Merge pull request #2275 from nybidari:iptables

PiperOrigin-RevId: 309783486

20 files changed, 745 insertions, 105 deletions

diff --git a/pkg/sentry/socket/netfilter/netfilter.go b/pkg/sentry/socket/netfilter/netfilter.go
index 72d093aa8..40736fb38 100644
--- a/pkg/sentry/socket/netfilter/netfilter.go
+++ b/pkg/sentry/socket/netfilter/netfilter.go
@@ -251,7 +251,7 @@ func marshalTarget(target stack.Target) []byte {
 	case stack.ReturnTarget:
 		return marshalStandardTarget(stack.RuleReturn)
 	case stack.RedirectTarget:
-		return marshalRedirectTarget()
+		return marshalRedirectTarget(tg)
 	case JumpTarget:
 		return marshalJumpTarget(tg)
 	default:
@@ -288,7 +288,7 @@ func marshalErrorTarget(errorName string) []byte {
 	return binary.Marshal(ret, usermem.ByteOrder, target)
 }
 
-func marshalRedirectTarget() []byte {
+func marshalRedirectTarget(rt stack.RedirectTarget) []byte {
 	// This is a redirect target named redirect
 	target := linux.XTRedirectTarget{
 		Target: linux.XTEntryTarget{
@@ -298,6 +298,16 @@ func marshalRedirectTarget() []byte {
 	copy(target.Target.Name[:], redirectTargetName)
 
 	ret := make([]byte, 0, linux.SizeOfXTRedirectTarget)
+	target.NfRange.RangeSize = 1
+	if rt.RangeProtoSpecified {
+		target.NfRange.RangeIPV4.Flags |= linux.NF_NAT_RANGE_PROTO_SPECIFIED
+	}
+	// Convert port from little endian to big endian.
+	port := make([]byte, 2)
+	binary.LittleEndian.PutUint16(port, rt.MinPort)
+	target.NfRange.RangeIPV4.MinPort = binary.BigEndian.Uint16(port)
+	binary.LittleEndian.PutUint16(port, rt.MaxPort)
+	target.NfRange.RangeIPV4.MaxPort = binary.BigEndian.Uint16(port)
 	return binary.Marshal(ret, usermem.ByteOrder, target)
 }
 
diff --git a/pkg/sentry/socket/netfilter/targets.go b/pkg/sentry/socket/netfilter/targets.go
index c948de876..84abe8d29 100644
--- a/pkg/sentry/socket/netfilter/targets.go
+++ b/pkg/sentry/socket/netfilter/targets.go
@@ -15,6 +15,7 @@
 package netfilter
 
 import (
+	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
 )
 
@@ -29,6 +30,6 @@ type JumpTarget struct {
 }
 
 // Action implements stack.Target.Action.
-func (jt JumpTarget) Action(stack.PacketBuffer) (stack.RuleVerdict, int) {
+func (jt JumpTarget) Action(*stack.PacketBuffer, *stack.ConnTrackTable, stack.Hook, *stack.GSO, *stack.Route, tcpip.Address) (stack.RuleVerdict, int) {
 	return stack.RuleJump, jt.RuleNum
 }
diff --git a/pkg/sentry/socket/netfilter/tcp_matcher.go b/pkg/sentry/socket/netfilter/tcp_matcher.go
index 55c0f04f3..57a1e1c12 100644
--- a/pkg/sentry/socket/netfilter/tcp_matcher.go
+++ b/pkg/sentry/socket/netfilter/tcp_matcher.go
@@ -120,14 +120,27 @@ func (tm *TCPMatcher) Match(hook stack.Hook, pkt stack.PacketBuffer, interfaceNa
 	if pkt.TransportHeader != nil {
 		tcpHeader = header.TCP(pkt.TransportHeader)
 	} else {
+		var length int
+		if hook == stack.Prerouting {
+			// The network header hasn't been parsed yet. We have to do it here.
+			hdr, ok := pkt.Data.PullUp(header.IPv4MinimumSize)
+			if !ok {
+				// There's no valid TCP header here, so we hotdrop the
+				// packet.
+				return false, true
+			}
+			h := header.IPv4(hdr)
+			pkt.NetworkHeader = hdr
+			length = int(h.HeaderLength())
+		}
 		// The TCP header hasn't been parsed yet. We have to do it here.
-		hdr, ok := pkt.Data.PullUp(header.TCPMinimumSize)
+		hdr, ok := pkt.Data.PullUp(length + header.TCPMinimumSize)
 		if !ok {
 			// There's no valid TCP header here, so we hotdrop the
 			// packet.
 			return false, true
 		}
-		tcpHeader = header.TCP(hdr)
+		tcpHeader = header.TCP(hdr[length:])
 	}
 
 	// Check whether the source and destination ports are within the
diff --git a/pkg/sentry/socket/netfilter/udp_matcher.go b/pkg/sentry/socket/netfilter/udp_matcher.go
index 04d03d494..cfa9e621d 100644
--- a/pkg/sentry/socket/netfilter/udp_matcher.go
+++ b/pkg/sentry/socket/netfilter/udp_matcher.go
@@ -119,14 +119,27 @@ func (um *UDPMatcher) Match(hook stack.Hook, pkt stack.PacketBuffer, interfaceNa
 	if pkt.TransportHeader != nil {
 		udpHeader = header.UDP(pkt.TransportHeader)
 	} else {
+		var length int
+		if hook == stack.Prerouting {
+			// The network header hasn't been parsed yet. We have to do it here.
+			hdr, ok := pkt.Data.PullUp(header.IPv4MinimumSize)
+			if !ok {
+				// There's no valid UDP header here, so we hotdrop the
+				// packet.
+				return false, true
+			}
+			h := header.IPv4(hdr)
+			pkt.NetworkHeader = hdr
+			length = int(h.HeaderLength())
+		}
 		// The UDP header hasn't been parsed yet. We have to do it here.
-		hdr, ok := pkt.Data.PullUp(header.UDPMinimumSize)
+		hdr, ok := pkt.Data.PullUp(length + header.UDPMinimumSize)
 		if !ok {
 			// There's no valid UDP header here, so we hotdrop the
 			// packet.
 			return false, true
 		}
-		udpHeader = header.UDP(hdr)
+		udpHeader = header.UDP(hdr[length:])
 	}
 
