Merge pull request #1943 from kevinGC:ipt-filter-ip

PiperOrigin-RevId: 301197007

9 files changed, 486 insertions, 12 deletions

diff --git a/pkg/abi/linux/netfilter.go b/pkg/abi/linux/netfilter.go
index e20c625f7..80dc09aa9 100644
--- a/pkg/abi/linux/netfilter.go
+++ b/pkg/abi/linux/netfilter.go
@@ -158,10 +158,32 @@ type IPTIP struct {
 	// Flags define matching behavior for the IP header.
 	Flags uint8
 
-	// InverseFlags invert the meaning of fields in struct IPTIP.
+	// InverseFlags invert the meaning of fields in struct IPTIP. See the
+	// IPT_INV_* flags.
 	InverseFlags uint8
 }
 
+// Flags in IPTIP.InverseFlags. Corresponding constants are in
+// include/uapi/linux/netfilter_ipv4/ip_tables.h.
+const (
+	// Invert the meaning of InputInterface.
+	IPT_INV_VIA_IN = 0x01
+	// Invert the meaning of OutputInterface.
+	IPT_INV_VIA_OUT = 0x02
+	// Unclear what this is, as no references to it exist in the kernel.
+	IPT_INV_TOS = 0x04
+	// Invert the meaning of Src.
+	IPT_INV_SRCIP = 0x08
+	// Invert the meaning of Dst.
+	IPT_INV_DSTIP = 0x10
+	// Invert the meaning of the IPT_F_FRAG flag.
+	IPT_INV_FRAG = 0x20
+	// Invert the meaning of the Protocol field.
+	IPT_INV_PROTO = 0x40
+	// Enable all flags.
+	IPT_INV_MASK = 0x7F
+)
+
 // SizeOfIPTIP is the size of an IPTIP.
 const SizeOfIPTIP = 84
 
diff --git a/pkg/sentry/socket/netfilter/netfilter.go b/pkg/sentry/socket/netfilter/netfilter.go
index e57ef02a1..b5b9be46f 100644
--- a/pkg/sentry/socket/netfilter/netfilter.go
+++ b/pkg/sentry/socket/netfilter/netfilter.go
@@ -703,25 +703,34 @@ func filterFromIPTIP(iptip linux.IPTIP) (iptables.IPHeaderFilter, error) {
 	if containsUnsupportedFields(iptip) {
 		return iptables.IPHeaderFilter{}, fmt.Errorf("unsupported fields in struct iptip: %+v", iptip)
 	}
+	if len(iptip.Dst) != header.IPv4AddressSize || len(iptip.DstMask) != header.IPv4AddressSize {
+		return iptables.IPHeaderFilter{}, fmt.Errorf("incorrect length of destination (%d) and/or destination mask (%d) fields", len(iptip.Dst), len(iptip.DstMask))
+	}
 	return iptables.IPHeaderFilter{
-		Protocol: tcpip.TransportProtocolNumber(iptip.Protocol),
+		Protocol:  tcpip.TransportProtocolNumber(iptip.Protocol),
+		Dst:       tcpip.Address(iptip.Dst[:]),
+		DstMask:   tcpip.Address(iptip.DstMask[:]),
+		DstInvert: iptip.InverseFlags&linux.IPT_INV_DSTIP != 0,
 	}, nil
 }
 
 func containsUnsupportedFields(iptip linux.IPTIP) bool {
-	// Currently we check that everything except protocol is zeroed.
+	// The following features are supported:
+	// - Protocol
+	// - Dst and DstMask
+	// - The inverse destination IP check flag
 	var emptyInetAddr = linux.InetAddr{}
 	var emptyInterface = [linux.IFNAMSIZ]byte{}
-	return iptip.Dst != emptyInetAddr ||
-		iptip.Src != emptyInetAddr ||
+	// Disable any supported inverse flags.
+	inverseMask := uint8(linux.IPT_INV_DSTIP)
+	return iptip.Src != emptyInetAddr ||
 		iptip.SrcMask != emptyInetAddr ||
-		iptip.DstMask != emptyInetAddr ||
 		iptip.InputInterface != emptyInterface ||
 		iptip.OutputInterface != emptyInterface ||
 		iptip.InputInterfaceMask != emptyInterface ||
 		iptip.OutputInterfaceMask != emptyInterface ||
 		iptip.Flags != 0 ||
-		iptip.InverseFlags != 0
+		iptip.InverseFlags&^inverseMask != 0
 }
 
 func validUnderflow(rule iptables.Rule) bool {
diff --git a/pkg/tcpip/iptables/iptables.go b/pkg/tcpip/iptables/iptables.go
index 2ab9e0675..d30571c74 100644
--- a/pkg/tcpip/iptables/iptables.go
+++ b/pkg/tcpip/iptables/iptables.go
@@ -267,9 +267,8 @@ func (it *IPTables) checkRule(hook Hook, pkt tcpip.PacketBuffer, table Table, ru
 		pkt.NetworkHeader = pkt.Data.First()
 	}
 
