iptables: refactor to make targets extendable

Like matchers, targets should use a module-like register/lookup system. This
replaces the brittle switch statements we had before.

The only behavior change is supporing IPT_GET_REVISION_TARGET. This makes it
much easier to add IPv6 redirect in the next change.

Updates #3549.

PiperOrigin-RevId: 334469418

12 files changed, 528 insertions, 234 deletions

diff --git a/pkg/abi/linux/netfilter.go b/pkg/abi/linux/netfilter.go
index 1c5b34711..b521144d9 100644
--- a/pkg/abi/linux/netfilter.go
+++ b/pkg/abi/linux/netfilter.go
@@ -265,6 +265,18 @@ type KernelXTEntryMatch struct {
 	Data []byte
 }
 
+// XTGetRevision corresponds to xt_get_revision in
+// include/uapi/linux/netfilter/x_tables.h
+//
+// +marshal
+type XTGetRevision struct {
+	Name     ExtensionName
+	Revision uint8
+}
+
+// SizeOfXTGetRevision is the size of an XTGetRevision.
+const SizeOfXTGetRevision = 30
+
 // XTEntryTarget holds a target for a rule. For example, it can specify that
 // packets matching the rule should DROP, ACCEPT, or use an extension target.
 // iptables-extension(8) has a list of possible targets.
@@ -285,6 +297,13 @@ type XTEntryTarget struct {
 // SizeOfXTEntryTarget is the size of an XTEntryTarget.
 const SizeOfXTEntryTarget = 32
 
+// KernelXTEntryTarget is identical to XTEntryTarget, but contains a
+// variable-length Data field.
+type KernelXTEntryTarget struct {
+	XTEntryTarget
+	Data []byte
+}
+
 // XTStandardTarget is a built-in target, one of ACCEPT, DROP, JUMP, QUEUE,
 // RETURN, or jump. It corresponds to struct xt_standard_target in
 // include/uapi/linux/netfilter/x_tables.h.
@@ -510,6 +529,8 @@ type IPTReplace struct {
 const SizeOfIPTReplace = 96
 
 // ExtensionName holds the name of a netfilter extension.
+//
+// +marshal
 type ExtensionName [XT_EXTENSION_MAXNAMELEN]byte
 
 // String implements fmt.Stringer.
diff --git a/pkg/sentry/socket/netfilter/extensions.go b/pkg/sentry/socket/netfilter/extensions.go
index 0336a32d8..549787955 100644
--- a/pkg/sentry/socket/netfilter/extensions.go
+++ b/pkg/sentry/socket/netfilter/extensions.go
@@ -19,6 +19,8 @@ import (
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/binary"
+	"gvisor.dev/gvisor/pkg/syserr"
+	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
 	"gvisor.dev/gvisor/pkg/usermem"
 )
@@ -37,7 +39,7 @@ type matchMaker interface {
 	// name is the matcher name as stored in the xt_entry_match struct.
 	name() string
 
-	// marshal converts from an stack.Matcher to an ABI struct.
+	// marshal converts from a stack.Matcher to an ABI struct.
 	marshal(matcher stack.Matcher) []byte
 
 	// unmarshal converts from the ABI matcher struct to an
@@ -93,3 +95,71 @@ func unmarshalMatcher(match linux.XTEntryMatch, filter stack.IPHeaderFilter, buf
 	}
 	return matchMaker.unmarshal(buf, filter)
 }
+
+// targetMaker knows how to (un)marshal a target. Once registered,
+// marshalTarget and unmarshalTarget can be used.
+type targetMaker interface {
+	// id uniquely identifies the target.
+	id() stack.TargetID
+
+	// marshal converts from a stack.Target to an ABI struct.
+	marshal(target stack.Target) []byte
+
+	// unmarshal converts from the ABI matcher struct to a stack.Target.
+	unmarshal(buf []byte, filter stack.IPHeaderFilter) (stack.Target, *syserr.Error)
+}
+
+// targetMakers maps the TargetID of supported targets to the targetMaker that
+// marshals and unmarshals it. It is immutable after package initialization.
+var targetMakers = map[stack.TargetID]targetMaker{}
+
+func targetRevision(name string, netProto tcpip.NetworkProtocolNumber, rev uint8) (uint8, bool) {
+	tid := stack.TargetID{
+		Name:            name,
+		NetworkProtocol: netProto,
+		Revision:        rev,
+	}
+	if _, ok := targetMakers[tid]; !ok {
+		return 0, false
+	}
+
+	// Return the highest supported revision unless rev is higher.
+	for _, other := range targetMakers {
+		otherID := other.id()
+		if name == otherID.Name && netProto == otherID.NetworkProtocol && otherID.Revision > rev {
+			rev = uint8(otherID.Revision)
+		}
+	}
+	return rev, true
+}
+
+// registerTargetMaker should be called by target extensions to register them
+// with the netfilter package.
+func registerTargetMaker(tm targetMaker) {
+	if _, ok := targetMakers[tm.id()]; ok {
+		panic(fmt.Sprintf("multiple targets registered with name %q.", tm.id()))
+	}
+	targetMakers[tm.id()] = tm
+}
+
+func marshalTarget(target stack.Target) []byte {
+	targetMaker, ok := targetMakers[target.ID()]
+	if !ok {
+		panic(fmt.Sprintf("unknown target of type %T with id %+v.", target, target.ID()))
+	}
+	return targetMaker.marshal(target)
+}
+
+func unmarshalTarget(target linux.XTEntryTarget, filter stack.IPHeaderFilter, buf []byte) (stack.Target, *syserr.Error) {
+	tid := stack.TargetID{
+		Name:            target.Name.String(),
+		NetworkProtocol: filter.NetworkProtocol(),
+		Revision:        target.Revision,
+	}
+	targetMaker, ok := targetMakers[tid]
+	if !ok {
+		nflog("unsupported target with name %q", target.Name.String())
+		return nil, syserr.ErrInvalidArgument
+	}
+	return targetMaker.unmarshal(buf, filter)
+}
diff --git a/pkg/sentry/socket/netfilter/ipv4.go b/pkg/sentry/socket/netfilter/ipv4.go
index e4c55a100..b560fae0d 100644
--- a/pkg/sentry/socket/netfilter/ipv4.go
+++ b/pkg/sentry/socket/netfilter/ipv4.go
@@ -181,18 +181,23 @@ func modifyEntries4(stk *stack.Stack, optVal []byte, replace *linux.IPTReplace,
 			nflog("entry doesn't have enough room for its target (only %d bytes remain)", len(optVal))
 			return nil, syserr.ErrInvalidArgument
 		}
-		target, err := parseTarget(filter, optVal[:targetSize])
-		if err != nil {
-			nflog("failed to parse target: %v", err)
-			return nil, syserr.ErrInvalidArgument
-		}
-		optVal = optVal[targetSize:]
 
