) (RuleVerdict, int) {
 	// Sanity check.
 	if mt.NetworkProtocol != pkt.NetworkProtocolNumber {
 		panic(fmt.Sprintf(
@@ -272,7 +256,7 @@ func (mt *MasqueradeTarget) Action(pkt *PacketBuffer, ct *ConnTrack, hook Hook,
 
 	address := ep.AddressWithPrefix().Address
 	ep.DecRef()
-	return snatAction(pkt, ct, hook, r, 0 /* port */, address)
+	return snatAction(pkt, hook, r, 0 /* port */, address)
 }
 
 func rewritePacket(n header.Network, t header.ChecksummableTransport, updateSRCFields, fullChecksum, updatePseudoHeader bool, newPort uint16, newAddr tcpip.Address) {
diff --git a/pkg/tcpip/stack/iptables_types.go b/pkg/tcpip/stack/iptables_types.go
index 50f73f173..b22024667 100644
--- a/pkg/tcpip/stack/iptables_types.go
+++ b/pkg/tcpip/stack/iptables_types.go
@@ -356,5 +356,5 @@ type Target interface {
 	// Action takes an action on the packet and returns a verdict on how
 	// traversal should (or should not) continue. If the return value is
 	// Jump, it also returns the index of the rule to jump to.
-	Action(*PacketBuffer, *ConnTrack, Hook, *Route, AddressableEndpoint) (RuleVerdict, int)
+	Action(*PacketBuffer, Hook, *Route, AddressableEndpoint) (RuleVerdict, int)
 }
diff --git a/pkg/tcpip/stack/packet_buffer.go b/pkg/tcpip/stack/packet_buffer.go
index 456b0cf80..888a8bd9d 100644
--- a/pkg/tcpip/stack/packet_buffer.go
+++ b/pkg/tcpip/stack/packet_buffer.go
@@ -143,6 +143,8 @@ type PacketBuffer struct {
 
 	// NetworkPacketInfo holds an incoming packet's network-layer information.
 	NetworkPacketInfo NetworkPacketInfo
+
+	tuple *tuple
 }
 
 // NewPacketBuffer creates a new PacketBuffer with opts.
@@ -302,6 +304,7 @@ func (pk *PacketBuffer) Clone() *PacketBuffer {
 		NICID:                        pk.NICID,
 		RXTransportChecksumValidated: pk.RXTransportChecksumValidated,
 		NetworkPacketInfo:            pk.NetworkPacketInfo,
+		tuple:                        pk.tuple,
 	}
 }
 
@@ -329,13 +332,8 @@ func (pk *PacketBuffer) CloneToInbound() *PacketBuffer {
 		buf: pk.buf.Clone(),
 		// Treat unfilled header portion as reserved.
 		reserved: pk.AvailableHeaderBytes(),
+		tuple:    pk.tuple,
 	}
-	// TODO(gvisor.dev/issue/5696): reimplement conntrack so that no need to
-	// maintain this flag in the packet. Currently conntrack needs this flag to
-	// tell if a noop connection should be inserted at Input hook. Once conntrack
-	// redefines the manipulation field as mutable, we won't need the special noop
-	// connection.
-	newPk.NatDone = pk.NatDone
 	return newPk
 }
 
@@ -367,12 +365,7 @@ func (pk *PacketBuffer) DeepCopyForForwarding(reservedHeaderBytes int) *PacketBu
 		newPk.TransportProtocolNumber = pk.TransportProtocolNumber
 	}
 
-	// TODO(gvisor.dev/issue/5696): reimplement conntrack so that no need to
-	// maintain this flag in the packet. Currently conntrack needs this flag to
-	// tell if a noop connection should be inserted at Input hook. Once conntrack
-	// redefines the manipulation field as mutable, we won't need the special noop
-	// connection.
-	newPk.NatDone = pk.NatDone
+	newPk.tuple = pk.tuple
 
 	return newPk
 }