-	// First check whether the packet matches the IP header filter.
-	// TODO(gvisor.dev/issue/170): Support other fields of the filter.
-	if rule.Filter.Protocol != 0 && rule.Filter.Protocol != header.IPv4(pkt.NetworkHeader).TransportProtocol() {
+	// Check whether the packet matches the IP header filter.
+	if !filterMatch(rule.Filter, header.IPv4(pkt.NetworkHeader)) {
 		// Continue on to the next rule.
 		return RuleJump, ruleIdx + 1
 	}
@@ -290,3 +289,26 @@ func (it *IPTables) checkRule(hook Hook, pkt tcpip.PacketBuffer, table Table, ru
 	// All the matchers matched, so run the target.
 	return rule.Target.Action(pkt)
 }
+
+func filterMatch(filter IPHeaderFilter, hdr header.IPv4) bool {
+	// TODO(gvisor.dev/issue/170): Support other fields of the filter.
+	// Check the transport protocol.
+	if filter.Protocol != 0 && filter.Protocol != hdr.TransportProtocol() {
+		return false
+	}
+
+	// Check the destination IP.
+	dest := hdr.DestinationAddress()
+	matches := true
+	for i := range filter.Dst {
+		if dest[i]&filter.DstMask[i] != filter.Dst[i] {
+			matches = false
+			break
+		}
+	}
+	if matches == filter.DstInvert {
+		return false
+	}
+
+	return true
+}
diff --git a/pkg/tcpip/iptables/types.go b/pkg/tcpip/iptables/types.go
index 7d032fd23..e7fcf6bff 100644
--- a/pkg/tcpip/iptables/types.go
+++ b/pkg/tcpip/iptables/types.go
@@ -144,6 +144,18 @@ type Rule struct {
 type IPHeaderFilter struct {
 	// Protocol matches the transport protocol.
 	Protocol tcpip.TransportProtocolNumber
+
+	// Dst matches the destination IP address.
+	Dst tcpip.Address
+
+	// DstMask masks bits of the destination IP address when comparing with
+	// Dst.
+	DstMask tcpip.Address
+
+	// DstInvert inverts the meaning of the destination IP check, i.e. when
+	// true the filter will match packets that fail the destination
+	// comparison.
+	DstInvert bool
 }
 
 // A Matcher is the interface for matching packets.
diff --git a/test/iptables/filter_input.go b/test/iptables/filter_input.go
index 141d20fbb..05647de33 100644
--- a/test/iptables/filter_input.go
+++ b/test/iptables/filter_input.go
@@ -47,6 +47,8 @@ func init() {
 	RegisterTestCase(FilterInputJumpReturnDrop{})
 	RegisterTestCase(FilterInputJumpBuiltin{})
 	RegisterTestCase(FilterInputJumpTwice{})
+	RegisterTestCase(FilterInputDestination{})
+	RegisterTestCase(FilterInputInvertDestination{})
 }
 
