From mboxrd@z Thu Jan 1 00:00:00 1970 From: Patrick McHardy Subject: Re: SYN,URG combination considered INVALID by tcp conntrack Date: Wed, 06 Sep 2006 20:20:58 +0200 Message-ID: <44FF118A.3090502@trash.net> References: <20060906144746.446d813c.m.posthuma@sangine.com> Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Cc: netfilter-devel@lists.netfilter.org Return-path: To: Martijn Posthuma In-Reply-To: <20060906144746.446d813c.m.posthuma@sangine.com> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: netfilter-devel-bounces@lists.netfilter.org Errors-To: netfilter-devel-bounces@lists.netfilter.org List-Id: netfilter-devel.vger.kernel.org Please break your lines at 72 characters .. Martijn Posthuma wrote: > We have been experiencing problems with tcp connection establishment when (for some reason) the URG flag and pointer were set in the SYN segment. The problem did not affect all machines that were contacted but a subset. All affected machines were using netfilter-conntrack and firewalling. Inspection of the conntrack code (on a 2.6 linux kernel source) reveiled that the above mentioned combination of flags is not considered valid. The code I am referring to is in nf_conntrack_proto_tcp.c version 2.2, lines 768 to 795: Not sure if I understand you correctly, did the machines running connection tracking emit these packets? > As you can see the SYN,URG combination is not considered valid. I can not find any justification for this in RFC793. I do realize that it is a rare combination of flags, but legal none the less. (In the RFC there is even an explicit mention of the fact that a SYN segment can contain data, so an URG flag is perfectly feasible in that case). > > Does someone know of a good reason that this flag combination is excluded? No, I guess it should be considered valid. Out of interest, did the SYN packets actually carry data?