* dhcpd: check if subnet mask is correct

dhcpd/dhcpd.go

@@ -127,7 +127,7 @@ func (s *Server) setConfig(config ServerConfig) error {
 	}
 
 	subnet, err := parseIPv4(config.SubnetMask)
-	if err != nil {
+	if err != nil || !isValidSubnetMask(subnet) {
 		return wrapErrPrint(err, "Failed to parse subnet mask %s", config.SubnetMask)
 	}
 

dhcpd/dhcpd_test.go

@@ -197,3 +197,15 @@ func TestDB(t *testing.T) {
 
 	os.Remove("leases.db")
 }
+
+func TestIsValidSubnetMask(t *testing.T) {
+	if !isValidSubnetMask([]byte{255, 255, 255, 0}) {
+		t.Fatalf("isValidSubnetMask([]byte{255,255,255,0})")
+	}
+	if isValidSubnetMask([]byte{255, 255, 253, 0}) {
+		t.Fatalf("isValidSubnetMask([]byte{255,255,253,0})")
+	}
+	if isValidSubnetMask([]byte{0, 255, 255, 255}) {
+		t.Fatalf("isValidSubnetMask([]byte{255,255,253,0})")
+	}
+}

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
+}