 // FilterInputDropUDP tests that we can drop UDP traffic.
@@ -596,3 +598,66 @@ func (FilterInputJumpTwice) ContainerAction(ip net.IP) error {
 func (FilterInputJumpTwice) LocalAction(ip net.IP) error {
 	return sendUDPLoop(ip, acceptPort, sendloopDuration)
 }
+
+// FilterInputDestination verifies that we can filter packets via `-d
+// <ipaddr>`.
+type FilterInputDestination struct{}
+
+// Name implements TestCase.Name.
+func (FilterInputDestination) Name() string {
+	return "FilterInputDestination"
+}
+
+// ContainerAction implements TestCase.ContainerAction.
+func (FilterInputDestination) ContainerAction(ip net.IP) error {
+	addrs, err := localAddrs()
+	if err != nil {
+		return err
+	}
+
+	// Make INPUT's default action DROP, then ACCEPT all packets bound for
+	// this machine.
+	rules := [][]string{{"-P", "INPUT", "DROP"}}
+	for _, addr := range addrs {
+		rules = append(rules, []string{"-A", "INPUT", "-d", addr, "-j", "ACCEPT"})
+	}
+	if err := filterTableRules(rules); err != nil {
+		return err
+	}
+
+	return listenUDP(acceptPort, sendloopDuration)
+}
+
+// LocalAction implements TestCase.LocalAction.
+func (FilterInputDestination) LocalAction(ip net.IP) error {
+	return sendUDPLoop(ip, acceptPort, sendloopDuration)
+}
+
+// FilterInputInvertDestination verifies that we can filter packets via `! -d
+// <ipaddr>`.
+type FilterInputInvertDestination struct{}
+
+// Name implements TestCase.Name.
+func (FilterInputInvertDestination) Name() string {
+	return "FilterInputInvertDestination"
+}
+
+// ContainerAction implements TestCase.ContainerAction.
+func (FilterInputInvertDestination) ContainerAction(ip net.IP) error {
+	// Make INPUT's default action DROP, then ACCEPT all packets not bound
+	// for 127.0.0.1.
+	rules := [][]string{
+		{"-P", "INPUT", "DROP"},
+		{"-A", "INPUT", "!", "-d", localIP, "-j", "ACCEPT"},
+	}
+	if err := filterTableRules(rules); err != nil {
+		return err
+	}
+
+	return listenUDP(acceptPort, sendloopDuration)
+}
+
+// LocalAction implements TestCase.LocalAction.
+func (FilterInputInvertDestination) LocalAction(ip net.IP) error {
+	return sendUDPLoop(ip, acceptPort, sendloopDuration)
+}
diff --git a/test/iptables/filter_output.go b/test/iptables/filter_output.go
index 1314a5a92..4582d514c 100644
--- a/test/iptables/filter_output.go
+++ b/test/iptables/filter_output.go
@@ -22,6 +22,8 @@ import (
 func init() {
 	RegisterTestCase(FilterOutputDropTCPDestPort{})
 	RegisterTestCase(FilterOutputDropTCPSrcPort{})
+	RegisterTestCase(FilterOutputDestination{})
+	RegisterTestCase(FilterOutputInvertDestination{})
 }
 
 // FilterOutputDropTCPDestPort tests that connections are not accepted on
@@ -87,3 +89,57 @@ func (FilterOutputDropTCPSrcPort) LocalAction(ip net.IP) error {
 