-		table.Rules = append(table.Rules, stack.Rule{
+		rule := stack.Rule{
 			Filter:   filter,
-			Target:   target,
 			Matchers: matchers,
-		})
+		}
+
+		{
+			target, err := parseTarget(filter, optVal[:targetSize], false /* ipv6 */)
+			if err != nil {
+				nflog("failed to parse target: %v", err)
+				return nil, err
+			}
+			rule.Target = target
+		}
+		optVal = optVal[targetSize:]
+
+		table.Rules = append(table.Rules, rule)
 		offsets[offset] = int(entryIdx)
 		offset += uint32(entry.NextOffset)
 
diff --git a/pkg/sentry/socket/netfilter/ipv6.go b/pkg/sentry/socket/netfilter/ipv6.go
index 3b2c1becd..4253f7bf4 100644
--- a/pkg/sentry/socket/netfilter/ipv6.go
+++ b/pkg/sentry/socket/netfilter/ipv6.go
@@ -184,18 +184,23 @@ func modifyEntries6(stk *stack.Stack, optVal []byte, replace *linux.IPTReplace,
 			nflog("entry doesn't have enough room for its target (only %d bytes remain)", len(optVal))
 			return nil, syserr.ErrInvalidArgument
 		}
-		target, err := parseTarget(filter, optVal[:targetSize])
-		if err != nil {
-			nflog("failed to parse target: %v", err)
-			return nil, syserr.ErrInvalidArgument
-		}
-		optVal = optVal[targetSize:]
 
-		table.Rules = append(table.Rules, stack.Rule{
+		rule := stack.Rule{
 			Filter:   filter,
-			Target:   target,
 			Matchers: matchers,
-		})
+		}
+
+		{
+			target, err := parseTarget(filter, optVal[:targetSize], true /* ipv6 */)
+			if err != nil {
+				nflog("failed to parse target: %v", err)
+				return nil, err
+			}
+			rule.Target = target
+		}
+		optVal = optVal[targetSize:]
+
+		table.Rules = append(table.Rules, rule)
 		offsets[offset] = int(entryIdx)
 		offset += uint32(entry.NextOffset)
 
diff --git a/pkg/sentry/socket/netfilter/netfilter.go b/pkg/sentry/socket/netfilter/netfilter.go
index 871ea80ee..94cb80437 100644
--- a/pkg/sentry/socket/netfilter/netfilter.go
+++ b/pkg/sentry/socket/netfilter/netfilter.go
@@ -25,6 +25,7 @@ import (
 	"gvisor.dev/gvisor/pkg/log"
 	"gvisor.dev/gvisor/pkg/sentry/kernel"
 	"gvisor.dev/gvisor/pkg/syserr"
+	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
 	"gvisor.dev/gvisor/pkg/usermem"
 )
@@ -199,7 +200,7 @@ func SetEntries(stk *stack.Stack, optVal []byte, ipv6 bool) *syserr.Error {
 
 	// Check the user chains.
 	for ruleIdx, rule := range table.Rules {
-		if _, ok := rule.Target.(stack.UserChainTarget); !ok {
+		if _, ok := rule.Target.(*stack.UserChainTarget); !ok {
 			continue
 		}
 
@@ -220,7 +221,7 @@ func SetEntries(stk *stack.Stack, optVal []byte, ipv6 bool) *syserr.Error {
 	// Set each jump to point to the appropriate rule. Right now they hold byte
 	// offsets.
 	for ruleIdx, rule := range table.Rules {
-		jump, ok := rule.Target.(JumpTarget)
+		jump, ok := rule.Target.(*JumpTarget)
 		if !ok {
 			continue
 		}
@@ -311,7 +312,7 @@ func validUnderflow(rule stack.Rule, ipv6 bool) bool {
 		return false
 	}
 	switch rule.Target.(type) {
-	case stack.AcceptTarget, stack.DropTarget:
+	case *stack.AcceptTarget, *stack.DropTarget:
 		return true
 	default:
 		return false
@@ -322,7 +323,7 @@ func isUnconditionalAccept(rule stack.Rule, ipv6 bool) bool {
 	if !validUnderflow(rule, ipv6) {
 		return false
 	}
-	_, ok := rule.Target.(stack.AcceptTarget)
+	_, ok := rule.Target.(*stack.AcceptTarget)
 	return ok
 }
 
@@ -341,3 +342,20 @@ func hookFromLinux(hook int) stack.Hook {
 	}
 	panic(fmt.Sprintf("Unknown hook %d does not correspond to a builtin chain", hook))
 }
+
+// TargetRevision returns a linux.XTGetRevision for a given target. It sets
+// Revision to the highest supported value, unless the provided revision number
+// is larger.
+func TargetRevision(t *kernel.Task, revPtr usermem.Addr, netProto tcpip.NetworkProtocolNumber) (linux.XTGetRevision, *syserr.Error) {
+	// Read in the target name and version.
+	var rev linux.XTGetRevision
+	if _, err := rev.CopyIn(t, revPtr); err != nil {
+		return linux.XTGetRevision{}, syserr.FromError(err)
+	}
+	maxSupported, ok := targetRevision(rev.Name.String(), netProto, rev.Revision)
+	if !ok {
+		return linux.XTGetRevision{}, syserr.ErrProtocolNotSupported
+	}
+	rev.Revision = maxSupported
+	return rev, nil
+}
diff --git a/pkg/sentry/socket/netfilter/targets.go b/pkg/sentry/socket/netfilter/targets.go
index 87e41abd8..e3b108e93 100644
--- a/pkg/sentry/socket/netfilter/targets.go
+++ b/pkg/sentry/socket/netfilter/targets.go
@@ -15,255 +15,276 @@
 package netfilter
 
 import (
-	"errors"
 	"fmt"
 
