Add unit tests for ExtractAddressFromReverse (#328)

This fix adds unit tests for `ExtractAddressFromReverse`. It also
updates the implementation of `ExtractAddressFromReverse` so that
only a valid IPv4 or IPv6 address (in string format) is returned.

If the addresses is not valid and can not be handled by golang's
`net.ParseIP` then an empty string (`""`) will be returned.

Signed-off-by: Yong Tang <yong.tang.github@outlook.com>

middleware/pkg/dnsutil/reverse.go

@@ -1,6 +1,9 @@
 package dnsutil
 
-import "strings"
+import (
+	"net"
+	"strings"
+)
 
 // ExtractAddressFromReverse turns a standard PTR reverse record name
 // into an IP address. This works for ipv4 or ipv6.
@@ -10,26 +13,51 @@ import "strings"
 func ExtractAddressFromReverse(reverseName string) string {
 	search := ""
 
+	f := reverse
+
 	switch {
 	case strings.HasSuffix(reverseName, v4arpaSuffix):
 		search = strings.TrimSuffix(reverseName, v4arpaSuffix)
 	case strings.HasSuffix(reverseName, v6arpaSuffix):
 		search = strings.TrimSuffix(reverseName, v6arpaSuffix)
+		f = reverse6
 	default:
 		return ""
 	}
 
 	// Reverse the segments and then combine them.
-	segments := reverse(strings.Split(search, "."))
-	return strings.Join(segments, ".")
+	return f(strings.Split(search, "."))
 }
 
-func reverse(slice []string) []string {
+func reverse(slice []string) string {
 	for i := 0; i < len(slice)/2; i++ {
 		j := len(slice) - i - 1
 		slice[i], slice[j] = slice[j], slice[i]
 	}
-	return slice
+	ip := net.ParseIP(strings.Join(slice, ".")).To4()
+	if ip == nil {
+		return ""
+	}
+	return ip.String()
+}
+
+// reverse6 reverse the segments and combine them according to RFC3596:
+// b.a.9.8.7.6.5.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.0.8.b.d.0.1.0.0.2
+// is reversed to 2001:db8::567:89ab
+func reverse6(slice []string) string {
+	for i := 0; i < len(slice)/2; i++ {
+		j := len(slice) - i - 1
+		slice[i], slice[j] = slice[j], slice[i]
+	}
+	slice6 := []string{}
+	for i := 0; i < len(slice)/4; i++ {
+		slice6 = append(slice6, strings.Join(slice[i*4:i*4+4], ""))
+	}
+	ip := net.ParseIP(strings.Join(slice6, ":")).To16()
+	if ip == nil {
+		return ""
+	}
+	return ip.String()
 }
 
 const (