 	return nil
 }
+
+// FilterOutputDestination tests that we can selectively allow packets to
+// certain destinations.
+type FilterOutputDestination struct{}
+
+// Name implements TestCase.Name.
+func (FilterOutputDestination) Name() string {
+	return "FilterOutputDestination"
+}
+
+// ContainerAction implements TestCase.ContainerAction.
+func (FilterOutputDestination) ContainerAction(ip net.IP) error {
+	rules := [][]string{
+		{"-A", "OUTPUT", "-d", ip.String(), "-j", "ACCEPT"},
+		{"-P", "OUTPUT", "DROP"},
+	}
+	if err := filterTableRules(rules); err != nil {
+		return err
+	}
+
+	return sendUDPLoop(ip, acceptPort, sendloopDuration)
+}
+
+// LocalAction implements TestCase.LocalAction.
+func (FilterOutputDestination) LocalAction(ip net.IP) error {
+	return listenUDP(acceptPort, sendloopDuration)
+}
+
+// FilterOutputInvertDestination tests that we can selectively allow packets
+// not headed for a particular destination.
+type FilterOutputInvertDestination struct{}
+
+// Name implements TestCase.Name.
+func (FilterOutputInvertDestination) Name() string {
+	return "FilterOutputInvertDestination"
+}
+
+// ContainerAction implements TestCase.ContainerAction.
+func (FilterOutputInvertDestination) ContainerAction(ip net.IP) error {
+	rules := [][]string{
+		{"-A", "OUTPUT", "!", "-d", localIP, "-j", "ACCEPT"},
+		{"-P", "OUTPUT", "DROP"},
+	}
+	if err := filterTableRules(rules); err != nil {
+		return err
+	}
+
+	return sendUDPLoop(ip, acceptPort, sendloopDuration)
+}
+
+// LocalAction implements TestCase.LocalAction.
+func (FilterOutputInvertDestination) LocalAction(ip net.IP) error {
+	return listenUDP(acceptPort, sendloopDuration)
+}
diff --git a/test/iptables/iptables_test.go b/test/iptables/iptables_test.go
index 56ba78107..73ba8b447 100644
--- a/test/iptables/iptables_test.go
+++ b/test/iptables/iptables_test.go
@@ -303,3 +303,69 @@ func TestJumpTwice(t *testing.T) {
 		t.Fatal(err)
 	}
 }
+
+func TestInputDestination(t *testing.T) {
+	if err := singleTest(FilterInputDestination{}); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestInputInvertDestination(t *testing.T) {
+	if err := singleTest(FilterInputInvertDestination{}); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestOutputDestination(t *testing.T) {
+	if err := singleTest(FilterOutputDestination{}); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestOutputInvertDestination(t *testing.T) {
+	if err := singleTest(FilterOutputInvertDestination{}); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestNATOutRedirectIP(t *testing.T) {
+	if err := singleTest(NATOutRedirectIP{}); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestNATOutDontRedirectIP(t *testing.T) {
+	if err := singleTest(NATOutDontRedirectIP{}); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestNATOutRedirectInvert(t *testing.T) {
+	if err := singleTest(NATOutRedirectInvert{}); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestNATPreRedirectIP(t *testing.T) {
+	if err := singleTest(NATPreRedirectIP{}); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestNATPreDontRedirectIP(t *testing.T) {
+	if err := singleTest(NATPreDontRedirectIP{}); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestNATPreRedirectInvert(t *testing.T) {
+	if err := singleTest(NATPreRedirectInvert{}); err != nil {
+		t.Fatal(err)
+	}
+}
+
+func TestNATRedirectRequiresProtocol(t *testing.T) {
+	if err := singleTest(NATRedirectRequiresProtocol{}); err != nil {
+		t.Fatal(err)
+	}
+}
diff --git a/test/iptables/iptables_util.go b/test/iptables/iptables_util.go
index 1f8dac4f1..e8ae65c5a 100644
--- a/test/iptables/iptables_util.go
+++ b/test/iptables/iptables_util.go
@@ -24,6 +24,7 @@ import (
 )
 
 const iptablesBinary = "iptables"
+const localIP = "127.0.0.1"
 
 // filterTable calls `iptables -t filter` with the given args.
 func filterTable(args ...string) error {
@@ -46,8 +47,17 @@ func tableCmd(table string, args []string) error {
 
 // filterTableRules is like filterTable, but runs multiple iptables commands.
 func filterTableRules(argsList [][]string) error {
+	return tableRules("filter", argsList)
+}
+
+// natTableRules is like natTable, but runs multiple iptables commands.
+func natTableRules(argsList [][]string) error {
+	return tableRules("nat", argsList)
+}
+
+func tableRules(table string, argsList [][]string) error {
 	for _, args := range argsList {
-		if err := filterTable(args...); err != nil {
+		if err := tableCmd(table, args); err != nil {
 			return err
 		}
 	}
@@ -146,3 +156,16 @@ func connectTCP(ip net.IP, port int, timeout time.Duration) error {
 
 	return nil
 }
+
+// localAddrs returns a list of local network interface addresses.
+func localAddrs() ([]string, error) {
+	addrs, err := net.InterfaceAddrs()
+	if err != nil {
+		return nil, err
+	}
+	addrStrs := make([]string, 0, len(addrs))
+	for _, addr := range addrs {
+		addrStrs = append(addrStrs, addr.String())
+	}
+	return addrStrs, nil
+}
diff --git a/test/iptables/nat.go b/test/iptables/nat.go
index 6ca6b46ca..40096901c 100644
--- a/test/iptables/nat.go
+++ b/test/iptables/nat.go
@@ -15,8 +15,10 @@
 package iptables
 
 import (
+	"errors"
 	"fmt"
 	"net"
+	"time"
 )
 
 const (
@@ -28,6 +30,13 @@ func init() {
 	RegisterTestCase(NATRedirectTCPPort{})
 	RegisterTestCase(NATDropUDP{})
 	RegisterTestCase(NATAcceptAll{})
+	RegisterTestCase(NATPreRedirectIP{})
+	RegisterTestCase(NATPreDontRedirectIP{})
+	RegisterTestCase(NATPreRedirectInvert{})
+	RegisterTestCase(NATOutRedirectIP{})
+	RegisterTestCase(NATOutDontRedirectIP{})
+	RegisterTestCase(NATOutRedirectInvert{})
+	RegisterTestCase(NATRedirectRequiresProtocol{})
 }
 
