From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jason Opperisano <opie@817west.com>
Subject: Re: broken SNAT with fixed external MTU
Date: Thu, 14 Oct 2004 14:17:47 -0400
Sender: netfilter-bounces@lists.netfilter.org
Message-ID: <20041014181747.GA3929@bender.817west.com>
References: <416EA61A.9030600@lucomp.net>
Mime-Version: 1.0
Return-path: <netfilter-bounces@lists.netfilter.org>
Content-Disposition: inline
In-Reply-To: <416EA61A.9030600@lucomp.net>
List-Id: <netfilter.vger.kernel.org>
List-Unsubscribe: <https://lists.netfilter.org/mailman/listinfo/netfilter>,
	<mailto:netfilter-request@lists.netfilter.org?subject=unsubscribe>
List-Archive: </pipermail/netfilter>
List-Post: <mailto:netfilter@lists.netfilter.org>
List-Help: <mailto:netfilter-request@lists.netfilter.org?subject=help>
List-Subscribe: <https://lists.netfilter.org/mailman/listinfo/netfilter>,
	<mailto:netfilter-request@lists.netfilter.org?subject=subscribe>
Errors-To: netfilter-bounces@lists.netfilter.org
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
To: netfilter@lists.netfilter.org

On Thu, Oct 14, 2004 at 06:15:22PM +0200, Mailing List Reader wrote:
> Hello everybody,
> 
> My DSL router (very bad firmwares) wants me to force the MTU
> (buggy MTU auto-setting). The firewall has a public IP and mtu 1500:
> 
> ---snip
> #route to the router net
> ip route add 192.168.1.0/24 dev eth1 mtu 1440
> # default to the router (mtu not needed here)
> ip route add default via 192.168.1.1 mtu 1440
> --snip
> 
> the firewall itself NATs one client back-to-back connected
> (cable checked!):
> 
> ---snip
> #mss clamping 1)
> iptables -A FORWARD -d $net1 -p tcp --tcp-flags SYN,RST SYN -j TCPMSS 
> --set-mss 1440
> iptables -A FORWARD -s $net1 -p tcp --tcp-flags SYN,RST SYN -j TCPMSS 
> --set-mss 1440

try not to confuse MTU with MSS, as they are not the same.  if you're
going for an MTU of 1440--you should be setting MSS to 1400 (MSS = MTU -
40).

-j

-- 
Jason Opperisano <opie@817west.com>