From 79a5c920a40180b7291d94535e50017d98eb3a63 Mon Sep 17 00:00:00 2001
From: Simon Zolin <s.zolin@adguard.com>
Date: Wed, 17 Jul 2019 11:55:21 +0300
Subject: [PATCH] * dhcpd: check if subnet mask is correct

---
 dhcpd/dhcpd.go      |  2 +-
 dhcpd/dhcpd_test.go | 12 ++++++++++++
 dhcpd/helpers.go    | 17 +++++++++++++++++
 3 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/dhcpd/dhcpd.go b/dhcpd/dhcpd.go
index 9ad0b08f..391389d4 100644
--- a/dhcpd/dhcpd.go
+++ b/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)
 	}
 
diff --git a/dhcpd/dhcpd_test.go b/dhcpd/dhcpd_test.go
index e5e16b0d..01fafb84 100644
--- a/dhcpd/dhcpd_test.go
+++ b/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})")
+	}
+}
diff --git a/dhcpd/helpers.go b/dhcpd/helpers.go
index c3e9f4fe..9553fca9 100644
--- a/dhcpd/helpers.go
+++ b/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
+}