From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-10.8 required=3.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id B6423C433B4 for ; Wed, 28 Apr 2021 02:29:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8FC4E613B1 for ; Wed, 28 Apr 2021 02:29:06 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S238082AbhD1C3t (ORCPT ); Tue, 27 Apr 2021 22:29:49 -0400 Received: from mail.as397444.net ([69.59.18.99]:35338 "EHLO mail.as397444.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S235901AbhD1C3s (ORCPT ); Tue, 27 Apr 2021 22:29:48 -0400 Received: by mail.as397444.net (Postfix) with UTF8SMTPSA id 530FE559C09; Wed, 28 Apr 2021 02:29:03 +0000 (UTC) X-DKIM-Note: Keys used to sign are likely public at https://as397444.net/dkim/ DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=mattcorallo.com; s=1619575263; t=1619576943; bh=uoElVMvzWf0yTWv6ifdIp3u4Ka8rvsdR/7kJRQft+Kk=; h=Date:To:Cc:From:Subject:From; b=xWFUiR+wsR8dwvp+aF43MOK8jGf+RyUIepieiQrKJtaUtanpyalnkpsN3eDDFCmXy nw+KJeKkAZ/0h47MyHNPlsEI2qKD5OFs7qw3pado16Qr2VB6gR1P8TtK8ZX1BDXxET E+tnBoaylilex6Uj7PYoYXKaNCgl4GzJSnb3mVErslUE1xRWYMv/DaWPr/6yCQ9E7z +sf2RkPlBO0qms2/EELGXtOiZb4RzVvJ+yhAkGlO6MeA3WMq0xXo4i5FA8niBQkaHZ riZQkCAapNUuV2h4uXDBAQM2LfH+RyC74SLxbqwWDSD35hD/TWxK0aE3cBSXY8sw5n FAVySmUG2xvzA== Message-ID: Date: Tue, 27 Apr 2021 22:29:03 -0400 MIME-Version: 1.0 Content-Language: en-US To: "David S. Miller" , netdev@vger.kernel.org, Alexey Kuznetsov , Hideaki YOSHIFUJI Cc: Eric Dumazet , Willy Tarreau , Keyu Man From: Matt Corallo Subject: [PATCH net-next] Reduce IP_FRAG_TIME fragment-reassembly timeout to 1s, from 30s Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: netdev@vger.kernel.org The default IP reassembly timeout of 30 seconds predates git history (and cursory web searches turn up nothing related to it). The only relevant source cited in net/ipv4/ip_fragment.c is RFC 791 defining IPv4 in 1981. RFC 791 suggests allowing the timer to increase on the receipt of each fragment (which Linux deliberately does not do), with a default timeout for each fragment of 15 seconds. It suggests 15s to cap a 10Kb/s flow to a 150Kb buffer of fragments. When Linux receives a fragment, if the total memory used for the fragment reassembly buffer (across all senders) exceeds net.ipv4.ipfrag_high_thresh (or the equivalent for IPv6), it silently drops all future fragments fragments until the timers on the original expire. All the way in 2021, these numbers feel almost comical. The default buffer size for fragmentation reassembly is hard-coded at 4MiB as `net->ipv4.fqdir->high_thresh = 4 * 1024 * 1024;` capping a host at 1.06Mb/s of lost fragments before all fragments received on the host are dropped (with independent limits for IPv6). At 1.06Mb/s of lost fragments, we move from DoS attack threshold to real-world scenarios - at moderate loss rates on consumer networks today its fairly easy to hit this, causing end hosts with their MTU (mis-)configured to fragment to have nearly 10-20 second blocks of 100% packet loss. Reducing the default fragment timeout to 1sec gives us 32Mb/s of fragments before we drop all fragments, which is certainly more in line with today's network speeds than 1.06Mb/s, though an optimal value may be still lower. Sadly, reducing it further requires a change to the sysctl interface, as net.ipv4.ipfrag_time is only specified in seconds. --- include/net/ip.h | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/include/net/ip.h b/include/net/ip.h index 2d6b985d11cc..f1473ac5a27c 100644 --- a/include/net/ip.h +++ b/include/net/ip.h @@ -135,7 +135,7 @@ struct ip_ra_chain { #define IP_MF 0x2000 /* Flag: "More Fragments" */ #define IP_OFFSET 0x1FFF /* "Fragment Offset" part */ -#define IP_FRAG_TIME (30 * HZ) /* fragment lifetime */ +#define IP_FRAG_TIME (1 * HZ) /* fragment lifetime */ struct msghdr; struct net_device; -- 2.30.2