 	// Check whether the source and destination ports are within the
diff --git a/pkg/tcpip/header/tcp.go b/pkg/tcpip/header/tcp.go
index 13480687d..21581257b 100644
--- a/pkg/tcpip/header/tcp.go
+++ b/pkg/tcpip/header/tcp.go
@@ -594,3 +594,20 @@ func AddTCPOptionPadding(options []byte, offset int) int {
 	}
 	return paddingToAdd
 }
+
+// Acceptable checks if a segment that starts at segSeq and has length segLen is
+// "acceptable" for arriving in a receive window that starts at rcvNxt and ends
+// before rcvAcc, according to the table on page 26 and 69 of RFC 793.
+func Acceptable(segSeq seqnum.Value, segLen seqnum.Size, rcvNxt, rcvAcc seqnum.Value) bool {
+	if rcvNxt == rcvAcc {
+		return segLen == 0 && segSeq == rcvNxt
+	}
+	if segLen == 0 {
+		// rcvWnd is incremented by 1 because that is Linux's behavior despite the
+		// RFC.
+		return segSeq.InRange(rcvNxt, rcvAcc.Add(1))
+	}
+	// Page 70 of RFC 793 allows packets that can be made "acceptable" by trimming
+	// the payload, so we'll accept any payload that overlaps the receieve window.
+	return rcvNxt.LessThan(segSeq.Add(segLen)) && segSeq.LessThan(rcvAcc)
+}
diff --git a/pkg/tcpip/network/ipv4/ipv4.go b/pkg/tcpip/network/ipv4/ipv4.go
index 1d61fddad..9db42b2a4 100644
--- a/pkg/tcpip/network/ipv4/ipv4.go
+++ b/pkg/tcpip/network/ipv4/ipv4.go
@@ -252,11 +252,31 @@ func (e *endpoint) WritePacket(r *stack.Route, gso *stack.GSO, params stack.Netw
 	// iptables filtering. All packets that reach here are locally
 	// generated.
 	ipt := e.stack.IPTables()
-	if ok := ipt.Check(stack.Output, pkt); !ok {
+	if ok := ipt.Check(stack.Output, &pkt, gso, r, ""); !ok {
 		// iptables is telling us to drop the packet.
 		return nil
 	}
 
