diff --git a/include/proto/dns.h b/include/proto/dns.h
index 328298e5b1..80887e86c3 100644
--- a/include/proto/dns.h
+++ b/include/proto/dns.h
@@ -25,7 +25,7 @@
 #include <types/dns.h>
 #include <types/proto_udp.h>
 
-char *dns_str_to_dn_label(char *string, char *dn, int dn_len);
+char *dns_str_to_dn_label(const char *string, char *dn, int dn_len);
 int dns_str_to_dn_label_len(const char *string);
 int dns_hostname_validation(const char *string, char **err);
 int dns_build_query(int query_id, int query_type, char *hostname_dn, int hostname_dn_len, char *buf, int bufsize);
diff --git a/src/dns.c b/src/dns.c
index 3d54b95f65..37e041cf0b 100644
--- a/src/dns.c
+++ b/src/dns.c
@@ -928,7 +928,7 @@ int dns_build_query(int query_id, int query_type, char *hostname_dn, int hostnam
  * In the second case, memory will be allocated.
  * in case of error, -1 is returned, otherwise, number of bytes copied in dn
  */
-char *dns_str_to_dn_label(char *string, char *dn, int dn_len)
+char *dns_str_to_dn_label(const char *string, char *dn, int dn_len)
 {
 	char *c, *d;
 	int i, offset;