fix prefixlen/local IP, incl. PtP addresses

From libnl addr.c comment:
IPv6 sends the local address as IFA_ADDRESS with no
IFA_LOCAL, IPv4 sends both IFA_LOCAL and IFA_ADDRESS
with IFA_ADDRESS being the peer address if they differ

But obviously, IPv6 sends IFA_LOCAL in case of PtP.

From iproute2 manual:
If a peer address is specified, the local address
cannot have a prefix length. The network prefix is
associated with the peer rather than with the local
address.

addr_linux.go

@@ -65,7 +65,11 @@ func (h *Handle) addrHandle(link Link, addr *Addr, req *nl.NetlinkRequest) error
 	msg := nl.NewIfAddrmsg(family)
 	msg.Index = uint32(base.Index)
 	msg.Scope = uint8(addr.Scope)
-	prefixlen, masklen := addr.Mask.Size()
+	mask := addr.Mask
+	if addr.Peer != nil {
+		mask = addr.Peer.Mask
+	}
+	prefixlen, masklen := mask.Size()
 	msg.Prefixlen = uint8(prefixlen)
 	req.AddData(msg)
 
@@ -107,7 +111,7 @@ func (h *Handle) addrHandle(link Link, addr *Addr, req *nl.NetlinkRequest) error
 		if addr.Broadcast == nil {
 			calcBroadcast := make(net.IP, masklen/8)
 			for i := range localAddrData {
-				calcBroadcast[i] = localAddrData[i] | ^addr.Mask[i]
+				calcBroadcast[i] = localAddrData[i] | ^mask[i]
 			}
 			addr.Broadcast = calcBroadcast
 		}
@@ -206,13 +210,17 @@ func parseAddr(m []byte) (addr Addr, family, index int, err error) {
 				IP:   attr.Value,
 				Mask: net.CIDRMask(int(msg.Prefixlen), 8*len(attr.Value)),
 			}
-			addr.Peer = dst
 		case unix.IFA_LOCAL:
+			// iproute2 manual:
+			// If a peer address is specified, the local address
+			// cannot have a prefix length. The network prefix is
+			// associated with the peer rather than with the local
+			// address.
+			n := 8 * len(attr.Value)
 			local = &net.IPNet{
 				IP:   attr.Value,
-				Mask: net.CIDRMask(int(msg.Prefixlen), 8*len(attr.Value)),
+				Mask: net.CIDRMask(n, n),
 			}
-			addr.IPNet = local
 		case unix.IFA_BROADCAST:
 			addr.Broadcast = attr.Value
 		case unix.IFA_LABEL:
@@ -226,12 +234,24 @@ func parseAddr(m []byte) (addr Addr, family, index int, err error) {
 		}
 	}
 
-	// IFA_LOCAL should be there but if not, fall back to IFA_ADDRESS
+	// libnl addr.c comment:
+	// IPv6 sends the local address as IFA_ADDRESS with no
+	// IFA_LOCAL, IPv4 sends both IFA_LOCAL and IFA_ADDRESS
+	// with IFA_ADDRESS being the peer address if they differ
+	//
+	// But obviously, as there are IPv6 PtP addresses, too,
+	// IFA_LOCAL should also be handled for IPv6.
 	if local != nil {
-		addr.IPNet = local
+		if family == FAMILY_V4 && local.IP.Equal(dst.IP) {
+			addr.IPNet = dst
+		} else {
+			addr.IPNet = local
+			addr.Peer = dst
+		}
 	} else {
 		addr.IPNet = dst
 	}
+
 	addr.Scope = int(msg.Scope)
 
 	return

addr_test.go

@@ -24,7 +24,7 @@ func DoTestAddr(t *testing.T, FunctionUndertest func(Link, *Addr) error) {
 		t.Skipf("Fails in travis with: addr_test.go:68: Address flags not set properly, got=0, expected=128")
 	}
 	// TODO: IFA_F_PERMANENT does not seem to be set by default on older kernels?
-	var address = &net.IPNet{IP: net.IPv4(127, 0, 0, 2), Mask: net.CIDRMask(24, 32)}
+	var address = &net.IPNet{IP: net.IPv4(127, 0, 0, 2), Mask: net.CIDRMask(32, 32)}
 	var peer = &net.IPNet{IP: net.IPv4(127, 0, 0, 3), Mask: net.CIDRMask(24, 32)}
 	var addrTests = []struct {
 		addr     *Addr