+	if pkt.NatDone {
+		// If the packet is manipulated as per NAT Ouput rules, handle packet
+		// based on destination address and do not send the packet to link layer.
+		netHeader := header.IPv4(pkt.NetworkHeader)
+		ep, err := e.stack.FindNetworkEndpoint(header.IPv4ProtocolNumber, netHeader.DestinationAddress())
+		if err == nil {
+			src := netHeader.SourceAddress()
+			dst := netHeader.DestinationAddress()
+			route := r.ReverseRoute(src, dst)
+
+			views := make([]buffer.View, 1, 1+len(pkt.Data.Views()))
+			views[0] = pkt.Header.View()
+			views = append(views, pkt.Data.Views()...)
+			packet := stack.PacketBuffer{
+				Data: buffer.NewVectorisedView(len(views[0])+pkt.Data.Size(), views)}
+			ep.HandlePacket(&route, packet)
+			return nil
+		}
+	}
+
 	if r.Loop&stack.PacketLoop != 0 {
 		// The inbound path expects the network header to still be in
 		// the PacketBuffer's Data field.
@@ -302,8 +322,8 @@ func (e *endpoint) WritePackets(r *stack.Route, gso *stack.GSO, pkts stack.Packe
 	// iptables filtering. All packets that reach here are locally
 	// generated.
 	ipt := e.stack.IPTables()
-	dropped := ipt.CheckPackets(stack.Output, pkts)
-	if len(dropped) == 0 {
+	dropped, natPkts := ipt.CheckPackets(stack.Output, pkts, gso, r)
+	if len(dropped) == 0 && len(natPkts) == 0 {
 		// Fast path: If no packets are to be dropped then we can just invoke the
 		// faster WritePackets API directly.
 		n, err := e.linkEP.WritePackets(r, gso, pkts, ProtocolNumber)
@@ -318,6 +338,24 @@ func (e *endpoint) WritePackets(r *stack.Route, gso *stack.GSO, pkts stack.Packe
 		if _, ok := dropped[pkt]; ok {
 			continue
 		}
+		if _, ok := natPkts[pkt]; ok {
+			netHeader := header.IPv4(pkt.NetworkHeader)
+			ep, err := e.stack.FindNetworkEndpoint(header.IPv4ProtocolNumber, netHeader.DestinationAddress())
+			if err == nil {
+				src := netHeader.SourceAddress()
+				dst := netHeader.DestinationAddress()
+				route := r.ReverseRoute(src, dst)
+
+				views := make([]buffer.View, 1, 1+len(pkt.Data.Views()))
+				views[0] = pkt.Header.View()
+				views = append(views, pkt.Data.Views()...)
+				packet := stack.PacketBuffer{
+					Data: buffer.NewVectorisedView(len(views[0])+pkt.Data.Size(), views)}
+				ep.HandlePacket(&route, packet)
+				n++
+				continue
+			}
+		}
 		if err := e.linkEP.WritePacket(r, gso, ProtocolNumber, *pkt); err != nil {
 			r.Stats().IP.PacketsSent.IncrementBy(uint64(n))
 			return n, err
@@ -407,7 +445,7 @@ func (e *endpoint) HandlePacket(r *stack.Route, pkt stack.PacketBuffer) {
 	// iptables filtering. All packets that reach here are intended for
 	// this machine and will not be forwarded.
 	ipt := e.stack.IPTables()
-	if ok := ipt.Check(stack.Input, pkt); !ok {
+	if ok := ipt.Check(stack.Input, &pkt, nil, nil, ""); !ok {
 		// iptables is telling us to drop the packet.
 		return
 	}
diff --git a/pkg/tcpip/stack/BUILD b/pkg/tcpip/stack/BUILD
index 5e963a4af..f71073207 100644
--- a/pkg/tcpip/stack/BUILD
+++ b/pkg/tcpip/stack/BUILD
@@ -30,6 +30,7 @@ go_template_instance(
 go_library(
     name = "stack",
     srcs = [
+        "conntrack.go",
         "dhcpv6configurationfromndpra_string.go",
         "forwarder.go",
         "icmp_rate_limit.go",
@@ -62,6 +63,7 @@ go_library(
         "//pkg/tcpip/header",
         "//pkg/tcpip/ports",
         "//pkg/tcpip/seqnum",
+        "//pkg/tcpip/transport/tcpconntrack",
         "//pkg/waiter",
         "@org_golang_x_time//rate:go_default_library",
     ],
diff --git a/pkg/tcpip/stack/conntrack.go b/pkg/tcpip/stack/conntrack.go
new file mode 100644
index 000000000..7d1ede1f2
--- /dev/null
+++ b/pkg/tcpip/stack/conntrack.go
@@ -0,0 +1,480 @@
+// Copyright 2020 The gVisor Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package stack
+
+import (
+	"encoding/binary"
+	"sync"
+	"time"
+
+	"gvisor.dev/gvisor/pkg/tcpip"
+	"gvisor.dev/gvisor/pkg/tcpip/buffer"
+	"gvisor.dev/gvisor/pkg/tcpip/hash/jenkins"
+	"gvisor.dev/gvisor/pkg/tcpip/header"
+	"gvisor.dev/gvisor/pkg/tcpip/transport/tcpconntrack"
+)
+
+// Connection tracking is used to track and manipulate packets for NAT rules.
+// The connection is created for a packet if it does not exist. Every connection
+// contains two tuples (original and reply). The tuples are manipulated if there
+// is a matching NAT rule. The packet is modified by looking at the tuples in the
+// Prerouting and Output hooks.
+
+// Direction of the tuple.
+type ctDirection int
+
+const (
+	dirOriginal ctDirection = iota
+	dirReply
+)
+
+// Status of connection.
+// TODO(gvisor.dev/issue/170): Add other states of connection.
+type connStatus int
+
+const (
+	connNew connStatus = iota
+	connEstablished
+)
+
+// Manipulation type for the connection.
+type manipType int
+
+const (
+	manipDstPrerouting manipType = iota
+	manipDstOutput
+)
+
+// connTrackMutable is the manipulatable part of the tuple.
+type connTrackMutable struct {
+	// addr is source address of the tuple.
+	addr tcpip.Address
+
+	// port is source port of the tuple.
+	port uint16
+
+	// protocol is network layer protocol.
+	protocol tcpip.NetworkProtocolNumber
+}
+
+// connTrackImmutable is the non-manipulatable part of the tuple.
+type connTrackImmutable struct {
+	// addr is destination address of the tuple.
+	addr tcpip.Address
+
+	// direction is direction (original or reply) of the tuple.
+	direction ctDirection
+
+	// port is destination port of the tuple.
+	port uint16
+
+	// protocol is transport layer protocol.
+	protocol tcpip.TransportProtocolNumber
+}
+
+// connTrackTuple represents the tuple which is created from the
+// packet.
+type connTrackTuple struct {
+	// dst is non-manipulatable part of the tuple.
+	dst connTrackImmutable
+
+	// src is manipulatable part of the tuple.
+	src connTrackMutable
+}
+
+// connTrackTupleHolder is the container of tuple and connection.
+type ConnTrackTupleHolder struct {
+	// conn is pointer to the connection tracking entry.
+	conn *connTrack
+
+	// tuple is original or reply tuple.
+	tuple connTrackTuple
+}
+
+// connTrack is the connection.
+type connTrack struct {
+	// originalTupleHolder contains tuple in original direction.
+	originalTupleHolder ConnTrackTupleHolder
+
+	// replyTupleHolder contains tuple in reply direction.
+	replyTupleHolder ConnTrackTupleHolder
+
+	// status indicates connection is new or established.
+	status connStatus
+
+	// timeout indicates the time connection should be active.
+	timeout time.Duration
+
+	// manip indicates if the packet should be manipulated.
+	manip manipType
+
+	// tcb is TCB control block. It is used to keep track of states
+	// of tcp connection.
+	tcb tcpconntrack.TCB
+
+	// tcbHook indicates if the packet is inbound or outbound to
+	// update the state of tcb.
+	tcbHook Hook
+}
+
+// ConnTrackTable contains a map of all existing connections created for
+// NAT rules.
+type ConnTrackTable struct {
+	// connMu protects connTrackTable.
+	connMu sync.RWMutex
+
+	// connTrackTable maintains a map of tuples needed for connection tracking
+	// for iptables NAT rules. The key for the map is an integer calculated
+	// using seed, source address, destination address, source port and
+	// destination port.
+	CtMap map[uint32]ConnTrackTupleHolder
+
+	// seed is a one-time random value initialized at stack startup
+	// and is used in calculation of hash key for connection tracking
+	// table.
+	Seed uint32
+}
+
+// parseHeaders sets headers in the packet.
+func parseHeaders(pkt *PacketBuffer) {
+	newPkt := pkt.Clone()
+
+	// Set network header.
+	hdr, ok := newPkt.Data.PullUp(header.IPv4MinimumSize)
+	if !ok {
+		return
+	}
+	netHeader := header.IPv4(hdr)
+	newPkt.NetworkHeader = hdr
+	length := int(netHeader.HeaderLength())
+
+	// TODO(gvisor.dev/issue/170): Need to support for other
+	// protocols as well.
+	// Set transport header.
+	switch protocol := netHeader.TransportProtocol(); protocol {
+	case header.UDPProtocolNumber:
+		if newPkt.TransportHeader == nil {
+			h, ok := newPkt.Data.PullUp(length + header.UDPMinimumSize)
+			if !ok {
+				return
+			}
+			newPkt.TransportHeader = buffer.View(header.UDP(h[length:]))
+		}
+	case header.TCPProtocolNumber:
+		if newPkt.TransportHeader == nil {
+			h, ok := newPkt.Data.PullUp(length + header.TCPMinimumSize)
+			if !ok {
+				return
+			}
+			newPkt.TransportHeader = buffer.View(header.TCP(h[length:]))
+		}
+	}
+	pkt.NetworkHeader = newPkt.NetworkHeader
+	pkt.TransportHeader = newPkt.TransportHeader
+}
+
+// packetToTuple converts packet to a tuple in original direction.
+func packetToTuple(pkt PacketBuffer, hook Hook) (connTrackTuple, *tcpip.Error) {
+	var tuple connTrackTuple
+
+	netHeader := header.IPv4(pkt.NetworkHeader)
+	// TODO(gvisor.dev/issue/170): Need to support for other
+	// protocols as well.
+	if netHeader == nil || netHeader.TransportProtocol() != header.TCPProtocolNumber {
+		return tuple, tcpip.ErrUnknownProtocol
+	}
+	tcpHeader := header.TCP(pkt.TransportHeader)
+	if tcpHeader == nil {
+		return tuple, tcpip.ErrUnknownProtocol
+	}
+
+	tuple.src.addr = netHeader.SourceAddress()
+	tuple.src.port = tcpHeader.SourcePort()
