Update LocalInterfaces() to comply with spec

Resolves #466

util.go

@@ -18,21 +18,34 @@ func localInterfaces() (ips []net.IP) {
 		if iface.Flags&net.FlagLoopback != 0 {
 			continue // loopback interface
 		}
+
 		addrs, err := iface.Addrs()
 		if err != nil {
 			return ips
 		}
+
 		for _, addr := range addrs {
 			var ip net.IP
-			switch v := addr.(type) {
+			switch addr := addr.(type) {
 			case *net.IPNet:
-				ip = v.IP
+				ip = addr.IP
 			case *net.IPAddr:
-				ip = v.IP
+				ip = addr.IP
+
 			}
+
 			if ip == nil || ip.IsLoopback() {
 				continue
 			}
+
+			// The conditions of invalidation written below are defined in
+			// https://tools.ietf.org/html/rfc8445#section-5.1.1.1
+			if ipv4 := ip.To4(); ipv4 == nil {
+				if !isSupportedIPv6(ip) {
+					continue
+				}
+			}
+
 			ips = append(ips, ip)
 		}
 	}
@@ -48,3 +61,23 @@ func (a *atomicError) Load() error {
 	err, _ := a.v.Load().(struct{ error })
 	return err.error
 }
+
+func isSupportedIPv6(ip net.IP) bool {
+	if len(ip) != net.IPv6len ||
+		!isZeros(ip[0:12]) || // !(IPv4-compatible IPv6)
+		ip[0] == 0xfe && ip[1]&0xc0 == 0xc0 || // !(IPv6 site-local unicast)
+		ip.IsLinkLocalUnicast() ||
+		ip.IsLinkLocalMulticast() {
+		return false
+	}
+	return true
+}
+
+func isZeros(ip net.IP) bool {
+	for i := 0; i < len(ip); i++ {
+		if ip[i] != 0 {
+			return false
+		}
+	}
+	return true
+}