 	"gvisor.dev/gvisor/pkg/abi/linux"
 	"gvisor.dev/gvisor/pkg/binary"
+	"gvisor.dev/gvisor/pkg/syserr"
 	"gvisor.dev/gvisor/pkg/tcpip"
 	"gvisor.dev/gvisor/pkg/tcpip/header"
 	"gvisor.dev/gvisor/pkg/tcpip/stack"
 	"gvisor.dev/gvisor/pkg/usermem"
 )
 
-// errorTargetName is used to mark targets as error targets. Error targets
-// shouldn't be reached - an error has occurred if we fall through to one.
-const errorTargetName = "ERROR"
+func init() {
+	// Standard targets include ACCEPT, DROP, RETURN, and JUMP.
+	registerTargetMaker(&standardTargetMaker{
+		NetworkProtocol: header.IPv4ProtocolNumber,
+	})
+	registerTargetMaker(&standardTargetMaker{
+		NetworkProtocol: header.IPv6ProtocolNumber,
+	})
+
+	// Both user chains and actual errors are represented in iptables by
+	// error targets.
+	registerTargetMaker(&errorTargetMaker{
+		NetworkProtocol: header.IPv4ProtocolNumber,
+	})
+	registerTargetMaker(&errorTargetMaker{
+		NetworkProtocol: header.IPv6ProtocolNumber,
+	})
+
+	registerTargetMaker(&redirectTargetMaker{
+		NetworkProtocol: header.IPv4ProtocolNumber,
+	})
+}
 
-// redirectTargetName is used to mark targets as redirect targets. Redirect
-// targets should be reached for only NAT and Mangle tables. These targets will
-// change the destination port/destination IP for packets.
-const redirectTargetName = "REDIRECT"
+type standardTargetMaker struct {
+	NetworkProtocol tcpip.NetworkProtocolNumber
+}
 
-func marshalTarget(target stack.Target) []byte {
+func (sm *standardTargetMaker) id() stack.TargetID {
+	// Standard targets have the empty string as a name and no revisions.
+	return stack.TargetID{
+		NetworkProtocol: sm.NetworkProtocol,
+	}
+}
+func (*standardTargetMaker) marshal(target stack.Target) []byte {
+	// Translate verdicts the same way as the iptables tool.
+	var verdict int32
 	switch tg := target.(type) {
-	case stack.AcceptTarget:
-		return marshalStandardTarget(stack.RuleAccept)
-	case stack.DropTarget:
-		return marshalStandardTarget(stack.RuleDrop)
-	case stack.ErrorTarget:
-		return marshalErrorTarget(errorTargetName)
-	case stack.UserChainTarget:
-		return marshalErrorTarget(tg.Name)
-	case stack.ReturnTarget:
-		return marshalStandardTarget(stack.RuleReturn)
-	case stack.RedirectTarget:
-		return marshalRedirectTarget(tg)
-	case JumpTarget:
-		return marshalJumpTarget(tg)
+	case *stack.AcceptTarget:
+		verdict = -linux.NF_ACCEPT - 1
+	case *stack.DropTarget:
+		verdict = -linux.NF_DROP - 1
+	case *stack.ReturnTarget:
+		verdict = linux.NF_RETURN
+	case *JumpTarget:
+		verdict = int32(tg.Offset)
 	default:
 		panic(fmt.Errorf("unknown target of type %T", target))
 	}
-}
-
-func marshalStandardTarget(verdict stack.RuleVerdict) []byte {
-	nflog("convert to binary: marshalling standard target")
 
 	// The target's name will be the empty string.
-	target := linux.XTStandardTarget{
+	xt := linux.XTStandardTarget{
 		Target: linux.XTEntryTarget{
 			TargetSize: linux.SizeOfXTStandardTarget,
 		},
-		Verdict: translateFromStandardVerdict(verdict),
+		Verdict: verdict,
 	}
 
 	ret := make([]byte, 0, linux.SizeOfXTStandardTarget)
-	return binary.Marshal(ret, usermem.ByteOrder, target)
+	return binary.Marshal(ret, usermem.ByteOrder, xt)
+}
+
+func (*standardTargetMaker) unmarshal(buf []byte, filter stack.IPHeaderFilter) (stack.Target, *syserr.Error) {
+	if len(buf) != linux.SizeOfXTStandardTarget {
+		nflog("buf has wrong size for standard target %d", len(buf))
+		return nil, syserr.ErrInvalidArgument
+	}
+	var standardTarget linux.XTStandardTarget
+	buf = buf[:linux.SizeOfXTStandardTarget]
+	binary.Unmarshal(buf, usermem.ByteOrder, &standardTarget)
+
+	if standardTarget.Verdict < 0 {
+		// A Verdict < 0 indicates a non-jump verdict.
+		return translateToStandardTarget(standardTarget.Verdict, filter.NetworkProtocol())
+	}
+	// A verdict >= 0 indicates a jump.
+	return &JumpTarget{
+		Offset:          uint32(standardTarget.Verdict),
+		NetworkProtocol: filter.NetworkProtocol(),
+	}, nil
+}
+
+type errorTargetMaker struct {
+	NetworkProtocol tcpip.NetworkProtocolNumber
+}
+
+func (em *errorTargetMaker) id() stack.TargetID {
+	// Error targets have no revision.
+	return stack.TargetID{
+		Name:            stack.ErrorTargetName,
+		NetworkProtocol: em.NetworkProtocol,
+	}
 }
 
