Relieve length check for NeighAttributes (#208)

* Remove length check for NeighAttributes

The attribute length has more context then previously presumed.
Especially when using tunnels and other link types, the attributes
can use different lengths then expected.

This completely removes any length check for neighbor attributes.
This places the burden of checking for correctness to the consumer
of this library.

neigh.go

@@ -13,9 +13,6 @@ import (
 var (
 	// errInvalidNeighMessage is returned when a LinkMessage is malformed.
 	errInvalidNeighMessage = errors.New("rtnetlink NeighMessage is invalid or too short")
-
-	// errInvalidNeighMessageAttr is returned when neigh attributes are malformed.
-	errInvalidNeighMessageAttr = errors.New("rtnetlink NeighMessage has a wrong attribute data length")
 )
 
 var _ Message = &NeighMessage{}
@@ -185,10 +182,6 @@ func (a *NeighAttributes) decode(ad *netlink.AttributeDecoder) error {
 		case unix.NDA_UNSPEC:
 			// unused attribute
 		case unix.NDA_DST:
-			l := len(ad.Bytes())
-			if l != 4 && l != 16 {
-				return errInvalidNeighMessageAttr
-			}
 			a.Address = ad.Bytes()
 		case unix.NDA_LLADDR:
 			// Allow IEEE 802 MAC-48, EUI-48, EUI-64, or 20-octet
@@ -198,9 +191,6 @@ func (a *NeighAttributes) decode(ad *netlink.AttributeDecoder) error {
 				// Ignore empty addresses.
 				continue
 			}
-			if l != 6 && l != 8 && l != 20 {
-				return errInvalidNeighMessageAttr
-			}
 			a.LLAddress = ad.Bytes()
 		case unix.NDA_CACHEINFO:
 			a.CacheInfo = &NeighCacheInfo{}

neigh_test.go

@@ -142,20 +142,6 @@ func TestNeighMessageUnmarshalBinary(t *testing.T) {
 			b:    make([]byte, 11),
 			err:  errInvalidNeighMessage,
 		},
-		{
-			name: "invalid attr",
-			b: []byte{
-				0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-				0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-				0x06, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00,
-				0x04, 0x00, 0x01, 0x00, 0x04, 0x00, 0x02, 0x00,
-				0x05, 0x00, 0x03, 0x00, 0x00, 0x00, 0x00, 0x00,
-				0x08, 0x00, 0x04, 0x00, 0x00, 0x00, 0x00, 0x00,
-				0x08, 0x00, 0x05, 0x00, 0x00, 0x00, 0x00, 0x00,
-				0x05, 0x00, 0x06, 0x00, 0x00, 0x00, 0x00, 0x00,
-			},
-			err: errInvalidNeighMessageAttr,
-		},
 		{
 			name: "data",
 			b: []byte{