* dhcpd: check if subnet mask is correct

dhcpd/helpers.go

@@ -1,6 +1,7 @@
 package dhcpd
 
 import (
+	"encoding/binary"
 	"fmt"
 	"net"
 
@@ -65,3 +66,19 @@ func parseIPv4(text string) (net.IP, error) {
 	}
 	return result.To4(), nil
 }
+
+// Return TRUE if subnet mask is correct (e.g. 255.255.255.0)
+func isValidSubnetMask(mask net.IP) bool {
+	var n uint32
+	n = binary.BigEndian.Uint32(mask)
+	for i := 0; i != 32; i++ {
+		if n == 0 {
+			break
+		}
+		if (n & 0x80000000) == 0 {
+			return false
+		}
+		n <<= 1
+	}
+	return true
+}