According to RFC 2616, whenever a connection to a upstream server fails due to a timeout (or DNS failure) the proxy server should return with status 504 (Gateway Timeout).
-- Davi Arnaut Index: mod_proxy_http.c =================================================================== --- mod_proxy_http.c (revision 398058) +++ mod_proxy_http.c (working copy) @@ -1689,7 +1689,7 @@ /* Step Two: Make the Connection */ if (ap_proxy_connect_backend(proxy_function, backend, worker, r->server)) { if (r->proxyreq == PROXYREQ_PROXY) - status = HTTP_NOT_FOUND; + status = HTTP_GATEWAY_TIME_OUT; else status = HTTP_SERVICE_UNAVAILABLE; goto cleanup;