+	tuple.src.protocol = header.IPv4ProtocolNumber
+
+	tuple.dst.addr = netHeader.DestinationAddress()
+	tuple.dst.port = tcpHeader.DestinationPort()
+	tuple.dst.protocol = netHeader.TransportProtocol()
+
+	return tuple, nil
+}
+
+// getReplyTuple creates reply tuple for the given tuple.
+func getReplyTuple(tuple connTrackTuple) connTrackTuple {
+	var replyTuple connTrackTuple
+	replyTuple.src.addr = tuple.dst.addr
+	replyTuple.src.port = tuple.dst.port
+	replyTuple.src.protocol = tuple.src.protocol
+	replyTuple.dst.addr = tuple.src.addr
+	replyTuple.dst.port = tuple.src.port
+	replyTuple.dst.protocol = tuple.dst.protocol
+	replyTuple.dst.direction = dirReply
+
+	return replyTuple
+}
+
+// makeNewConn creates new connection.
+func makeNewConn(tuple, replyTuple connTrackTuple) connTrack {
+	var conn connTrack
+	conn.status = connNew
+	conn.originalTupleHolder.tuple = tuple
+	conn.originalTupleHolder.conn = &conn
+	conn.replyTupleHolder.tuple = replyTuple
+	conn.replyTupleHolder.conn = &conn
+
+	return conn
+}
+
+// getTupleHash returns hash of the tuple. The fields used for
+// generating hash are seed (generated once for stack), source address,
+// destination address, source port and destination ports.
+func (ct *ConnTrackTable) getTupleHash(tuple connTrackTuple) uint32 {
+	h := jenkins.Sum32(ct.Seed)
+	h.Write([]byte(tuple.src.addr))
+	h.Write([]byte(tuple.dst.addr))
+	portBuf := make([]byte, 2)
+	binary.LittleEndian.PutUint16(portBuf, tuple.src.port)
+	h.Write([]byte(portBuf))
+	binary.LittleEndian.PutUint16(portBuf, tuple.dst.port)
+	h.Write([]byte(portBuf))
+
+	return h.Sum32()
+}
+
+// connTrackForPacket returns connTrack for packet.
+// TODO(gvisor.dev/issue/170): Only TCP packets are supported. Need to support other
+// transport protocols.
+func (ct *ConnTrackTable) connTrackForPacket(pkt *PacketBuffer, hook Hook, createConn bool) (*connTrack, ctDirection) {
+	if hook == Prerouting {
+		// Headers will not be set in Prerouting.
+		// TODO(gvisor.dev/issue/170): Change this after parsing headers
+		// code is added.
+		parseHeaders(pkt)
+	}
+
+	var dir ctDirection
+	tuple, err := packetToTuple(*pkt, hook)
+	if err != nil {
+		return nil, dir
+	}
+
+	ct.connMu.Lock()
+	defer ct.connMu.Unlock()
+
+	connTrackTable := ct.CtMap
+	hash := ct.getTupleHash(tuple)
+
+	var conn *connTrack
+	switch createConn {
+	case true:
+		// If connection does not exist for the hash, create a new
+		// connection.
+		replyTuple := getReplyTuple(tuple)
+		replyHash := ct.getTupleHash(replyTuple)
+		newConn := makeNewConn(tuple, replyTuple)
+		conn = &newConn
+
+		// Add tupleHolders to the map.
+		// TODO(gvisor.dev/issue/170): Need to support collisions using linked list.
+		ct.CtMap[hash] = conn.originalTupleHolder
+		ct.CtMap[replyHash] = conn.replyTupleHolder
+	default:
+		tupleHolder, ok := connTrackTable[hash]
+		if !ok {
+			return nil, dir
+		}
+
+		// If this is the reply of new connection, set the connection
+		// status as ESTABLISHED.
+		conn = tupleHolder.conn
+		if conn.status == connNew && tupleHolder.tuple.dst.direction == dirReply {
+			conn.status = connEstablished
+		}
+		if tupleHolder.conn == nil {
+			panic("tupleHolder has null connection tracking entry")
+		}
+
+		dir = tupleHolder.tuple.dst.direction
+	}
+	return conn, dir
+}
+
+// SetNatInfo will manipulate the tuples according to iptables NAT rules.
+func (ct *ConnTrackTable) SetNatInfo(pkt *PacketBuffer, rt RedirectTarget, hook Hook) {
+	// Get the connection. Connection is always created before this
+	// function is called.
+	conn, _ := ct.connTrackForPacket(pkt, hook, false)
+	if conn == nil {
+		panic("connection should be created to manipulate tuples.")
+	}
+	replyTuple := conn.replyTupleHolder.tuple
+	replyHash := ct.getTupleHash(replyTuple)
+
+	// TODO(gvisor.dev/issue/170): Support only redirect of ports. Need to
+	// support changing of address for Prerouting.
+
+	// Change the port as per the iptables rule. This tuple will be used
+	// to manipulate the packet in HandlePacket.
+	conn.replyTupleHolder.tuple.src.addr = rt.MinIP
+	conn.replyTupleHolder.tuple.src.port = rt.MinPort
+	newHash := ct.getTupleHash(conn.replyTupleHolder.tuple)
+
+	// Add the changed tuple to the map.
+	ct.connMu.Lock()
+	defer ct.connMu.Unlock()
+	ct.CtMap[newHash] = conn.replyTupleHolder
+	if hook == Output {
+		conn.replyTupleHolder.conn.manip = manipDstOutput
+	}
+
+	// Delete the old tuple.
+	delete(ct.CtMap, replyHash)
+}
+
+// handlePacketPrerouting manipulates ports for packets in Prerouting hook.
+// TODO(gvisor.dev/issue/170): Change address for Prerouting hook..
+func handlePacketPrerouting(pkt *PacketBuffer, conn *connTrack, dir ctDirection) {
+	netHeader := header.IPv4(pkt.NetworkHeader)
+	tcpHeader := header.TCP(pkt.TransportHeader)
+
+	// For prerouting redirection, packets going in the original direction
+	// have their destinations modified and replies have their sources
+	// modified.
+	switch dir {
+	case dirOriginal:
+		port := conn.replyTupleHolder.tuple.src.port
+		tcpHeader.SetDestinationPort(port)
+		netHeader.SetDestinationAddress(conn.replyTupleHolder.tuple.src.addr)
+	case dirReply:
+		port := conn.originalTupleHolder.tuple.dst.port
+		tcpHeader.SetSourcePort(port)
+		netHeader.SetSourceAddress(conn.originalTupleHolder.tuple.dst.addr)
+	}
+
+	netHeader.SetChecksum(0)
+	netHeader.SetChecksum(^netHeader.CalculateChecksum())
+}
+
+// handlePacketOutput manipulates ports for packets in Output hook.
+func handlePacketOutput(pkt *PacketBuffer, conn *connTrack, gso *GSO, r *Route, dir ctDirection) {
+	netHeader := header.IPv4(pkt.NetworkHeader)
+	tcpHeader := header.TCP(pkt.TransportHeader)
+
+	// For output redirection, packets going in the original direction
+	// have their destinations modified and replies have their sources
+	// modified. For prerouting redirection, we only reach this point
+	// when replying, so packet sources are modified.
+	if conn.manip == manipDstOutput && dir == dirOriginal {
+		port := conn.replyTupleHolder.tuple.src.port
+		tcpHeader.SetDestinationPort(port)
+		netHeader.SetDestinationAddress(conn.replyTupleHolder.tuple.src.addr)
+	} else {
+		port := conn.originalTupleHolder.tuple.dst.port
+		tcpHeader.SetSourcePort(port)
+		netHeader.SetSourceAddress(conn.originalTupleHolder.tuple.dst.addr)
+	}
+
+	// Calculate the TCP checksum and set it.
+	tcpHeader.SetChecksum(0)
+	hdr := &pkt.Header
+	length := uint16(pkt.Data.Size()+hdr.UsedLength()) - uint16(netHeader.HeaderLength())
+	xsum := r.PseudoHeaderChecksum(header.TCPProtocolNumber, length)
+	if gso != nil && gso.NeedsCsum {
+		tcpHeader.SetChecksum(xsum)
+	} else if r.Capabilities()&CapabilityTXChecksumOffload == 0 {
+		xsum = header.ChecksumVVWithOffset(pkt.Data, xsum, int(tcpHeader.DataOffset()), pkt.Data.Size())
+		tcpHeader.SetChecksum(^tcpHeader.CalculateChecksum(xsum))
+	}
+
+	netHeader.SetChecksum(0)
+	netHeader.SetChecksum(^netHeader.CalculateChecksum())
+}
+
+// HandlePacket will manipulate the port and address of the packet if the
+// connection exists.
+func (ct *ConnTrackTable) HandlePacket(pkt *PacketBuffer, hook Hook, gso *GSO, r *Route) {
+	if pkt.NatDone {
+		return
+	}
+
+	if hook != Prerouting && hook != Output {
+		return
+	}
+
+	conn, dir := ct.connTrackForPacket(pkt, hook, false)
+	// Connection or Rule not found for the packet.
+	if conn == nil {
+		return
+	}
+
+	netHeader := header.IPv4(pkt.NetworkHeader)
+	// TODO(gvisor.dev/issue/170): Need to support for other transport
+	// protocols as well.
+	if netHeader == nil || netHeader.TransportProtocol() != header.TCPProtocolNumber {
+		return
+	}
+
+	tcpHeader := header.TCP(pkt.TransportHeader)
+	if tcpHeader == nil {
+		return
+	}
+
+	switch hook {
+	case Prerouting:
+		handlePacketPrerouting(pkt, conn, dir)
+	case Output:
+		handlePacketOutput(pkt, conn, gso, r, dir)
+	}
+	pkt.NatDone = true
+
+	// Update the state of tcb.
+	// TODO(gvisor.dev/issue/170): Add support in tcpcontrack to handle
+	// other tcp states.
+	var st tcpconntrack.Result
+	if conn.tcb.IsEmpty() {
+		conn.tcb.Init(tcpHeader)
+		conn.tcbHook = hook
+	} else {
+		switch hook {
+		case conn.tcbHook:
+			st = conn.tcb.UpdateStateOutbound(tcpHeader)
+		default:
+			st = conn.tcb.UpdateStateInbound(tcpHeader)
+		}
+	}
+
+	// Delete conntrack if tcp connection is closed.
+	if st == tcpconntrack.ResultClosedByPeer || st == tcpconntrack.ResultClosedBySelf || st == tcpconntrack.ResultReset {
+		ct.deleteConnTrack(conn)
+	}
+}
+
+// deleteConnTrack deletes the connection.
+func (ct *ConnTrackTable) deleteConnTrack(conn *connTrack) {
+	if conn == nil {
+		return
+	}
+
+	tuple := conn.originalTupleHolder.tuple
+	hash := ct.getTupleHash(tuple)
+	replyTuple := conn.replyTupleHolder.tuple
+	replyHash := ct.getTupleHash(replyTuple)
+
+	ct.connMu.Lock()
+	defer ct.connMu.Unlock()
+
+	delete(ct.CtMap, hash)
+	delete(ct.CtMap, replyHash)
+}
diff --git a/pkg/tcpip/stack/iptables.go b/pkg/tcpip/stack/iptables.go
index 6b91159d4..7c3c47d50 100644
--- a/pkg/tcpip/stack/iptables.go
+++ b/pkg/tcpip/stack/iptables.go
@@ -17,6 +17,7 @@ package stack
 import (
 	"fmt"
 