-func marshalErrorTarget(errorName string) []byte {
+func (*errorTargetMaker) marshal(target stack.Target) []byte {
+	var errorName string
+	switch tg := target.(type) {
+	case *stack.ErrorTarget:
+		errorName = stack.ErrorTargetName
+	case *stack.UserChainTarget:
+		errorName = tg.Name
+	default:
+		panic(fmt.Sprintf("errorMakerTarget cannot marshal unknown type %T", target))
+	}
+
 	// This is an error target named error
-	target := linux.XTErrorTarget{
+	xt := linux.XTErrorTarget{
 		Target: linux.XTEntryTarget{
 			TargetSize: linux.SizeOfXTErrorTarget,
 		},
 	}
-	copy(target.Name[:], errorName)
-	copy(target.Target.Name[:], errorTargetName)
+	copy(xt.Name[:], errorName)
+	copy(xt.Target.Name[:], stack.ErrorTargetName)
 
 	ret := make([]byte, 0, linux.SizeOfXTErrorTarget)
-	return binary.Marshal(ret, usermem.ByteOrder, target)
+	return binary.Marshal(ret, usermem.ByteOrder, xt)
+}
+
+func (*errorTargetMaker) unmarshal(buf []byte, filter stack.IPHeaderFilter) (stack.Target, *syserr.Error) {
+	if len(buf) != linux.SizeOfXTErrorTarget {
+		nflog("buf has insufficient size for error target %d", len(buf))
+		return nil, syserr.ErrInvalidArgument
+	}
+	var errorTarget linux.XTErrorTarget
+	buf = buf[:linux.SizeOfXTErrorTarget]
+	binary.Unmarshal(buf, usermem.ByteOrder, &errorTarget)
+
+	// Error targets are used in 2 cases:
+	// * An actual error case. These rules have an error
+	//   named stack.ErrorTargetName. The last entry of the table
+	//   is usually an error case to catch any packets that
+	//   somehow fall through every rule.
+	// * To mark the start of a user defined chain. These
+	//   rules have an error with the name of the chain.
+	switch name := errorTarget.Name.String(); name {
+	case stack.ErrorTargetName:
+		return &stack.ErrorTarget{NetworkProtocol: filter.NetworkProtocol()}, nil
+	default:
+		// User defined chain.
+		return &stack.UserChainTarget{
+			Name:            name,
+			NetworkProtocol: filter.NetworkProtocol(),
+		}, nil
+	}
 }
 
-func marshalRedirectTarget(rt stack.RedirectTarget) []byte {
+type redirectTargetMaker struct {
+	NetworkProtocol tcpip.NetworkProtocolNumber
+}
+
+func (rm *redirectTargetMaker) id() stack.TargetID {
+	return stack.TargetID{
+		Name:            stack.RedirectTargetName,
+		NetworkProtocol: rm.NetworkProtocol,
+	}
+}
+
+func (*redirectTargetMaker) marshal(target stack.Target) []byte {
+	rt := target.(*stack.RedirectTarget)
 	// This is a redirect target named redirect
-	target := linux.XTRedirectTarget{
+	xt := linux.XTRedirectTarget{
 		Target: linux.XTEntryTarget{
 			TargetSize: linux.SizeOfXTRedirectTarget,
 		},
 	}
-	copy(target.Target.Name[:], redirectTargetName)
+	copy(xt.Target.Name[:], stack.RedirectTargetName)
 
 	ret := make([]byte, 0, linux.SizeOfXTRedirectTarget)
-	target.NfRange.RangeSize = 1
+	xt.NfRange.RangeSize = 1
 	if rt.RangeProtoSpecified {
-		target.NfRange.RangeIPV4.Flags |= linux.NF_NAT_RANGE_PROTO_SPECIFIED
+		xt.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)
+	xt.NfRange.RangeIPV4.MinPort = htons(rt.MinPort)
+	xt.NfRange.RangeIPV4.MaxPort = htons(rt.MaxPort)
+	return binary.Marshal(ret, usermem.ByteOrder, xt)
 }
 
-func marshalJumpTarget(jt JumpTarget) []byte {
-	nflog("convert to binary: marshalling jump target")
+func (*redirectTargetMaker) unmarshal(buf []byte, filter stack.IPHeaderFilter) (stack.Target, *syserr.Error) {
+	if len(buf) < linux.SizeOfXTRedirectTarget {
+		nflog("redirectTargetMaker: buf has insufficient size for redirect target %d", len(buf))
+		return nil, syserr.ErrInvalidArgument
+	}
 
-	// The target's name will be the empty string.
-	target := linux.XTStandardTarget{
-		Target: linux.XTEntryTarget{
-			TargetSize: linux.SizeOfXTStandardTarget,
-		},
-		// Verdict is overloaded by the ABI. When positive, it holds
-		// the jump offset from the start of the table.
-		Verdict: int32(jt.Offset),
+	if p := filter.Protocol; p != header.TCPProtocolNumber && p != header.UDPProtocolNumber {
+		nflog("redirectTargetMaker: bad proto %d", p)
+		return nil, syserr.ErrInvalidArgument
 	}
 
-	ret := make([]byte, 0, linux.SizeOfXTStandardTarget)
-	return binary.Marshal(ret, usermem.ByteOrder, target)
-}
+	var redirectTarget linux.XTRedirectTarget
+	buf = buf[:linux.SizeOfXTRedirectTarget]
+	binary.Unmarshal(buf, usermem.ByteOrder, &redirectTarget)
 
-// translateFromStandardVerdict translates verdicts the same way as the iptables
-// tool.
-func translateFromStandardVerdict(verdict stack.RuleVerdict) int32 {
-	switch verdict {
-	case stack.RuleAccept:
-		return -linux.NF_ACCEPT - 1
-	case stack.RuleDrop:
-		return -linux.NF_DROP - 1
-	case stack.RuleReturn:
-		return linux.NF_RETURN
-	default:
-		// TODO(gvisor.dev/issue/170): Support Jump.
-		panic(fmt.Sprintf("unknown standard verdict: %d", verdict))
+	// Copy linux.XTRedirectTarget to stack.RedirectTarget.
+	target := stack.RedirectTarget{NetworkProtocol: filter.NetworkProtocol()}
+
+	// RangeSize should be 1.
+	nfRange := redirectTarget.NfRange
+	if nfRange.RangeSize != 1 {
+		nflog("redirectTargetMaker: bad rangesize %d", nfRange.RangeSize)
+		return nil, syserr.ErrInvalidArgument
 	}
+
+	// TODO(gvisor.dev/issue/170): Check if the flags are valid.
+	// Also check if we need to map ports or IP.
+	// For now, redirect target only supports destination port change.
+	// Port range and IP range are not supported yet.
+	if nfRange.RangeIPV4.Flags&linux.NF_NAT_RANGE_PROTO_SPECIFIED == 0 {
+		nflog("redirectTargetMaker: invalid range flags %d", nfRange.RangeIPV4.Flags)
+		return nil, syserr.ErrInvalidArgument
+	}
+	target.RangeProtoSpecified = true
+
+	target.MinIP = tcpip.Address(nfRange.RangeIPV4.MinIP[:])
+	target.MaxIP = tcpip.Address(nfRange.RangeIPV4.MaxIP[:])
+
+	// TODO(gvisor.dev/issue/170): Port range is not supported yet.
+	if nfRange.RangeIPV4.MinPort != nfRange.RangeIPV4.MaxPort {
+		nflog("redirectTargetMaker: MinPort != MaxPort (%d, %d)", nfRange.RangeIPV4.MinPort, nfRange.RangeIPV4.MaxPort)
+		return nil, syserr.ErrInvalidArgument
+	}
+
+	target.MinPort = ntohs(nfRange.RangeIPV4.MinPort)
+	target.MaxPort = ntohs(nfRange.RangeIPV4.MaxPort)
+
+	return &target, nil
 }
 
