diff --git a/src/http/modules/ngx_http_not_modified_filter_module.c b/src/http/modules/ngx_http_not_modified_filter_module.c
index 389a3a9..feed9cf 100644
--- a/src/http/modules/ngx_http_not_modified_filter_module.c
+++ b/src/http/modules/ngx_http_not_modified_filter_module.c
@@ -61,6 +61,12 @@
         return ngx_http_next_header_filter(r);
     }
 
+    clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);
+
+    if (clcf->if_modified_since == NGX_HTTP_IMS_OFF) {
+        return ngx_http_next_header_filter(r);
+    }
+
     ims = ngx_http_parse_time(r->headers_in.if_modified_since->value.data,
                               r->headers_in.if_modified_since->value.len);
 
@@ -69,9 +75,7 @@
 
     if (ims != r->headers_out.last_modified_time) {
 
-        clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);
-
-        if (clcf->if_modified_since == 0
+        if (clcf->if_modified_since == NGX_HTTP_IMS_EXACT
             || ims < r->headers_out.last_modified_time)
         {
             return ngx_http_next_header_filter(r);
diff --git a/src/http/ngx_http_core_module.c b/src/http/ngx_http_core_module.c
index 9890fbb..a21da0f 100644
--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -112,8 +112,9 @@
 
 
 static ngx_conf_enum_t  ngx_http_core_if_modified_since[] = {
-    { ngx_string("exact"), 0 },
-    { ngx_string("before"), 1 },
+    { ngx_string("off"), NGX_HTTP_IMS_OFF },
+    { ngx_string("exact"), NGX_HTTP_IMS_EXACT },
+    { ngx_string("before"), NGX_HTTP_IMS_BEFORE },
     { ngx_null_string, 0 }
 };
 
@@ -3052,7 +3053,7 @@
     ngx_conf_merge_uint_value(conf->satisfy, prev->satisfy,
                               NGX_HTTP_SATISFY_ALL);
     ngx_conf_merge_uint_value(conf->if_modified_since, prev->if_modified_since,
-                              0);
+                              NGX_HTTP_IMS_EXACT);
     ngx_conf_merge_value(conf->internal, prev->internal, 0);
     ngx_conf_merge_value(conf->client_body_in_file_only,
                               prev->client_body_in_file_only, 0);
diff --git a/src/http/ngx_http_core_module.h b/src/http/ngx_http_core_module.h
index 46acb2b..21a5355 100644
--- a/src/http/ngx_http_core_module.h
+++ b/src/http/ngx_http_core_module.h
@@ -28,6 +28,11 @@
 #define NGX_HTTP_SATISFY_ANY            1
 
 
+#define NGX_HTTP_IMS_OFF                0
+#define NGX_HTTP_IMS_EXACT              1
+#define NGX_HTTP_IMS_BEFORE             2
+
+
 typedef struct ngx_http_location_tree_node_s  ngx_http_location_tree_node_t;
 typedef struct ngx_http_core_loc_conf_s  ngx_http_core_loc_conf_t;