+	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/header"
 )
 
@@ -110,6 +111,10 @@ func DefaultTables() IPTables {
 			Prerouting: []string{TablenameMangle, TablenameNat},
 			Output:     []string{TablenameMangle, TablenameNat, TablenameFilter},
 		},
+		connections: ConnTrackTable{
+			CtMap: make(map[uint32]ConnTrackTupleHolder),
+			Seed:  generateRandUint32(),
+		},
 	}
 }
 
@@ -173,12 +178,16 @@ const (
 // dropped.
 //
 // Precondition: pkt.NetworkHeader is set.
-func (it *IPTables) Check(hook Hook, pkt PacketBuffer) bool {
+func (it *IPTables) Check(hook Hook, pkt *PacketBuffer, gso *GSO, r *Route, address tcpip.Address) bool {
+	// Packets are manipulated only if connection and matching
+	// NAT rule exists.
+	it.connections.HandlePacket(pkt, hook, gso, r)
+
 	// Go through each table containing the hook.
 	for _, tablename := range it.Priorities[hook] {
 		table := it.Tables[tablename]
 		ruleIdx := table.BuiltinChains[hook]
-		switch verdict := it.checkChain(hook, pkt, table, ruleIdx); verdict {
+		switch verdict := it.checkChain(hook, pkt, table, ruleIdx, gso, r, address); verdict {
 		// If the table returns Accept, move on to the next table.
 		case chainAccept:
 			continue
@@ -189,7 +198,7 @@ func (it *IPTables) Check(hook Hook, pkt PacketBuffer) bool {
 			// 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); v {
+			switch v, _ := underflow.Target.Action(pkt, &it.connections, hook, gso, r, address); v {
 			case RuleAccept:
 				continue
 			case RuleDrop:
@@ -219,26 +228,34 @@ func (it *IPTables) Check(hook Hook, pkt PacketBuffer) bool {
 //
 // NOTE: unlike the Check API the returned map contains packets that should be
 // dropped.
-func (it *IPTables) CheckPackets(hook Hook, pkts PacketBufferList) (drop map[*PacketBuffer]struct{}) {
+func (it *IPTables) CheckPackets(hook Hook, pkts PacketBufferList, gso *GSO, r *Route) (drop map[*PacketBuffer]struct{}, natPkts map[*PacketBuffer]struct{}) {
 	for pkt := pkts.Front(); pkt != nil; pkt = pkt.Next() {
-		if ok := it.Check(hook, *pkt); !ok {
-			if drop == nil {
-				drop = make(map[*PacketBuffer]struct{})
+		if !pkt.NatDone {
+			if ok := it.Check(hook, pkt, gso, r, ""); !ok {
+				if drop == nil {
+					drop = make(map[*PacketBuffer]struct{})
+				}
+				drop[pkt] = struct{}{}
+			}
+			if pkt.NatDone {
+				if natPkts == nil {
+					natPkts = make(map[*PacketBuffer]struct{})
+				}
+				natPkts[pkt] = struct{}{}
 			}
-			drop[pkt] = struct{}{}
 		}
 	}
-	return drop
+	return drop, natPkts
 }
 
 // Precondition: pkt is a IPv4 packet of at least length header.IPv4MinimumSize.
-// TODO(gvisor.dev/issue/170): pk.NetworkHeader will always be set as a
+// TODO(gvisor.dev/issue/170): pkt.NetworkHeader will always be set as a
 // precondition.
-func (it *IPTables) checkChain(hook Hook, pkt PacketBuffer, table Table, ruleIdx int) chainVerdict {
+func (it *IPTables) checkChain(hook Hook, pkt *PacketBuffer, table Table, ruleIdx int, gso *GSO, r *Route, address tcpip.Address) chainVerdict {
 	// Start from ruleIdx and walk the list of rules until a rule gives us
 	// a verdict.
 	for ruleIdx < len(table.Rules) {
-		switch verdict, jumpTo := it.checkRule(hook, pkt, table, ruleIdx); verdict {
+		switch verdict, jumpTo := it.checkRule(hook, pkt, table, ruleIdx, gso, r, address); verdict {
 		case RuleAccept:
 			return chainAccept
 
