diff --git a/src/core/dns.c b/src/core/dns.c
index d18c51f6..3529844d 100644
--- a/src/core/dns.c
+++ b/src/core/dns.c
@@ -653,7 +653,7 @@ dns_compare_name(const char *query, struct pbuf* p, u16_t start_offset)
 
   do {
     n = pbuf_try_get_at(p, response_offset++);
-    if (n < 0) {
+    if ((n < 0) || (response_offset == 0)) {
       return 0xFFFF;
     }
     /** @see RFC 1035 - 4.1.4. Message compression */
@@ -671,6 +671,9 @@ dns_compare_name(const char *query, struct pbuf* p, u16_t start_offset)
           return 0xFFFF;
         }
         ++response_offset;
+        if (response_offset == 0) {
+          return 0xFFFF;
+        }
         ++query;
         --n;
       }
@@ -682,7 +685,10 @@ dns_compare_name(const char *query, struct pbuf* p, u16_t start_offset)
     }
   } while (n != 0);
 
-  return response_offset + 1;
+  if (response_offset == 0xFFFF) {
+    return 0xFFFF;
+  }
+  return (u16_t)(response_offset + 1);
 }
 
 /**
@@ -700,7 +706,7 @@ dns_skip_name(struct pbuf* p, u16_t query_idx)
 
   do {
     n = pbuf_try_get_at(p, offset++);
-    if (n < 0) {
+    if ((n < 0) || (offset == 0)) {
       return 0xFFFF;
     }
     /** @see RFC 1035 - 4.1.4. Message compression */
@@ -720,7 +726,10 @@ dns_skip_name(struct pbuf* p, u16_t query_idx)
     }
   } while (n != 0);
 
-  return offset + 1;
+  if (offset == 0xFFFF) {
+    return 0xFFFF;
+  }
+  return (u16_t)(offset + 1);
 }
 
 /**
@@ -782,9 +791,13 @@ dns_send(u8_t idx)
         ++n;
       }
       copy_len = (u16_t)(hostname - hostname_part);
+      if (query_idx + n + 1 > 0xFFFF) {
+        /* u16_t overflow */
+        goto overflow_return;
+      }
       pbuf_put_at(p, query_idx, n);
-      pbuf_take_at(p, hostname_part, copy_len, query_idx + 1);
-      query_idx += n + 1;
+      pbuf_take_at(p, hostname_part, copy_len, (u16_t)(query_idx + 1));
+      query_idx = (u16_t)(query_idx + n + 1);
     } while (*hostname != 0);
     pbuf_put_at(p, query_idx, 0);
     query_idx++;
@@ -838,6 +851,9 @@ dns_send(u8_t idx)
   }
 
   return err;
+overflow_return:
+  pbuf_free(p);
+  return ERR_VAL;
 }
 
 #if ((LWIP_DNS_SECURE & LWIP_DNS_SECURE_RAND_SRC_PORT) != 0)
@@ -895,9 +911,9 @@ dns_alloc_pcb(void)
     }
   }
   /* if we come here, creating a new UDP pcb failed, so we have to use
-     an already existing one */
-  for (i = 0, idx = dns_last_pcb_idx + 1; i < DNS_MAX_SOURCE_PORTS; i++, idx++) {
-    if (idx >= DNS_MAX_SOURCE_PORTS) {
+     an already existing one (so overflow is no issue) */
+  for (i = 0, idx = (u8_t)(dns_last_pcb_idx + 1); i < DNS_MAX_SOURCE_PORTS; i++, idx++) {
+    if (idx >= LWIP_MAX(0xFF, DNS_MAX_SOURCE_PORTS)) {
       idx = 0;
     }
     if (dns_pcbs[idx] != NULL) {
@@ -1203,7 +1219,10 @@ dns_recv(void *arg, struct udp_pcb *pcb, struct pbuf *p, const ip_addr_t *addr,
           goto memerr; /* ignore this packet */
         }
         /* skip the rest of the "question" part */
-        res_idx += SIZEOF_DNS_QUERY;
+        if (res_idx + SIZEOF_DNS_QUERY > 0xFFFF) {
+          goto memerr;
+        }
+        res_idx = (u16_t)(res_idx + SIZEOF_DNS_QUERY);
 
