--- a/src/http/ngx_http_core_module.c
+++ b/src/http/ngx_http_core_module.c
@@ -114,6 +114,14 @@ static ngx_conf_enum_t  ngx_http_core_sa
 };
 
 
+static ngx_conf_enum_t  ngx_http_core_if_modified_since[] = {
+    { 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 }
+};
+
+
 #if (NGX_HTTP_GZIP)
 
 static ngx_conf_enum_t  ngx_http_gzip_http_version[] = {
@@ -507,6 +515,13 @@ static ngx_command_t  ngx_http_core_comm
       offsetof(ngx_http_core_loc_conf_t, server_tokens),
       NULL },
 
+    { ngx_string("if_modified_since"),
+      NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_CONF_TAKE1,
+      ngx_conf_set_enum_slot,
+      NGX_HTTP_LOC_CONF_OFFSET,
+      offsetof(ngx_http_core_loc_conf_t, if_modified_since),
+      &ngx_http_core_if_modified_since },
+
     { ngx_string("error_page"),
       NGX_HTTP_MAIN_CONF|NGX_HTTP_SRV_CONF|NGX_HTTP_LOC_CONF|NGX_HTTP_LIF_CONF
                         |NGX_CONF_2MORE,
@@ -2684,6 +2699,7 @@ ngx_http_core_create_loc_conf(ngx_conf_t
     lcf->client_body_buffer_size = NGX_CONF_UNSET_SIZE;
     lcf->client_body_timeout = NGX_CONF_UNSET_MSEC;
     lcf->satisfy = NGX_CONF_UNSET_UINT;
+    lcf->if_modified_since = NGX_CONF_UNSET_UINT;
     lcf->internal = NGX_CONF_UNSET;
     lcf->client_body_in_file_only = NGX_CONF_UNSET;
     lcf->sendfile = NGX_CONF_UNSET;
@@ -2870,6 +2886,8 @@ ngx_http_core_merge_loc_conf(ngx_conf_t 
 
     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,
+                              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);