@@ -255,7 +272,7 @@ func (it *IPTables) checkChain(hook Hook, pkt PacketBuffer, table Table, ruleIdx
 				ruleIdx++
 				continue
 			}
-			switch verdict := it.checkChain(hook, pkt, table, jumpTo); verdict {
+			switch verdict := it.checkChain(hook, pkt, table, jumpTo, gso, r, address); verdict {
 			case chainAccept:
 				return chainAccept
 			case chainDrop:
@@ -279,9 +296,9 @@ func (it *IPTables) checkChain(hook Hook, pkt PacketBuffer, table Table, ruleIdx
 }
 
 // Precondition: pkt is a IPv4 packet of at least length header.IPv4MinimumSize.
-// TODO(gvisor.dev/issue/170): pk.NetworkHeader will always be set as a
+// TODO(gvisor.dev/issue/170): pkt.NetworkHeader will always be set as a
 // precondition.
-func (it *IPTables) checkRule(hook Hook, pkt PacketBuffer, table Table, ruleIdx int) (RuleVerdict, int) {
+func (it *IPTables) checkRule(hook Hook, pkt *PacketBuffer, table Table, ruleIdx int, gso *GSO, r *Route, address tcpip.Address) (RuleVerdict, int) {
 	rule := table.Rules[ruleIdx]
 
 	// If pkt.NetworkHeader hasn't been set yet, it will be contained in
@@ -304,7 +321,7 @@ func (it *IPTables) checkRule(hook Hook, pkt PacketBuffer, table Table, ruleIdx
 	// Go through each rule matcher. If they all match, run
 	// the rule target.
 	for _, matcher := range rule.Matchers {
-		matches, hotdrop := matcher.Match(hook, pkt, "")
+		matches, hotdrop := matcher.Match(hook, *pkt, "")
 		if hotdrop {
 			return RuleDrop, 0
 		}
@@ -315,7 +332,7 @@ func (it *IPTables) checkRule(hook Hook, pkt PacketBuffer, table Table, ruleIdx
 	}
 
 	// All the matchers matched, so run the target.
-	return rule.Target.Action(pkt)
+	return rule.Target.Action(pkt, &it.connections, hook, gso, r, address)
 }
 
 func filterMatch(filter IPHeaderFilter, hdr header.IPv4) bool {
diff --git a/pkg/tcpip/stack/iptables_targets.go b/pkg/tcpip/stack/iptables_targets.go
index 8be61f4b1..36cc6275d 100644
--- a/pkg/tcpip/stack/iptables_targets.go
+++ b/pkg/tcpip/stack/iptables_targets.go
@@ -24,7 +24,7 @@ import (
 type AcceptTarget struct{}
 
 // Action implements Target.Action.
-func (AcceptTarget) Action(packet PacketBuffer) (RuleVerdict, int) {
+func (AcceptTarget) Action(*PacketBuffer, *ConnTrackTable, Hook, *GSO, *Route, tcpip.Address) (RuleVerdict, int) {
 	return RuleAccept, 0
 }
 
@@ -32,7 +32,7 @@ func (AcceptTarget) Action(packet PacketBuffer) (RuleVerdict, int) {
 type DropTarget struct{}
 
 // Action implements Target.Action.
-func (DropTarget) Action(packet PacketBuffer) (RuleVerdict, int) {
+func (DropTarget) Action(*PacketBuffer, *ConnTrackTable, Hook, *GSO, *Route, tcpip.Address) (RuleVerdict, int) {
 	return RuleDrop, 0
 }
 
@@ -41,7 +41,7 @@ func (DropTarget) Action(packet PacketBuffer) (RuleVerdict, int) {
 type ErrorTarget struct{}
 
 // Action implements Target.Action.
-func (ErrorTarget) Action(packet PacketBuffer) (RuleVerdict, int) {
+func (ErrorTarget) Action(*PacketBuffer, *ConnTrackTable, Hook, *GSO, *Route, tcpip.Address) (RuleVerdict, int) {
 	log.Debugf("ErrorTarget triggered.")
 	return RuleDrop, 0
 }
@@ -52,7 +52,7 @@ type UserChainTarget struct {
 }
 
 // Action implements Target.Action.
-func (UserChainTarget) Action(PacketBuffer) (RuleVerdict, int) {
+func (UserChainTarget) Action(*PacketBuffer, *ConnTrackTable, Hook, *GSO, *Route, tcpip.Address) (RuleVerdict, int) {
 	panic("UserChainTarget should never be called.")
 }
 
@@ -61,7 +61,7 @@ func (UserChainTarget) Action(PacketBuffer) (RuleVerdict, int) {
 type ReturnTarget struct{}
 
 // Action implements Target.Action.
-func (ReturnTarget) Action(PacketBuffer) (RuleVerdict, int) {
+func (ReturnTarget) Action(*PacketBuffer, *ConnTrackTable, Hook, *GSO, *Route, tcpip.Address) (RuleVerdict, int) {
 	return RuleReturn, 0
 }
 
@@ -75,16 +75,16 @@ type RedirectTarget struct {
 	// redirect.
 	RangeProtoSpecified bool
 
-	// Min address used to redirect.
+	// MinIP indicates address used to redirect.
 	MinIP tcpip.Address
 
-	// Max address used to redirect.
+	// MaxIP indicates address used to redirect.
 	MaxIP tcpip.Address
 
-	// Min port used to redirect.
+	// MinPort indicates port used to redirect.
 	MinPort uint16
 
-	// Max port used to redirect.
+	// MaxPort indicates port used to redirect.
 	MaxPort uint16
 }
 
@@ -92,61 +92,76 @@ type RedirectTarget struct {
 // TODO(gvisor.dev/issue/170): Parse headers without copying. The current
 // implementation only works for PREROUTING and calls pkt.Clone(), neither
 // of which should be the case.
-func (rt RedirectTarget) Action(pkt PacketBuffer) (RuleVerdict, int) {
-	newPkt := pkt.Clone()
+func (rt RedirectTarget) Action(pkt *PacketBuffer, ct *ConnTrackTable, hook Hook, gso *GSO, r *Route, address tcpip.Address) (RuleVerdict, int) {
+	// Packet is already manipulated.
+	if pkt.NatDone {
+		return RuleAccept, 0
+	}
 
 	// Set network header.
-	headerView, ok := newPkt.Data.PullUp(header.IPv4MinimumSize)
-	if !ok {
-		return RuleDrop, 0
+	if hook == Prerouting {
+		parseHeaders(pkt)
 	}
-	netHeader := header.IPv4(headerView)
-	newPkt.NetworkHeader = headerView
 
-	hlen := int(netHeader.HeaderLength())
-	tlen := int(netHeader.TotalLength())
-	newPkt.Data.TrimFront(hlen)
-	newPkt.Data.CapLength(tlen - hlen)
+	// Drop the packet if network and transport header are not set.
+	if pkt.NetworkHeader == nil || pkt.TransportHeader == nil {
+		return RuleDrop, 0
+	}
 