 // NATRedirectUDPPort tests that packets are redirected to different port.
@@ -79,7 +88,8 @@ func (NATRedirectTCPPort) LocalAction(ip net.IP) error {
 	return connectTCP(ip, dropPort, sendloopDuration)
 }
 
-// NATDropUDP tests that packets are not received in ports other than redirect port.
+// NATDropUDP tests that packets are not received in ports other than redirect
+// port.
 type NATDropUDP struct{}
 
 // Name implements TestCase.Name.
@@ -130,3 +140,192 @@ func (NATAcceptAll) ContainerAction(ip net.IP) error {
 func (NATAcceptAll) LocalAction(ip net.IP) error {
 	return sendUDPLoop(ip, acceptPort, sendloopDuration)
 }
+
+// NATOutRedirectIP uses iptables to select packets based on destination IP and
+// redirects them.
+type NATOutRedirectIP struct{}
+
+// Name implements TestCase.Name.
+func (NATOutRedirectIP) Name() string {
+	return "NATOutRedirectIP"
+}
+
+// ContainerAction implements TestCase.ContainerAction.
+func (NATOutRedirectIP) ContainerAction(ip net.IP) error {
+	// Redirect OUTPUT packets to a listening localhost port.
+	dest := net.IP([]byte{200, 0, 0, 2})
+	return loopbackTest(dest, "-A", "OUTPUT", "-d", dest.String(), "-p", "udp", "-j", "REDIRECT", "--to-port", fmt.Sprintf("%d", acceptPort))
+}
+
+// LocalAction implements TestCase.LocalAction.
+func (NATOutRedirectIP) LocalAction(ip net.IP) error {
+	// No-op.
+	return nil
+}
+
+// NATOutDontRedirectIP tests that iptables matching with "-d" does not match
+// packets it shouldn't.
+type NATOutDontRedirectIP struct{}
+
+// Name implements TestCase.Name.
+func (NATOutDontRedirectIP) Name() string {
+	return "NATOutDontRedirectIP"
+}
+
+// ContainerAction implements TestCase.ContainerAction.
+func (NATOutDontRedirectIP) ContainerAction(ip net.IP) error {
+	if err := natTable("-A", "OUTPUT", "-d", localIP, "-p", "udp", "-j", "REDIRECT", "--to-port", fmt.Sprintf("%d", dropPort)); err != nil {
+		return err
+	}
+	return sendUDPLoop(ip, acceptPort, sendloopDuration)
+}
+
+// LocalAction implements TestCase.LocalAction.
+func (NATOutDontRedirectIP) LocalAction(ip net.IP) error {
+	return listenUDP(acceptPort, sendloopDuration)
+}
+
+// NATOutRedirectInvert tests that iptables can match with "! -d".
+type NATOutRedirectInvert struct{}
+
+// Name implements TestCase.Name.
+func (NATOutRedirectInvert) Name() string {
+	return "NATOutRedirectInvert"
+}
+
+// ContainerAction implements TestCase.ContainerAction.
+func (NATOutRedirectInvert) ContainerAction(ip net.IP) error {
+	// Redirect OUTPUT packets to a listening localhost port.
+	dest := []byte{200, 0, 0, 3}
+	destStr := "200.0.0.2"
+	return loopbackTest(dest, "-A", "OUTPUT", "!", "-d", destStr, "-p", "udp", "-j", "REDIRECT", "--to-port", fmt.Sprintf("%d", acceptPort))
+}
+
+// LocalAction implements TestCase.LocalAction.
+func (NATOutRedirectInvert) LocalAction(ip net.IP) error {
+	// No-op.
+	return nil
+}
+
+// NATPreRedirectIP tests that we can use iptables to select packets based on
+// destination IP and redirect them.
+type NATPreRedirectIP struct{}
+
+// Name implements TestCase.Name.
+func (NATPreRedirectIP) Name() string {
+	return "NATPreRedirectIP"
+}
+
+// ContainerAction implements TestCase.ContainerAction.
+func (NATPreRedirectIP) ContainerAction(ip net.IP) error {
+	addrs, err := localAddrs()
+	if err != nil {
+		return err
+	}
+
+	var rules [][]string
+	for _, addr := range addrs {