         /* Check for error. If so, call callback to inform. */
         if (hdr.flags2 & DNS_FLAG2_ERR_MASK) {
@@ -1220,7 +1239,10 @@ dns_recv(void *arg, struct udp_pcb *pcb, struct pbuf *p, const ip_addr_t *addr,
             if (pbuf_copy_partial(p, &ans, SIZEOF_DNS_ANSWER, res_idx) != SIZEOF_DNS_ANSWER) {
               goto memerr; /* ignore this packet */
             }
-            res_idx += SIZEOF_DNS_ANSWER;
+            if (res_idx + SIZEOF_DNS_ANSWER > 0xFFFF) {
+              goto memerr;
+            }
+            res_idx = (u16_t)(res_idx + SIZEOF_DNS_ANSWER);
 
             if (ans.cls == PP_HTONS(DNS_RRCLASS_IN)) {
 #if LWIP_IPV4
@@ -1266,7 +1288,7 @@ dns_recv(void *arg, struct udp_pcb *pcb, struct pbuf *p, const ip_addr_t *addr,
             if ((int)(res_idx + lwip_htons(ans.len)) > 0xFFFF) {
               goto memerr; /* ignore this packet */
             }
-            res_idx += lwip_htons(ans.len);
+            res_idx = (u16_t)(res_idx + lwip_htons(ans.len));
             --nanswers;
           }
 #if LWIP_IPV4 && LWIP_IPV6
@@ -1362,7 +1384,7 @@ dns_enqueue(const char *name, size_t hostnamelen, dns_found_callback found,
     }
     /* check if this is the oldest completed entry */
     if (entry->state == DNS_STATE_DONE) {
-      u8_t age = dns_seqno - entry->seqno;
+      u8_t age = (u8_t)(dns_seqno - entry->seqno);
       if (age > lseq) {
         lseq = age;
         lseqi = i;
diff --git a/src/core/netif.c b/src/core/netif.c
index 72c658ce..8f5c5c21 100644
--- a/src/core/netif.c
+++ b/src/core/netif.c
@@ -161,7 +161,7 @@ netif_loopif_init(struct netif *netif)
   netif->output_ip6 = netif_loop_output_ipv6;
 #endif
 #if LWIP_LOOPIF_MULTICAST
-  netif->flags |= NETIF_FLAG_IGMP;
+  netif_set_flags(netif, NETIF_FLAG_IGMP);
 #endif
   return ERR_OK;
 }
@@ -380,7 +380,11 @@ netif_add(struct netif *netif,
       }
     } while (netif2 != NULL);
   }
-  netif_num = netif->num + 1;
+  if (netif->num == 254) {
+    netif_num = 0;
+  } else {
+    netif_num = (u8_t)(netif->num + 1);
+  }
 
   /* add this netif to the list */
   netif->next = netif_list;
@@ -724,7 +728,7 @@ void
 netif_set_up(struct netif *netif)
 {
   if (!(netif->flags & NETIF_FLAG_UP)) {
-    netif->flags |= NETIF_FLAG_UP;
+    netif_set_flags(netif, NETIF_FLAG_UP);
 
     MIB2_COPY_SYSUPTIME_TO(&netif->ts);
 
@@ -798,7 +802,7 @@ netif_set_down(struct netif *netif)
     }
 #endif
 
-    netif->flags &= ~NETIF_FLAG_UP;
+    netif_clear_flags(netif, NETIF_FLAG_UP);
     MIB2_COPY_SYSUPTIME_TO(&netif->ts);
 
 #if LWIP_IPV4 && LWIP_ARP
@@ -851,7 +855,7 @@ void
 netif_set_link_up(struct netif *netif)
 {
   if (!(netif->flags & NETIF_FLAG_LINK_UP)) {
-    netif->flags |= NETIF_FLAG_LINK_UP;
+    netif_set_flags(netif, NETIF_FLAG_LINK_UP);
 