-	// TODO(gvisor.dev/issue/170): Change destination address to
-	// loopback or interface address on which the packet was
-	// received.
+	// Change the address to localhost (127.0.0.1) in Output and
+	// to primary address of the incoming interface in Prerouting.
+	switch hook {
+	case Output:
+		rt.MinIP = tcpip.Address([]byte{127, 0, 0, 1})
+		rt.MaxIP = tcpip.Address([]byte{127, 0, 0, 1})
+	case Prerouting:
+		rt.MinIP = address
+		rt.MaxIP = address
+	default:
+		panic("redirect target is supported only on output and prerouting hooks")
+	}
 
 	// TODO(gvisor.dev/issue/170): Check Flags in RedirectTarget if
 	// we need to change dest address (for OUTPUT chain) or ports.
+	netHeader := header.IPv4(pkt.NetworkHeader)
 	switch protocol := netHeader.TransportProtocol(); protocol {
 	case header.UDPProtocolNumber:
-		var udpHeader header.UDP
-		if newPkt.TransportHeader != nil {
-			udpHeader = header.UDP(newPkt.TransportHeader)
-		} else {
-			if pkt.Data.Size() < header.UDPMinimumSize {
-				return RuleDrop, 0
-			}
-			hdr, ok := newPkt.Data.PullUp(header.UDPMinimumSize)
-			if !ok {
-				return RuleDrop, 0
+		udpHeader := header.UDP(pkt.TransportHeader)
+		udpHeader.SetDestinationPort(rt.MinPort)
+
+		// Calculate UDP checksum and set it.
+		if hook == Output {
+			udpHeader.SetChecksum(0)
+			hdr := &pkt.Header
+			length := uint16(pkt.Data.Size()+hdr.UsedLength()) - uint16(netHeader.HeaderLength())
+
+			// Only calculate the checksum if offloading isn't supported.
+			if r.Capabilities()&CapabilityTXChecksumOffload == 0 {
+				xsum := r.PseudoHeaderChecksum(protocol, length)
+				for _, v := range pkt.Data.Views() {
+					xsum = header.Checksum(v, xsum)
+				}
+				udpHeader.SetChecksum(0)
+				udpHeader.SetChecksum(^udpHeader.CalculateChecksum(xsum))
 			}
-			udpHeader = header.UDP(hdr)
 		}
-		udpHeader.SetDestinationPort(rt.MinPort)
+		// Change destination address.
+		netHeader.SetDestinationAddress(rt.MinIP)
+		netHeader.SetChecksum(0)
+		netHeader.SetChecksum(^netHeader.CalculateChecksum())
+		pkt.NatDone = true
 	case header.TCPProtocolNumber:
-		var tcpHeader header.TCP
-		if newPkt.TransportHeader != nil {
-			tcpHeader = header.TCP(newPkt.TransportHeader)
-		} else {
-			if pkt.Data.Size() < header.TCPMinimumSize {
-				return RuleDrop, 0
-			}
-			hdr, ok := newPkt.Data.PullUp(header.TCPMinimumSize)
-			if !ok {
-				return RuleDrop, 0
-			}
-			tcpHeader = header.TCP(hdr)
+		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.connTrackForPacket(pkt, hook, true); conn != nil {
+			ct.SetNatInfo(pkt, rt, hook)
+			ct.HandlePacket(pkt, hook, gso, r)
 		}
-		// TODO(gvisor.dev/issue/170): Need to recompute checksum
-		// and implement nat connection tracking to support TCP.
-		tcpHeader.SetDestinationPort(rt.MinPort)
 	default:
 		return RuleDrop, 0
 	}
diff --git a/pkg/tcpip/stack/iptables_types.go b/pkg/tcpip/stack/iptables_types.go
index 2ffb55f2a..1bb0ba1bd 100644
--- a/pkg/tcpip/stack/iptables_types.go
+++ b/pkg/tcpip/stack/iptables_types.go
@@ -82,6 +82,8 @@ type IPTables struct {
 	// list is the order in which each table should be visited for that
 	// hook.
 	Priorities map[Hook][]string
+
+	connections ConnTrackTable
 }
 
 // A Table defines a set of chains and hooks into the network stack. It is
@@ -176,5 +178,5 @@ 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(packet PacketBuffer) (RuleVerdict, int)
+	Action(packet *PacketBuffer, connections *ConnTrackTable, hook Hook, gso *GSO, r *Route, address tcpip.Address) (RuleVerdict, int)
 }
diff --git a/pkg/tcpip/stack/nic.go b/pkg/tcpip/stack/nic.go
index 7b54919bb..8f4c1fe42 100644
--- a/pkg/tcpip/stack/nic.go
+++ b/pkg/tcpip/stack/nic.go
@@ -1230,8 +1230,10 @@ func (n *NIC) DeliverNetworkPacket(linkEP LinkEndpoint, remote, local tcpip.Link
 
 	// TODO(gvisor.dev/issue/170): Not supporting iptables for IPv6 yet.
 	if protocol == header.IPv4ProtocolNumber {
+		// iptables filtering.
 		ipt := n.stack.IPTables()
-		if ok := ipt.Check(Prerouting, pkt); !ok {
+		address := n.primaryAddress(protocol)
+		if ok := ipt.Check(Prerouting, &pkt, nil, nil, address.Address); !ok {
 			// iptables is telling us to drop the packet.
 			return
 		}
diff --git a/pkg/tcpip/stack/packet_buffer.go b/pkg/tcpip/stack/packet_buffer.go
index 9ff80ab24..926df4d7b 100644
--- a/pkg/tcpip/stack/packet_buffer.go
+++ b/pkg/tcpip/stack/packet_buffer.go
@@ -72,6 +72,10 @@ type PacketBuffer struct {
 	EgressRoute           *Route
 	GSOOptions            *GSO
 	NetworkProtocolNumber tcpip.NetworkProtocolNumber
+
+	// NatDone indicates if the packet has been manipulated as per NAT
+	// iptables rule.
+	NatDone bool
 }
 
