diff --git a/src/core/ngx_string.h b/src/core/ngx_string.h
index 81e1ea1..0525b0e 100644
--- a/src/core/ngx_string.h
+++ b/src/core/ngx_string.h
@@ -158,6 +158,7 @@
 ngx_int_t ngx_rstrncmp(u_char *s1, u_char *s2, size_t n);
 ngx_int_t ngx_rstrncasecmp(u_char *s1, u_char *s2, size_t n);
 ngx_int_t ngx_memn2cmp(u_char *s1, u_char *s2, size_t n1, size_t n2);
+ngx_int_t ngx_dns_strcmp(u_char *s1, u_char *s2);
 
 ngx_int_t ngx_atoi(u_char *line, size_t n);
 ssize_t ngx_atosz(u_char *line, size_t n);