diff --git a/src/core/dns.c b/src/core/dns.c
index 11df6922..866319a9 100644
--- a/src/core/dns.c
+++ b/src/core/dns.c
@@ -96,7 +96,6 @@
 #include "lwip/prot/dns.h"
 
 #include <string.h>
-#include <ctype.h>
 
 /** Random generator function to create random TXIDs and source ports for queries */
 #ifndef DNS_RAND_TXID
@@ -667,7 +666,7 @@ dns_compare_name(const char *query, struct pbuf *p, u16_t start_offset)
         if (c < 0) {
           return 0xFFFF;
         }
-        if (tolower((*query)) != tolower((u8_t)c)) {
+        if (lwip_tolower((*query)) != lwip_tolower((u8_t)c)) {
           return 0xFFFF;
         }
         if (response_offset == 0xFFFF) {
diff --git a/src/include/lwip/arch.h b/src/include/lwip/arch.h
index 069060ff..b4181610 100644
--- a/src/include/lwip/arch.h
+++ b/src/include/lwip/arch.h
@@ -221,12 +221,15 @@ typedef int ssize_t;
 #define lwip_isxdigit(c)          (lwip_isdigit(c) || lwip_in_range((c), 'a', 'f') || lwip_in_range((c), 'A', 'F'))
 #define lwip_islower(c)           lwip_in_range((c), 'a', 'z')
 #define lwip_isspace(c)           ((c) == ' ' || (c) == '\f' || (c) == '\n' || (c) == '\r' || (c) == '\t' || (c) == '\v')
+#define lwip_isupper(c)           lwip_in_range((c), 'A', 'Z')
+#define lwip_tolower(c)           (lwip_isupper(c) ? (c) - 'A' + 'a' : c)
 #else
 #include <ctype.h>
 #define lwip_isdigit(c)           isdigit((unsigned char)(c))
 #define lwip_isxdigit(c)          isxdigit((unsigned char)(c))
 #define lwip_islower(c)           islower((unsigned char)(c))
 #define lwip_isspace(c)           isspace((unsigned char)(c))
+#define lwip_tolower(c)           tolower(c)
 #endif
 
 /** C++ const_cast<target_type>(val) equivalent to remove constness from a value (GCC -Wcast-qual) */