nat: Add flag for specified range in DNAT

Example: meta l4proto tcp dnat to 10.200.45.6:9080

  [ meta load l4proto => reg 1 ]
  [ cmp eq reg 1 0x00000006 ]
  [ immediate reg 1 0x062dc80a ]
  [ immediate reg 2 0x00007823 ]
  [ nat dnat ip addr_min reg 1 proto_min reg 2 flags 0x2 ]

expr/expr.go

@@ -336,6 +336,8 @@ const (
 	NF_NAT_RANGE_PERSISTENT = unix.NF_NAT_RANGE_PERSISTENT
 	// NF_NAT_RANGE_PREFIX defines flag for a prefix masquerade
 	NF_NAT_RANGE_PREFIX = unix.NF_NAT_RANGE_NETMAP
+	// NF_NAT_RANGE_PROTO_SPECIFIED defines flag for a specified range
+	NF_NAT_RANGE_PROTO_SPECIFIED = unix.NF_NAT_RANGE_PROTO_SPECIFIED
 )
 
 func (e *Masq) marshal(fam byte) ([]byte, error) {

expr/nat.go

@@ -41,6 +41,7 @@ type NAT struct {
 	FullyRandom bool
 	Persistent  bool
 	Prefix      bool
+	Specified   bool
 }
 
 // |00048|N-|00001|	|len |flags| type|
@@ -97,6 +98,9 @@ func (e *NAT) marshalData(fam byte) ([]byte, error) {
 	if e.Prefix {
 		flags |= NF_NAT_RANGE_PREFIX
 	}
+	if e.Specified {
+		flags |= NF_NAT_RANGE_PROTO_SPECIFIED
+	}
 	if flags != 0 {
 		attrs = append(attrs, netlink.Attribute{Type: unix.NFTA_NAT_FLAGS, Data: binaryutil.BigEndian.PutUint32(flags)})
 	}
@@ -130,6 +134,7 @@ func (e *NAT) unmarshal(fam byte, data []byte) error {
 			e.Random = (flags & NF_NAT_RANGE_PROTO_RANDOM) != 0
 			e.FullyRandom = (flags & NF_NAT_RANGE_PROTO_RANDOM_FULLY) != 0
 			e.Prefix = (flags & NF_NAT_RANGE_PREFIX) != 0
+			e.Specified = (flags & NF_NAT_RANGE_PROTO_SPECIFIED) != 0
 		}
 	}
 	return ad.Err()

expr/nat_test.go

@@ -0,0 +1,66 @@
+package expr
+
+import (
+	"encoding/binary"
+	"reflect"
+	"testing"
+
+	"github.com/mdlayher/netlink"
+	"golang.org/x/sys/unix"
+)
+
+func TestNat(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		name string
+		nat  NAT
+	}{
+		{
+			name: "Unmarshal DNAT specified case",
+			nat: NAT{
+				Type:        NATTypeDestNAT,
+				Family:      unix.NFPROTO_IPV4,
+				RegAddrMin:  1,
+				RegProtoMin: 2,
+				Specified:   true,
+			},
+		},
+		{
+			name: "Unmarshal SNAT persistent case",
+			nat: NAT{
+				Type:        NATTypeSourceNAT,
+				Family:      unix.NFPROTO_IPV4,
+				RegAddrMin:  1,
+				RegProtoMin: 2,
+				Persistent:  true,
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			nnat := NAT{}
+			data, err := tt.nat.marshal(0 /* don't care in this test */)
+			if err != nil {
+				t.Fatalf("marshal error: %+v", err)
+
+			}
+			ad, err := netlink.NewAttributeDecoder(data)
+			if err != nil {
+				t.Fatalf("NewAttributeDecoder() error: %+v", err)
+			}
+			ad.ByteOrder = binary.BigEndian
+			for ad.Next() {
+				if ad.Type() == unix.NFTA_EXPR_DATA {
+					if err := nnat.unmarshal(0, ad.Bytes()); err != nil {
+						t.Errorf("unmarshal error: %+v", err)
+						break
+					}
+				}
+			}
+			if !reflect.DeepEqual(tt.nat, nnat) {
+				t.Fatalf("original %+v and recovered %+v Ct structs are different", tt.nat, nnat)
+			}
+		})
+	}
+}