 // Clone makes a copy of pk. It clones the Data field, which creates a new
diff --git a/pkg/tcpip/stack/route.go b/pkg/tcpip/stack/route.go
index 53148dc03..150297ab9 100644
--- a/pkg/tcpip/stack/route.go
+++ b/pkg/tcpip/stack/route.go
@@ -261,3 +261,16 @@ func (r *Route) MakeLoopedRoute() Route {
 func (r *Route) Stack() *Stack {
 	return r.ref.stack()
 }
+
+// ReverseRoute returns new route with given source and destination address.
+func (r *Route) ReverseRoute(src tcpip.Address, dst tcpip.Address) Route {
+	return Route{
+		NetProto:          r.NetProto,
+		LocalAddress:      dst,
+		LocalLinkAddress:  r.RemoteLinkAddress,
+		RemoteAddress:     src,
+		RemoteLinkAddress: r.LocalLinkAddress,
+		ref:               r.ref,
+		Loop:              r.Loop,
+	}
+}
diff --git a/pkg/tcpip/stack/stack.go b/pkg/tcpip/stack/stack.go
index 4a2dc3dc6..e33fae4eb 100644
--- a/pkg/tcpip/stack/stack.go
+++ b/pkg/tcpip/stack/stack.go
@@ -1885,3 +1885,22 @@ func generateRandInt64() int64 {
 	}
 	return v
 }
+
+// FindNetworkEndpoint returns the network endpoint for the given address.
+func (s *Stack) FindNetworkEndpoint(netProto tcpip.NetworkProtocolNumber, address tcpip.Address) (NetworkEndpoint, *tcpip.Error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	for _, nic := range s.nics {
+		id := NetworkEndpointID{address}
+
+		if ref, ok := nic.mu.endpoints[id]; ok {
+			nic.mu.RLock()
+			defer nic.mu.RUnlock()
+
+			// An endpoint with this id exists, check if it can be used and return it.
+			return ref.ep, nil
+		}
+	}
+	return nil, tcpip.ErrBadAddress
+}
diff --git a/pkg/tcpip/transport/tcp/BUILD b/pkg/tcpip/transport/tcp/BUILD
index f2aa69069..f38eb6833 100644
--- a/pkg/tcpip/transport/tcp/BUILD
+++ b/pkg/tcpip/transport/tcp/BUILD
@@ -115,7 +115,7 @@ go_test(
     size = "small",
     srcs = ["rcv_test.go"],
     deps = [
-        ":tcp",
+        "//pkg/tcpip/header",
         "//pkg/tcpip/seqnum",
     ],
 )
diff --git a/pkg/tcpip/transport/tcp/rcv.go b/pkg/tcpip/transport/tcp/rcv.go
index a4b73b588..6fe97fefd 100644
--- a/pkg/tcpip/transport/tcp/rcv.go
+++ b/pkg/tcpip/transport/tcp/rcv.go
@@ -70,24 +70,7 @@ func newReceiver(ep *endpoint, irs seqnum.Value, rcvWnd seqnum.Size, rcvWndScale
 // acceptable checks if the segment sequence number range is acceptable
 // according to the table on page 26 of RFC 793.
 func (r *receiver) acceptable(segSeq seqnum.Value, segLen seqnum.Size) bool {
-	return Acceptable(segSeq, segLen, r.rcvNxt, r.rcvAcc)
-}
-
-// Acceptable checks if a segment that starts at segSeq and has length segLen is
-// "acceptable" for arriving in a receive window that starts at rcvNxt and ends
-// before rcvAcc, according to the table on page 26 and 69 of RFC 793.
-func Acceptable(segSeq seqnum.Value, segLen seqnum.Size, rcvNxt, rcvAcc seqnum.Value) bool {
-	if rcvNxt == rcvAcc {
-		return segLen == 0 && segSeq == rcvNxt
-	}
-	if segLen == 0 {
-		// rcvWnd is incremented by 1 because that is Linux's behavior despite the
-		// RFC.
-		return segSeq.InRange(rcvNxt, rcvAcc.Add(1))
-	}
-	// Page 70 of RFC 793 allows packets that can be made "acceptable" by trimming
-	// the payload, so we'll accept any payload that overlaps the receieve window.
-	return rcvNxt.LessThan(segSeq.Add(segLen)) && segSeq.LessThan(rcvAcc)
+	return header.Acceptable(segSeq, segLen, r.rcvNxt, r.rcvAcc)
 }
 
 // getSendParams returns the parameters needed by the sender when building
diff --git a/pkg/tcpip/transport/tcp/rcv_test.go b/pkg/tcpip/transport/tcp/rcv_test.go
index dc02729ce..c9eeff935 100644
--- a/pkg/tcpip/transport/tcp/rcv_test.go
+++ b/pkg/tcpip/transport/tcp/rcv_test.go
@@ -17,8 +17,8 @@ package rcv_test
 import (
 	"testing"
 
+	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/tcpip/seqnum"
-	"gvisor.dev/gvisor/pkg/tcpip/transport/tcp"
 )
 
 func TestAcceptable(t *testing.T) {
@@ -67,8 +67,8 @@ func TestAcceptable(t *testing.T) {
 		{105, 2, 108, 108, false},
 		{105, 2, 109, 109, false},
 	} {
-		if got := tcp.Acceptable(tt.segSeq, tt.segLen, tt.rcvNxt, tt.rcvAcc); got != tt.want {
-			t.Errorf("tcp.Acceptable(%d, %d, %d, %d) = %t, want %t", tt.segSeq, tt.segLen, tt.rcvNxt, tt.rcvAcc, got, tt.want)
+		if got := header.Acceptable(tt.segSeq, tt.segLen, tt.rcvNxt, tt.rcvAcc); got != tt.want {
+			t.Errorf("header.Acceptable(%d, %d, %d, %d) = %t, want %t", tt.segSeq, tt.segLen, tt.rcvNxt, tt.rcvAcc, got, tt.want)
 		}
 	}
 }
diff --git a/pkg/tcpip/transport/tcpconntrack/BUILD b/pkg/tcpip/transport/tcpconntrack/BUILD
index 2025ff757..3ad6994a7 100644
--- a/pkg/tcpip/transport/tcpconntrack/BUILD
+++ b/pkg/tcpip/transport/tcpconntrack/BUILD
@@ -9,7 +9,6 @@ go_library(
     deps = [
         "//pkg/tcpip/header",
         "//pkg/tcpip/seqnum",
-        "//pkg/tcpip/transport/tcp",
     ],
 )
 
diff --git a/pkg/tcpip/transport/tcpconntrack/tcp_conntrack.go b/pkg/tcpip/transport/tcpconntrack/tcp_conntrack.go
index 30d05200f..12bc1b5b5 100644
--- a/pkg/tcpip/transport/tcpconntrack/tcp_conntrack.go
+++ b/pkg/tcpip/transport/tcpconntrack/tcp_conntrack.go
@@ -20,7 +20,6 @@ package tcpconntrack
 import (
 	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/tcpip/seqnum"
-	"gvisor.dev/gvisor/pkg/tcpip/transport/tcp"
 )
 
 // Result is returned when the state of a TCB is updated in response to an
@@ -312,7 +311,7 @@ type stream struct {
 // the window is zero, if it's a packet with no payload and sequence number
 // equal to una.
 func (s *stream) acceptable(segSeq seqnum.Value, segLen seqnum.Size) bool {
-	return tcp.Acceptable(segSeq, segLen, s.una, s.end)
+	return header.Acceptable(segSeq, segLen, s.una, s.end)
 }
 
 // closed determines if the stream has already been closed. This happens when
@@ -338,3 +337,16 @@ func logicalLen(tcp header.TCP) seqnum.Size {
 	}
 	return l
 }
+
+// IsEmpty returns true if tcb is not initialized.
+func (t *TCB) IsEmpty() bool {
+	if t.inbound != (stream{}) || t.outbound != (stream{}) {
+		return false
+	}
+
+	if t.firstFin != nil || t.state != ResultDrop {
+		return false
+	}
+
+	return true
+}