 // translateToStandardTarget translates from the value in a
 // linux.XTStandardTarget to an stack.Verdict.
-func translateToStandardTarget(val int32) (stack.Target, error) {
+func translateToStandardTarget(val int32, netProto tcpip.NetworkProtocolNumber) (stack.Target, *syserr.Error) {
 	// TODO(gvisor.dev/issue/170): Support other verdicts.
 	switch val {
 	case -linux.NF_ACCEPT - 1:
-		return stack.AcceptTarget{}, nil
+		return &stack.AcceptTarget{NetworkProtocol: netProto}, nil
 	case -linux.NF_DROP - 1:
-		return stack.DropTarget{}, nil
+		return &stack.DropTarget{NetworkProtocol: netProto}, nil
 	case -linux.NF_QUEUE - 1:
-		return nil, errors.New("unsupported iptables verdict QUEUE")
+		nflog("unsupported iptables verdict QUEUE")
+		return nil, syserr.ErrInvalidArgument
 	case linux.NF_RETURN:
-		return stack.ReturnTarget{}, nil
+		return &stack.ReturnTarget{NetworkProtocol: netProto}, nil
 	default:
-		return nil, fmt.Errorf("unknown iptables verdict %d", val)
+		nflog("unknown iptables verdict %d", val)
+		return nil, syserr.ErrInvalidArgument
 	}
 }
 
 // parseTarget parses a target from optVal. optVal should contain only the
 // target.
-func parseTarget(filter stack.IPHeaderFilter, optVal []byte) (stack.Target, error) {
+func parseTarget(filter stack.IPHeaderFilter, optVal []byte, ipv6 bool) (stack.Target, *syserr.Error) {
 	nflog("set entries: parsing target of size %d", len(optVal))
 	if len(optVal) < linux.SizeOfXTEntryTarget {
-		return nil, fmt.Errorf("optVal has insufficient size for entry target %d", len(optVal))
+		nflog("optVal has insufficient size for entry target %d", len(optVal))
+		return nil, syserr.ErrInvalidArgument
 	}
 	var target linux.XTEntryTarget
 	buf := optVal[:linux.SizeOfXTEntryTarget]
 	binary.Unmarshal(buf, usermem.ByteOrder, &target)
-	switch target.Name.String() {
-	case "":
-		// Standard target.
-		if len(optVal) != linux.SizeOfXTStandardTarget {
-			return nil, fmt.Errorf("optVal has wrong size for standard target %d", len(optVal))
-		}
-		var standardTarget linux.XTStandardTarget
-		buf = optVal[:linux.SizeOfXTStandardTarget]
-		binary.Unmarshal(buf, usermem.ByteOrder, &standardTarget)
-
-		if standardTarget.Verdict < 0 {
-			// A Verdict < 0 indicates a non-jump verdict.
-			return translateToStandardTarget(standardTarget.Verdict)
-		}
-		// A verdict >= 0 indicates a jump.
-		return JumpTarget{Offset: uint32(standardTarget.Verdict)}, nil
-
-	case errorTargetName:
-		// Error target.
-		if len(optVal) != linux.SizeOfXTErrorTarget {
-			return nil, fmt.Errorf("optVal has insufficient size for error target %d", len(optVal))
-		}
-		var errorTarget linux.XTErrorTarget
-		buf = optVal[:linux.SizeOfXTErrorTarget]
-		binary.Unmarshal(buf, usermem.ByteOrder, &errorTarget)
-
-		// Error targets are used in 2 cases:
-		// * An actual error case. These rules have an error
-		//   named errorTargetName. The last entry of the table
-		//   is usually an error case to catch any packets that
-		//   somehow fall through every rule.
-		// * To mark the start of a user defined chain. These
-		//   rules have an error with the name of the chain.
-		switch name := errorTarget.Name.String(); name {
-		case errorTargetName:
-			nflog("set entries: error target")
-			return stack.ErrorTarget{}, nil
-		default:
-			// User defined chain.
-			nflog("set entries: user-defined target %q", name)
-			return stack.UserChainTarget{Name: name}, nil
-		}
-
-	case redirectTargetName:
-		// Redirect target.
-		if len(optVal) < linux.SizeOfXTRedirectTarget {
-			return nil, fmt.Errorf("netfilter.SetEntries: optVal has insufficient size for redirect target %d", len(optVal))
-		}
-
-		if p := filter.Protocol; p != header.TCPProtocolNumber && p != header.UDPProtocolNumber {
-			return nil, fmt.Errorf("netfilter.SetEntries: bad proto %d", p)
-		}
-
-		var redirectTarget linux.XTRedirectTarget
-		buf = optVal[:linux.SizeOfXTRedirectTarget]
-		binary.Unmarshal(buf, usermem.ByteOrder, &redirectTarget)
-
-		// Copy linux.XTRedirectTarget to stack.RedirectTarget.
-		var target stack.RedirectTarget
-		nfRange := redirectTarget.NfRange
-
-		// RangeSize should be 1.
-		if nfRange.RangeSize != 1 {
-			return nil, fmt.Errorf("netfilter.SetEntries: bad rangesize %d", nfRange.RangeSize)
-		}
-
-		// TODO(gvisor.dev/issue/170): Check if the flags are valid.
-		// Also check if we need to map ports or IP.
-		// For now, redirect target only supports destination port change.
-		// Port range and IP range are not supported yet.
-		if nfRange.RangeIPV4.Flags&linux.NF_NAT_RANGE_PROTO_SPECIFIED == 0 {
-			return nil, fmt.Errorf("netfilter.SetEntries: invalid range flags %d", nfRange.RangeIPV4.Flags)
-		}
-		target.RangeProtoSpecified = true
-
-		target.MinIP = tcpip.Address(nfRange.RangeIPV4.MinIP[:])
-		target.MaxIP = tcpip.Address(nfRange.RangeIPV4.MaxIP[:])
-
-		// TODO(gvisor.dev/issue/170): Port range is not supported yet.
-		if nfRange.RangeIPV4.MinPort != nfRange.RangeIPV4.MaxPort {
-			return nil, fmt.Errorf("netfilter.SetEntries: minport != maxport (%d, %d)", nfRange.RangeIPV4.MinPort, nfRange.RangeIPV4.MaxPort)
-		}
-
-		// Convert port from big endian to little endian.
-		port := make([]byte, 2)
-		binary.BigEndian.PutUint16(port, nfRange.RangeIPV4.MinPort)
-		target.MinPort = binary.LittleEndian.Uint16(port)
-
-		binary.BigEndian.PutUint16(port, nfRange.RangeIPV4.MaxPort)
-		target.MaxPort = binary.LittleEndian.Uint16(port)
-		return target, nil
-	}
 