 #if LWIP_DHCP
     dhcp_network_changed(netif);
@@ -883,7 +887,7 @@ void
 netif_set_link_down(struct netif *netif )
 {
   if (netif->flags & NETIF_FLAG_LINK_UP) {
-    netif->flags &= ~NETIF_FLAG_LINK_UP;
+    netif_clear_flags(netif, NETIF_FLAG_LINK_UP);
     NETIF_LINK_CALLBACK(netif);
 #if LWIP_NETIF_EXT_STATUS_CALLBACK
     {
@@ -963,7 +967,7 @@ netif_loop_output(struct netif *netif, struct pbuf *p)
     MIB2_STATS_NETIF_INC(stats_if, ifoutdiscards);
     return ERR_MEM;
   }
-  netif->loop_cnt_current += clen;
+  netif->loop_cnt_current = (u16_t)(netif->loop_cnt_current + clen);
 #endif /* LWIP_LOOPBACK_MAX_PBUFS */
 
   /* Copy the whole pbuf queue p into the single pbuf r */
@@ -1067,7 +1071,7 @@ netif_poll(struct netif *netif)
     /* adjust the number of pbufs on queue */
     LWIP_ASSERT("netif->loop_cnt_current underflow",
       ((netif->loop_cnt_current - clen) < netif->loop_cnt_current));
-    netif->loop_cnt_current -= clen;
+    netif->loop_cnt_current = (u16_t)(netif->loop_cnt_current - clen);
 #endif /* LWIP_LOOPBACK_MAX_PBUFS */
 
     /* 'in_end' now points to the last pbuf from 'in' */
@@ -1126,8 +1130,11 @@ netif_alloc_client_data_id(void)
   u8_t result = netif_client_id;
   netif_client_id++;
 
+#if LWIP_NUM_NETIF_CLIENT_DATA > 256
+#error LWIP_NUM_NETIF_CLIENT_DATA must be <= 256
+#endif
   LWIP_ASSERT("Increase LWIP_NUM_NETIF_CLIENT_DATA in lwipopts.h", result < LWIP_NUM_NETIF_CLIENT_DATA);
-  return result + LWIP_NETIF_CLIENT_DATA_INDEX_MAX;
+  return (u8_t)(result + LWIP_NETIF_CLIENT_DATA_INDEX_MAX);
 }
 #endif
 
diff --git a/src/include/lwip/netif.h b/src/include/lwip/netif.h
index fc787299..13af5c44 100644
--- a/src/include/lwip/netif.h
+++ b/src/include/lwip/netif.h
@@ -427,6 +427,10 @@ void netif_set_gw(struct netif *netif, const ip4_addr_t *gw);
 #define netif_ip_gw4(netif)      ((const ip_addr_t*)&((netif)->gw))
 #endif /* LWIP_IPV4 */
 
+#define netif_set_flags(netif, set_flags)     do { (netif)->flags = (u8_t)((netif)->flags |  (set_flags)); } while(0)
+#define netif_clear_flags(netif, set_flags)   do { (netif)->flags = (u8_t)((netif)->flags & ~(set_flags)); } while(0)
+#define netif_is_flag_set(nefif, flag)        (((netif)->flags & (flag)) != 0)
+
 void netif_set_up(struct netif *netif);
 void netif_set_down(struct netif *netif);
 /** @ingroup netif
@@ -521,8 +525,8 @@ u8_t netif_name_to_index(const char *name);
 char * netif_index_to_name(u8_t idx, char *name);
 struct netif* netif_get_by_index(u8_t idx);
 
-/* Interface indexes always start at 1 per RFC 3493, section 4, num starts at 0 */
-#define netif_get_index(netif)      ((netif)->num + 1)
+/* Interface indexes always start at 1 per RFC 3493, section 4, num starts at 0 (internal index is 0..254)*/
+#define netif_get_index(netif)      ((u8_t)((netif)->num + 1))
 #define NETIF_NO_INDEX              (0)
 
 /**
diff --git a/src/include/lwip/opt.h b/src/include/lwip/opt.h
index db2322b6..c87e794c 100644
--- a/src/include/lwip/opt.h
+++ b/src/include/lwip/opt.h
@@ -1546,7 +1546,7 @@
 
 /**
  * LWIP_NUM_NETIF_CLIENT_DATA: Number of clients that may store
- * data in client_data member array of struct netif.
+ * data in client_data member array of struct netif (max. 256).
  */
 #if !defined LWIP_NUM_NETIF_CLIENT_DATA || defined __DOXYGEN__
 #define LWIP_NUM_NETIF_CLIENT_DATA            0