fix loopback/link-local checks

1. Ensure we check the entire addr, not just a prefix.
2. Make the LinkLocal test skip zones.
3. Consider link-local multicast to be link-local.
4. Defer to *go* to determine if something is, in fact, a loopback/link-local address.

ip.go

 package manet
 
 import (
-	"bytes"
+	"net"
 
 	ma "github.com/multiformats/go-multiaddr"
 )
@@ -24,14 +24,6 @@ var (
 	IP6Unspecified = ma.StringCast("/ip6/::")
 )
 
-// Loopback multiaddr prefixes. Any multiaddr beginning with one of the
-// following byte sequences is considered a loopback multiaddr.
-var loopbackPrefixes = [][]byte{
-	{ma.P_IP4, 127}, // 127.*
-	{ma.P_IP6, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0xff, 0xff, 127}, // ::ffff:127.*
-	IP6Loopback.Bytes(), // ::1
-}
-
 // IsThinWaist returns whether a Multiaddr starts with "Thin Waist" Protocols.
 // This means: /{IP4, IP6}[/{TCP, UDP}]
 func IsThinWaist(m ma.Multiaddr) bool {
@@ -62,22 +54,41 @@ func IsThinWaist(m ma.Multiaddr) bool {
 // IsIPLoopback returns whether a Multiaddr is a "Loopback" IP address
 // This means either /ip4/127.*.*.*, /ip6/::1, or /ip6/::ffff:127.*.*.*.*
 func IsIPLoopback(m ma.Multiaddr) bool {
-	b := m.Bytes()
-	for _, prefix := range loopbackPrefixes {
-		if bytes.HasPrefix(b, prefix) {
-			return true
+	c, rest := ma.SplitFirst(m)
+	if rest != nil {
+		// Not *just* an IPv4 addr
+		return false
 	}
+	switch c.Protocol().Code {
+	case ma.P_IP4, ma.P_IP6:
+		return net.IP(c.RawValue()).IsLoopback()
 	}
 	return false
 }
 
-// IsIP6LinkLocal returns if a multiaddress is an IPv6 local link. These
-// addresses are non routable. The prefix is technically
-// fe80::/10, but we test fe80::/16 for simplicity (no need to mask).
-// So far, no hardware interfaces exist long enough to use those 2 bits.
-// Send a PR if there is.
+// IsIP6LinkLocal returns if a an IPv6 link-local multiaddress (with zero or
+// more leading zones). These addresses are non routable.
 func IsIP6LinkLocal(m ma.Multiaddr) bool {
-	return bytes.HasPrefix(m.Bytes(), []byte{ma.P_IP6, 0xfe, 0x80})
+	matched := false
+	ma.ForEach(m, func(c ma.Component) bool {
+		// Too much.
+		if matched {
+			matched = false
+			return false
+		}
+
+		switch c.Protocol().Code {
+		case ma.P_IP6ZONE:
+			return true
+		case ma.P_IP6:
+			ip := net.IP(c.RawValue())
+			matched = ip.IsLinkLocalMulticast() || ip.IsLinkLocalUnicast()
+			return true
+		default:
+			return false
+		}
+	})
+	return matched
 }
 
 // IsIPUnspecified returns whether a Multiaddr is am Unspecified IP address

net_test.go

@@ -322,6 +322,10 @@ func TestIPLoopback(t *testing.T) {
 		t.Error("IsIPLoopback false positive (/ip4/112.123.11.1)")
 	}
 
+	if IsIPLoopback(newMultiaddr(t, "/ip4/127.0.0.1/ip4/127.0.0.1")) {
+		t.Error("IsIPLoopback false positive (/ip4/127.0.0.1/127.0.0.1)")
+	}
+
 	if IsIPLoopback(newMultiaddr(t, "/ip4/192.168.0.1/ip6/::1")) {
 		t.Error("IsIPLoopback false positive (/ip4/192.168.0.1/ip6/::1)")
 	}
@@ -363,7 +367,7 @@ func TestIPUnspecified(t *testing.T) {
 
 func TestIP6LinkLocal(t *testing.T) {
 	for a := 0; a < 65536; a++ {
-		isLinkLocal := (a == 0xfe80)
+		isLinkLocal := (a&0xffc0 == 0xfe80 || a&0xff0f == 0xff02)
 		m := newMultiaddr(t, fmt.Sprintf("/ip6/%x::1", a))
 		if IsIP6LinkLocal(m) != isLinkLocal {
 			t.Errorf("IsIP6LinkLocal failed (%s != %v)", m, isLinkLocal)