+		rules = append(rules, []string{"-A", "PREROUTING", "-p", "udp", "-d", addr, "-j", "REDIRECT", "--to-ports", fmt.Sprintf("%d", acceptPort)})
+	}
+	if err := natTableRules(rules); err != nil {
+		return err
+	}
+	return listenUDP(acceptPort, sendloopDuration)
+}
+
+// LocalAction implements TestCase.LocalAction.
+func (NATPreRedirectIP) LocalAction(ip net.IP) error {
+	return sendUDPLoop(ip, dropPort, sendloopDuration)
+}
+
+// NATPreDontRedirectIP tests that iptables matching with "-d" does not match
+// packets it shouldn't.
+type NATPreDontRedirectIP struct{}
+
+// Name implements TestCase.Name.
+func (NATPreDontRedirectIP) Name() string {
+	return "NATPreDontRedirectIP"
+}
+
+// ContainerAction implements TestCase.ContainerAction.
+func (NATPreDontRedirectIP) ContainerAction(ip net.IP) error {
+	if err := natTable("-A", "PREROUTING", "-p", "udp", "-d", localIP, "-j", "REDIRECT", "--to-ports", fmt.Sprintf("%d", dropPort)); err != nil {
+		return err
+	}
+	return listenUDP(acceptPort, sendloopDuration)
+}
+
+// LocalAction implements TestCase.LocalAction.
+func (NATPreDontRedirectIP) LocalAction(ip net.IP) error {
+	return sendUDPLoop(ip, acceptPort, sendloopDuration)
+}
+
+// NATPreRedirectInvert tests that iptables can match with "! -d".
+type NATPreRedirectInvert struct{}
+
+// Name implements TestCase.Name.
+func (NATPreRedirectInvert) Name() string {
+	return "NATPreRedirectInvert"
+}
+
+// ContainerAction implements TestCase.ContainerAction.
+func (NATPreRedirectInvert) ContainerAction(ip net.IP) error {
+	if err := natTable("-A", "PREROUTING", "-p", "udp", "!", "-d", localIP, "-j", "REDIRECT", "--to-ports", fmt.Sprintf("%d", acceptPort)); err != nil {
+		return err
+	}
+	return listenUDP(acceptPort, sendloopDuration)
+}
+
+// LocalAction implements TestCase.LocalAction.
+func (NATPreRedirectInvert) LocalAction(ip net.IP) error {
+	return sendUDPLoop(ip, dropPort, sendloopDuration)
+}
+
+// NATRedirectRequiresProtocol tests that use of the --to-ports flag requires a
+// protocol to be specified with -p.
+type NATRedirectRequiresProtocol struct{}
+
+// Name implements TestCase.Name.
+func (NATRedirectRequiresProtocol) Name() string {
+	return "NATRedirectRequiresProtocol"
+}
+
+// ContainerAction implements TestCase.ContainerAction.
+func (NATRedirectRequiresProtocol) ContainerAction(ip net.IP) error {
+	if err := natTable("-A", "PREROUTING", "-d", localIP, "-j", "REDIRECT", "--to-ports", fmt.Sprintf("%d", acceptPort)); err == nil {
+		return errors.New("expected an error using REDIRECT --to-ports without a protocol")
+	}
+	return nil
+}
+
+// LocalAction implements TestCase.LocalAction.
+func (NATRedirectRequiresProtocol) LocalAction(ip net.IP) error {
+	// No-op.
+	return nil
+}
+
+// loopbackTests runs an iptables rule and ensures that packets sent to
+// dest:dropPort are received by localhost:acceptPort.
+func loopbackTest(dest net.IP, args ...string) error {
+	if err := natTable(args...); err != nil {
+		return err
+	}
+	sendCh := make(chan error)
+	listenCh := make(chan error)
+	go func() {
+		sendCh <- sendUDPLoop(dest, dropPort, sendloopDuration)
+	}()
+	go func() {
+		listenCh <- listenUDP(acceptPort, sendloopDuration)
+	}()
+	select {
+	case err := <-listenCh:
+		if err != nil {
+			return err
+		}
+	case <-time.After(sendloopDuration):
+		return errors.New("timed out")
+	}
+	// sendCh will always take the full sendloop time.
+	return <-sendCh
+}