From mboxrd@z Thu Jan 1 00:00:00 1970 From: Patrick McHardy Subject: Re: [PATCH] netfilter: nf_nat: avoid double nat for loopback Date: Tue, 07 Jun 2011 11:37:43 +0200 Message-ID: <4DEDF167.1010202@trash.net> References: Mime-Version: 1.0 Content-Type: text/plain; charset=ISO-8859-15 Content-Transfer-Encoding: 7bit Cc: Pablo Neira Ayuso , netfilter-devel@vger.kernel.org, netdev@vger.kernel.org To: Julian Anastasov Return-path: In-Reply-To: Sender: netfilter-devel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On 04.06.2011 16:02, Julian Anastasov wrote: > > Avoid double NAT and seq adjustment for loopback > traffic because it causes silent repetition of TCP data. One > example is passive FTP with DNAT rule and difference in the > length of IP addresses. > > This patch adds checks if packet is sent and > received via loopback device. As the same conntrack is used > both for outgoing and incoming direction, we restrict NAT, > seq adjustment and confirmation to happen only in > outgoing direction (OUTPUT and POSTROUTING). > > Signed-off-by: Julian Anastasov > --- > > As the check is not so cheap, another alternative > is to add new skb flag, eg. "loopback", that can be set in > drivers/net/loopback.c, loopback_xmit(). May be there is space > for it in flags2? I don't think we should be adding code specifically needed for netfilter to the loopback driver if we can avoid it. I don't think we need to actually avoid calling nf_nat_packet twice, that shouldn't do any harm, just the sequence number adjustment. So we could add the loopback check to the IPS_SEQ_ADJUST_BIT case to at least avoid it in some cases. Would that work or am I missing something?