-	// Unknown target.
-	return nil, fmt.Errorf("unknown target %q doesn't exist or isn't supported yet", target.Name.String())
+	return unmarshalTarget(target, filter, optVal)
 }
 
 // JumpTarget implements stack.Target.
@@ -274,9 +295,31 @@ type JumpTarget struct {
 
 	// RuleNum is the rule to jump to.
 	RuleNum int
+
+	// NetworkProtocol is the network protocol the target is used with.
+	NetworkProtocol tcpip.NetworkProtocolNumber
+}
+
+// ID implements Target.ID.
+func (jt *JumpTarget) ID() stack.TargetID {
+	return stack.TargetID{
+		NetworkProtocol: jt.NetworkProtocol,
+	}
 }
 
 // Action implements stack.Target.Action.
 func (jt JumpTarget) Action(*stack.PacketBuffer, *stack.ConnTrack, stack.Hook, *stack.GSO, *stack.Route, tcpip.Address) (stack.RuleVerdict, int) {
 	return stack.RuleJump, jt.RuleNum
 }
+
+func ntohs(port uint16) uint16 {
+	buf := make([]byte, 2)
+	binary.BigEndian.PutUint16(buf, port)
+	return usermem.ByteOrder.Uint16(buf)
+}
+
+func htons(port uint16) uint16 {
+	buf := make([]byte, 2)
+	usermem.ByteOrder.PutUint16(buf, port)
+	return binary.BigEndian.Uint16(buf)
+}
diff --git a/pkg/sentry/socket/netstack/netstack.go b/pkg/sentry/socket/netstack/netstack.go
index 6fede181a..20f66fbf1 100644
--- a/pkg/sentry/socket/netstack/netstack.go
+++ b/pkg/sentry/socket/netstack/netstack.go
@@ -1719,6 +1719,26 @@ func getSockOptIP(t *kernel.Task, s socket.SocketOps, ep commonEndpoint, name in
 		}
 		return &entries, nil
 
