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=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no 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 C152EC4338F for ; Tue, 27 Jul 2021 15:10:22 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 9DEAE61B26 for ; Tue, 27 Jul 2021 15:10:22 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S232768AbhG0PKV (ORCPT ); Tue, 27 Jul 2021 11:10:21 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:57908 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S232465AbhG0PKV (ORCPT ); Tue, 27 Jul 2021 11:10:21 -0400 Received: from Chamillionaire.breakpoint.cc (Chamillionaire.breakpoint.cc [IPv6:2a0a:51c0:0:12e:520::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 36906C061757; Tue, 27 Jul 2021 08:10:21 -0700 (PDT) Received: from fw by Chamillionaire.breakpoint.cc with local (Exim 4.92) (envelope-from ) id 1m8Oio-0000Q2-Bl; Tue, 27 Jul 2021 17:10:18 +0200 Date: Tue, 27 Jul 2021 17:10:18 +0200 From: Florian Westphal To: Tom Yan Cc: netfilter@vger.kernel.org, netfilter-devel@vger.kernel.org Subject: Re: [nft] Regarding `tcp flags` (and a potential bug) Message-ID: <20210727151018.GA15121@breakpoint.cc> References: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: netfilter-devel@vger.kernel.org Tom Yan wrote: > Just noticed something that is even worse: > > # nft add rule meh tcp_flags 'tcp flags { fin, rst, ack }' > # nft add rule meh tcp_flags 'tcp flags == { fin, rst, ack }' These two are identical. > # nft add rule meh tcp_flags 'tcp flags & ( fin | rst | ack ) != 0' This matches if any one of fin/rst/ack is set. > # nft add rule meh tcp_flags 'tcp flags & ( fin | rst | ack ) == 0' This matches if fin/rst/ack are all 0 (not set). > # nft list table meh > table ip meh { > chain tcp_flags { > tcp flags { fin, rst, ack } > tcp flags { fin, rst, ack } > tcp flags fin,rst,ack > tcp flags ! fin,rst,ack > } > } Can you elaborate? This looks correct to me. > > # nft add rule meh tcp_flags 'tcp flags & (fin | syn | rst | ack) ! syn' Its unfortunate nft accepts this. The trailing ! syn is nonsensical. This is equal to tcp flags ! syn. > > # nft add rule meh tcp_flags 'tcp flags & (fin | syn | rst | ack) == syn' > > # nft add rule meh tcp_flags 'tcp flags & (fin | syn | rst | ack) != syn' > > # nft list table meh > > table ip meh { > > chain tcp_flags { > > tcp flags & (fin | syn | rst | ack) syn > > tcp flags & (fin | syn | rst | ack) ! syn > > tcp flags syn / fin,syn,rst,ack > > tcp flags syn / fin,syn,rst,ack > > } > > } > > > > I don't suppose the mask in the first two rules would matter. And with > > `tcp flags syn / fin,syn,rst,ack`, I assume it would be false when > > "syn is cleared and/or any/all of fin/rst/ack is/are set"? > > > > Also, as you can see, for the last two rules, `nft` interpreted them > > as an identical rule, which I assume to be a bug. These does NOT seem > > to workaround it either: > > > > # nft flush chain meh tcp_flags > > # nft add rule meh tcp_flags 'tcp flags == syn / fin,syn,rst,ack' > > # nft add rule meh tcp_flags 'tcp flags != syn / fin,syn,rst,ack' > > # nft list table meh > > table ip meh { > > chain tcp_flags { > > tcp flags syn / fin,syn,rst,ack > > tcp flags syn / fin,syn,rst,ack Seems the reverse translation is broken, the negation is lost. The rule is added correctly (i.e., flags == syn vs. != syn adds different rules, see nft --debug=netlink add ..