--- a/src/http/modules/ngx_http_not_modified_filter_module.c
+++ b/src/http/modules/ngx_http_not_modified_filter_module.c
@@ -9,7 +9,8 @@
 #include <ngx_http.h>
 
 
-
+static ngx_int_t ngx_http_test_precondition(ngx_http_request_t *r);
+static ngx_int_t ngx_http_test_not_modified(ngx_http_request_t *r);
 static ngx_int_t ngx_http_not_modified_filter_init(ngx_conf_t *cf);
 
 
@@ -50,17 +51,51 @@ static ngx_http_output_header_filter_pt 
 static ngx_int_t
 ngx_http_not_modified_header_filter(ngx_http_request_t *r)
 {
-    time_t                     ims;
-    ngx_http_core_loc_conf_t  *clcf;
-
     if (r->headers_out.status != NGX_HTTP_OK
         || r != r->main
-        || r->headers_in.if_modified_since == NULL
         || r->headers_out.last_modified_time == -1)
     {
         return ngx_http_next_header_filter(r);
     }
 
+    if (r->headers_in.if_unmodified_since) {
+        return ngx_http_test_precondition(r);
+    }
+
+    if (r->headers_in.if_modified_since) {
+        return ngx_http_test_not_modified(r);
+    }
+
+    return ngx_http_next_header_filter(r);
+}
+
+
+static ngx_int_t
+ngx_http_test_precondition(ngx_http_request_t *r)
+{
+    time_t  iums;
+
+    iums = ngx_http_parse_time(r->headers_in.if_unmodified_since->value.data,
+                               r->headers_in.if_unmodified_since->value.len);
+
+    ngx_log_debug2(NGX_LOG_DEBUG_HTTP, r->connection->log, 0,
+                 "http iums:%d lm:%d", iums, r->headers_out.last_modified_time);
+
+    if (iums >= r->headers_out.last_modified_time) {
+        return ngx_http_next_header_filter(r);
+    }
+
+    return ngx_http_filter_finalize_request(r, NULL,
+                                            NGX_HTTP_PRECONDITION_FAILED);
+}
+
+
+static ngx_int_t
+ngx_http_test_not_modified(ngx_http_request_t *r)
+{
+    time_t                     ims;
+    ngx_http_core_loc_conf_t  *clcf;
+
     clcf = ngx_http_get_module_loc_conf(r, ngx_http_core_module);
 
     if (clcf->if_modified_since == NGX_HTTP_IMS_OFF) {