+	case linux.IPT_SO_GET_REVISION_TARGET:
+		if outLen < linux.SizeOfXTGetRevision {
+			return nil, syserr.ErrInvalidArgument
+		}
+
+		// Only valid for raw IPv4 sockets.
+		if family, skType, _ := s.Type(); family != linux.AF_INET || skType != linux.SOCK_RAW {
+			return nil, syserr.ErrProtocolNotAvailable
+		}
+
+		stack := inet.StackFromContext(t)
+		if stack == nil {
+			return nil, syserr.ErrNoDevice
+		}
+		ret, err := netfilter.TargetRevision(t, outPtr, header.IPv4ProtocolNumber)
+		if err != nil {
+			return nil, err
+		}
+		return &ret, nil
+
 	default:
 		emitUnimplementedEventIP(t, name)
 	}
diff --git a/pkg/tcpip/network/ipv4/ipv4_test.go b/pkg/tcpip/network/ipv4/ipv4_test.go
index 0b3ed9483..277560e35 100644
--- a/pkg/tcpip/network/ipv4/ipv4_test.go
+++ b/pkg/tcpip/network/ipv4/ipv4_test.go
@@ -1040,7 +1040,7 @@ func TestWriteStats(t *testing.T) {
 					t.Fatalf("failed to find filter table")
 				}
 				ruleIdx := filter.BuiltinChains[stack.Output]
-				filter.Rules[ruleIdx].Target = stack.DropTarget{}
+				filter.Rules[ruleIdx].Target = &stack.DropTarget{}
 				if err := ipt.ReplaceTable(stack.FilterTable, filter, false /* ipv6 */); err != nil {
 					t.Fatalf("failed to replace table: %s", err)
 				}
@@ -1062,10 +1062,10 @@ func TestWriteStats(t *testing.T) {
 				}
 				// We'll match and DROP the last packet.
 				ruleIdx := filter.BuiltinChains[stack.Output]
-				filter.Rules[ruleIdx].Target = stack.DropTarget{}
+				filter.Rules[ruleIdx].Target = &stack.DropTarget{}
 				filter.Rules[ruleIdx].Matchers = []stack.Matcher{&limitedMatcher{nPackets - 1}}
 				// Make sure the next rule is ACCEPT.
-				filter.Rules[ruleIdx+1].Target = stack.AcceptTarget{}
+				filter.Rules[ruleIdx+1].Target = &stack.AcceptTarget{}
 				if err := ipt.ReplaceTable(stack.FilterTable, filter, false /* ipv6 */); err != nil {
 					t.Fatalf("failed to replace table: %s", err)
 				}
diff --git a/pkg/tcpip/network/ipv6/ipv6_test.go b/pkg/tcpip/network/ipv6/ipv6_test.go
index d85b5c00f..94344057e 100644
--- a/pkg/tcpip/network/ipv6/ipv6_test.go
+++ b/pkg/tcpip/network/ipv6/ipv6_test.go
@@ -1748,7 +1748,7 @@ func TestWriteStats(t *testing.T) {
 					t.Fatalf("failed to find filter table")
 				}
 				ruleIdx := filter.BuiltinChains[stack.Output]
-				filter.Rules[ruleIdx].Target = stack.DropTarget{}
+				filter.Rules[ruleIdx].Target = &stack.DropTarget{}
 				if err := ipt.ReplaceTable(stack.FilterTable, filter, true /* ipv6 */); err != nil {
 					t.Fatalf("failed to replace table: %v", err)
 				}
@@ -1770,10 +1770,10 @@ func TestWriteStats(t *testing.T) {
 				}
 				// We'll match and DROP the last packet.
 				ruleIdx := filter.BuiltinChains[stack.Output]
-				filter.Rules[ruleIdx].Target = stack.DropTarget{}
+				filter.Rules[ruleIdx].Target = &stack.DropTarget{}
 				filter.Rules[ruleIdx].Matchers = []stack.Matcher{&limitedMatcher{nPackets - 1}}
 				// Make sure the next rule is ACCEPT.
-				filter.Rules[ruleIdx+1].Target = stack.AcceptTarget{}
+				filter.Rules[ruleIdx+1].Target = &stack.AcceptTarget{}
 				if err := ipt.ReplaceTable(stack.FilterTable, filter, true /* ipv6 */); err != nil {
 					t.Fatalf("failed to replace table: %v", err)
 				}
diff --git a/pkg/tcpip/stack/iptables.go b/pkg/tcpip/stack/iptables.go
index 4a521eca9..faa503b00 100644
--- a/pkg/tcpip/stack/iptables.go
+++ b/pkg/tcpip/stack/iptables.go
@@ -60,11 +60,11 @@ func DefaultTables() *IPTables {
 		v4Tables: [numTables]Table{
 			natID: Table{
 				Rules: []Rule{
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: ErrorTarget{}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv4ProtocolNumber}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv4ProtocolNumber}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv4ProtocolNumber}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv4ProtocolNumber}},
+					Rule{Target: &ErrorTarget{NetworkProtocol: header.IPv4ProtocolNumber}},
 				},
 				BuiltinChains: [NumHooks]int{
 					Prerouting:  0,
@@ -83,9 +83,9 @@ func DefaultTables() *IPTables {
 			},
 			mangleID: Table{
 				Rules: []Rule{
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: ErrorTarget{}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv4ProtocolNumber}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv4ProtocolNumber}},
+					Rule{Target: &ErrorTarget{NetworkProtocol: header.IPv4ProtocolNumber}},
 				},
 				BuiltinChains: [NumHooks]int{
 					Prerouting: 0,
@@ -101,10 +101,10 @@ func DefaultTables() *IPTables {
 			},
 			filterID: Table{
 				Rules: []Rule{
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: ErrorTarget{}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv4ProtocolNumber}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv4ProtocolNumber}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv4ProtocolNumber}},
+					Rule{Target: &ErrorTarget{NetworkProtocol: header.IPv4ProtocolNumber}},
 				},
 				BuiltinChains: [NumHooks]int{
 					Prerouting:  HookUnset,
@@ -125,11 +125,11 @@ func DefaultTables() *IPTables {
 		v6Tables: [numTables]Table{
 			natID: Table{
 				Rules: []Rule{
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: ErrorTarget{}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv6ProtocolNumber}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv6ProtocolNumber}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv6ProtocolNumber}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv6ProtocolNumber}},
+					Rule{Target: &ErrorTarget{NetworkProtocol: header.IPv6ProtocolNumber}},
 				},
 				BuiltinChains: [NumHooks]int{
 					Prerouting:  0,
@@ -148,9 +148,9 @@ func DefaultTables() *IPTables {
 			},
 			mangleID: Table{
 				Rules: []Rule{
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: ErrorTarget{}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv6ProtocolNumber}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv6ProtocolNumber}},
+					Rule{Target: &ErrorTarget{NetworkProtocol: header.IPv6ProtocolNumber}},
 				},
 				BuiltinChains: [NumHooks]int{
 					Prerouting: 0,
@@ -166,10 +166,10 @@ func DefaultTables() *IPTables {
 			},
 			filterID: Table{
 				Rules: []Rule{
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: AcceptTarget{}},
-					Rule{Target: ErrorTarget{}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv6ProtocolNumber}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv6ProtocolNumber}},
+					Rule{Target: &AcceptTarget{NetworkProtocol: header.IPv6ProtocolNumber}},
+					Rule{Target: &ErrorTarget{NetworkProtocol: header.IPv6ProtocolNumber}},
 				},
 				BuiltinChains: [NumHooks]int{
 					Prerouting:  HookUnset,
diff --git a/pkg/tcpip/stack/iptables_targets.go b/pkg/tcpip/stack/iptables_targets.go
index 5f1b2af64..8581dd5e8 100644
--- a/pkg/tcpip/stack/iptables_targets.go
+++ b/pkg/tcpip/stack/iptables_targets.go
@@ -21,7 +21,17 @@ import (
 )
 
 // AcceptTarget accepts packets.
-type AcceptTarget struct{}
+type AcceptTarget struct {
+	// NetworkProtocol is the network protocol the target is used with.
+	NetworkProtocol tcpip.NetworkProtocolNumber
+}
+
+// ID implements Target.ID.
+func (at *AcceptTarget) ID() TargetID {
+	return TargetID{
+		NetworkProtocol: at.NetworkProtocol,
+	}
+}
 
 // Action implements Target.Action.
 func (AcceptTarget) Action(*PacketBuffer, *ConnTrack, Hook, *GSO, *Route, tcpip.Address) (RuleVerdict, int) {
@@ -29,16 +39,41 @@ func (AcceptTarget) Action(*PacketBuffer, *ConnTrack, Hook, *GSO, *Route, tcpip.
 }
 
 // DropTarget drops packets.
-type DropTarget struct{}
+type DropTarget struct {
+	// NetworkProtocol is the network protocol the target is used with.
+	NetworkProtocol tcpip.NetworkProtocolNumber
+}
+
+// ID implements Target.ID.
+func (dt *DropTarget) ID() TargetID {
+	return TargetID{
+		NetworkProtocol: dt.NetworkProtocol,
+	}
+}
 
 // Action implements Target.Action.
 func (DropTarget) Action(*PacketBuffer, *ConnTrack, Hook, *GSO, *Route, tcpip.Address) (RuleVerdict, int) {
 	return RuleDrop, 0
 }
 
