dhcpserver: make subnet mask configurable

Closes https://github.com/espressif/esp-idf/issues/4988

components/lwip/apps/dhcpserver/dhcpserver.c

@@ -87,6 +87,13 @@ static ip4_addr_t server_address;
 static ip4_addr_t dns_server = {0};
 static ip4_addr_t client_address;        //added
 static ip4_addr_t client_address_plus;
+static ip4_addr_t s_dhcps_mask = {
+#ifdef USE_CLASS_B_NET
+        .addr = PP_HTONL(LWIP_MAKEU32(255, 240, 0, 0))
+#else
+        .addr = PP_HTONL(LWIP_MAKEU32(255, 255, 255, 0))
+#endif
+    };
 
 static list_node *plist = NULL;
 static bool renew = false;
@@ -136,7 +143,12 @@ void *dhcps_option_info(u8_t op_id, u32_t opt_len)
             }
 
             break;
+        case SUBNET_MASK:
+            if (opt_len == sizeof(s_dhcps_mask)) {
+                option_arg = &s_dhcps_mask;
+            }
 
+            break;
         default:
             break;
     }
@@ -185,6 +197,12 @@ void dhcps_set_option_info(u8_t op_id, void *opt_info, u32_t opt_len)
             }
             break;
 
+        case SUBNET_MASK:
+            if (opt_len == sizeof(s_dhcps_mask)) {
+                s_dhcps_mask = *(ip4_addr_t *)opt_info;
+            }
+
+
         default:
             break;
     }
@@ -296,21 +314,12 @@ static u8_t *add_offer_options(u8_t *optptr)
 
     ipadd.addr = *((u32_t *) &server_address);
 
-#ifdef USE_CLASS_B_NET
-    *optptr++ = DHCP_OPTION_SUBNET_MASK;
-    *optptr++ = 4;  //length
-    *optptr++ = 255;
-    *optptr++ = 240;
-    *optptr++ = 0;
-    *optptr++ = 0;
-#else
     *optptr++ = DHCP_OPTION_SUBNET_MASK;
     *optptr++ = 4;
-    *optptr++ = 255;
-    *optptr++ = 255;
-    *optptr++ = 255;
-    *optptr++ = 0;
-#endif
+    *optptr++ = ip4_addr1(&s_dhcps_mask);
+    *optptr++ = ip4_addr2(&s_dhcps_mask);
+    *optptr++ = ip4_addr3(&s_dhcps_mask);
+    *optptr++ = ip4_addr4(&s_dhcps_mask);
 
     *optptr++ = DHCP_OPTION_LEASE_TIME;
     *optptr++ = 4;
@@ -357,21 +366,13 @@ static u8_t *add_offer_options(u8_t *optptr)
         *optptr++ = ip4_addr4(&ipadd);
     }
 
-#ifdef CLASS_B_NET
-    *optptr++ = DHCP_OPTION_BROADCAST_ADDRESS;
-    *optptr++ = 4;
-    *optptr++ = ip4_addr1(&ipadd);
-    *optptr++ = 255;
-    *optptr++ = 255;
-    *optptr++ = 255;
-#else
+    ip4_addr_t broadcast_addr = { .addr = (ipadd.addr & s_dhcps_mask.addr) | ~s_dhcps_mask.addr };
     *optptr++ = DHCP_OPTION_BROADCAST_ADDRESS;
     *optptr++ = 4;
-    *optptr++ = ip4_addr1(&ipadd);
-    *optptr++ = ip4_addr2(&ipadd);
-    *optptr++ = ip4_addr3(&ipadd);
-    *optptr++ = 255;
-#endif
+    *optptr++ = ip4_addr1(&broadcast_addr);
+    *optptr++ = ip4_addr2(&broadcast_addr);
+    *optptr++ = ip4_addr3(&broadcast_addr);
+    *optptr++ = ip4_addr4(&broadcast_addr);
 
     *optptr++ = DHCP_OPTION_INTERFACE_MTU;
     *optptr++ = 2;