1 files changed, 40 insertions, 0 deletions
diff --git a/middleware/pkg/dnsutil/reverse.go b/middleware/pkg/dnsutil/reverse.go
new file mode 100644
index 000000000..a360432f3
--- /dev/null
+++ b/middleware/pkg/dnsutil/reverse.go
@@ -0,0 +1,40 @@
+package dnsutil
+
+import "strings"
+
+// ExtractAddressFromReverse turns a standard PTR reverse record name
+// into an IP address. This works for ipv4 or ipv6.
+//
+// 54.119.58.176.in-addr.arpa. becomes 176.58.119.54. If the conversion
+// failes the empty string is returned.
+func ExtractAddressFromReverse(reverseName string) string {
+	search := ""
+
+	switch {
+	case strings.HasSuffix(reverseName, v4arpaSuffix):
+		search = strings.TrimSuffix(reverseName, v4arpaSuffix)
+	case strings.HasSuffix(reverseName, v6arpaSuffix):
+		search = strings.TrimSuffix(reverseName, v6arpaSuffix)
+	default:
+		return ""
+	}
+
+	// Reverse the segments and then combine them.
+	segments := reverse(strings.Split(search, "."))
+	return strings.Join(segments, ".")
+}
+
+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
+}
+
+const (
+	// v4arpaSuffix is the reverse tree suffix for v4 IP addresses.
+	v4arpaSuffix = ".in-addr.arpa."
+	// v6arpaSuffix is the reverse tree suffix for v6 IP addresses.
+	v6arpaSuffix = ".ip6.arpa."
+)