+// ErrorTargetName is used to mark targets as error targets. Error targets
+// shouldn't be reached - an error has occurred if we fall through to one.
+const ErrorTargetName = "ERROR"
+
 // ErrorTarget logs an error and drops the packet. It represents a target that
 // should be unreachable.
-type ErrorTarget struct{}
+type ErrorTarget struct {
+	// NetworkProtocol is the network protocol the target is used with.
+	NetworkProtocol tcpip.NetworkProtocolNumber
+}
+
+// ID implements Target.ID.
+func (et *ErrorTarget) ID() TargetID {
+	return TargetID{
+		Name:            ErrorTargetName,
+		NetworkProtocol: et.NetworkProtocol,
+	}
+}
 
 // Action implements Target.Action.
 func (ErrorTarget) Action(*PacketBuffer, *ConnTrack, Hook, *GSO, *Route, tcpip.Address) (RuleVerdict, int) {
@@ -48,7 +83,19 @@ func (ErrorTarget) Action(*PacketBuffer, *ConnTrack, Hook, *GSO, *Route, tcpip.A
 
 // UserChainTarget marks a rule as the beginning of a user chain.
 type UserChainTarget struct {
+	// Name is the chain name.
 	Name string
+
+	// NetworkProtocol is the network protocol the target is used with.
+	NetworkProtocol tcpip.NetworkProtocolNumber
+}
+
+// ID implements Target.ID.
+func (uc *UserChainTarget) ID() TargetID {
+	return TargetID{
+		Name:            ErrorTargetName,
+		NetworkProtocol: uc.NetworkProtocol,
+	}
 }
 
 // Action implements Target.Action.
@@ -58,13 +105,28 @@ func (UserChainTarget) Action(*PacketBuffer, *ConnTrack, Hook, *GSO, *Route, tcp
 
 // ReturnTarget returns from the current chain. If the chain is a built-in, the
 // hook's underflow should be called.
-type ReturnTarget struct{}
+type ReturnTarget struct {
+	// NetworkProtocol is the network protocol the target is used with.
+	NetworkProtocol tcpip.NetworkProtocolNumber
+}
+
+// ID implements Target.ID.
+func (rt *ReturnTarget) ID() TargetID {
+	return TargetID{
+		NetworkProtocol: rt.NetworkProtocol,
+	}
+}
 
 // Action implements Target.Action.
 func (ReturnTarget) Action(*PacketBuffer, *ConnTrack, Hook, *GSO, *Route, tcpip.Address) (RuleVerdict, int) {
 	return RuleReturn, 0
 }
 
+// RedirectTargetName is used to mark targets as redirect targets. Redirect
+// targets should be reached for only NAT and Mangle tables. These targets will
+// change the destination port/destination IP for packets.
+const RedirectTargetName = "REDIRECT"
+
 // RedirectTarget redirects the packet by modifying the destination port/IP.
 // Min and Max values for IP and Ports in the struct indicate the range of
 // values which can be used to redirect.
@@ -86,6 +148,17 @@ type RedirectTarget struct {
 
 	// MaxPort indicates port used to redirect.
 	MaxPort uint16
+
+	// NetworkProtocol is the network protocol the target is used with.
+	NetworkProtocol tcpip.NetworkProtocolNumber
+}
+
+// ID implements Target.ID.
+func (rt *RedirectTarget) ID() TargetID {
+	return TargetID{
+		Name:            RedirectTargetName,
+		NetworkProtocol: rt.NetworkProtocol,
+	}
 }
 
 // Action implements Target.Action.
diff --git a/pkg/tcpip/stack/iptables_types.go b/pkg/tcpip/stack/iptables_types.go
index 093ee6881..7b3f3e88b 100644
--- a/pkg/tcpip/stack/iptables_types.go
+++ b/pkg/tcpip/stack/iptables_types.go
@@ -104,8 +104,20 @@ type IPTables struct {
 	reaperDone chan struct{}
 }
 
-// A Table defines a set of chains and hooks into the network stack. It is
-// really just a list of rules.
+// A Table defines a set of chains and hooks into the network stack.
+//
+// It is a list of Rules, entry points (BuiltinChains), and error handlers
+// (Underflows). As packets traverse netstack, they hit hooks. When a packet
+// hits a hook, iptables compares it to Rules starting from that hook's entry
+// point. So if a packet hits the Input hook, we look up the corresponding
+// entry point in BuiltinChains and jump to that point.
+//
+// If the Rule doesn't match the packet, iptables continues to the next Rule.
+// If a Rule does match, it can issue a verdict on the packet (e.g. RuleAccept
+// or RuleDrop) that causes the packet to stop traversing iptables. It can also
+// jump to other rules or perform custom actions based on Rule.Target.
+//
+// Underflow Rules are invoked when a chain returns without reaching a verdict.
 //
 // +stateify savable
 type Table struct {
@@ -260,6 +272,18 @@ func (fl IPHeaderFilter) match(pkt *PacketBuffer, hook Hook, nicName string) boo
 	return true
 }
 
+// NetworkProtocol returns the protocol (IPv4 or IPv6) on to which the header
+// applies.
+func (fl IPHeaderFilter) NetworkProtocol() tcpip.NetworkProtocolNumber {
+	switch len(fl.Src) {
+	case header.IPv4AddressSize:
+		return header.IPv4ProtocolNumber
+	case header.IPv6AddressSize:
+		return header.IPv6ProtocolNumber
+	}
+	panic(fmt.Sprintf("invalid address in IPHeaderFilter: %s", fl.Src))
+}
+
 // filterAddress returns whether addr matches the filter.
 func filterAddress(addr, mask, filterAddr tcpip.Address, invert bool) bool {
 	matches := true
@@ -285,8 +309,23 @@ type Matcher interface {
 	Match(hook Hook, packet *PacketBuffer, interfaceName string) (matches bool, hotdrop bool)
 }
 
+// A TargetID uniquely identifies a target.
+type TargetID struct {
+	// Name is the target name as stored in the xt_entry_target struct.
+	Name string
+
+	// NetworkProtocol is the protocol to which the target applies.
+	NetworkProtocol tcpip.NetworkProtocolNumber
+
+	// Revision is the version of the target.
+	Revision uint8
+}
+
 // A Target is the interface for taking an action for a packet.
 type Target interface {
+	// ID uniquely identifies the Target.
+	ID() TargetID
+
 	// 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.