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Date: Thu, 9 Jun 2016 16:06:01 +0300
From: Slawa Olhovchenkov <slw@zxy.spb.ru>
To: Kristof Provost <kp@FreeBSD.org>
Cc: stable@freebsd.org, freebsd-net@freebsd.org
Subject: Re: ipfw fwd to closed port
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References: <20160608230240.GA51364@zxy.spb.ru>
 <20160609130017.GA4071@vega.codepro.be>
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On Thu, Jun 09, 2016 at 03:00:17PM +0200, Kristof Provost wrote:

> On 2016-06-09 02:02:40 (+0300), Slawa Olhovchenkov <slw@zxy.spb.ru> wrote:
> > Forwarding by ipfw to closed local port generating RST packet with
> > incorrect checksun. Is this know ussuse? Need open PR?
> 
> Where did you capture the packet? If you've captured the packet on the
> machine that generated it tcpdump may indeed claim that the checksum is
> wrong, because it's computed by the hardware (so after tcpdump captured
> it).

On the tun0 (destination of RST packet routed to tun0).
tun0: flags=8051<UP,POINTOPOINT,RUNNING,MULTICAST> metric 0 mtu 1500
        options=80000<LINKSTATE>
        inet 192.168.4.1 --> 192.168.4.1 netmask 0xffffff00 
        inet6 fe80::240:63ff:fedc:ac9e%tun0 prefixlen 64 scopeid 0x9 
        nd6 options=21<PERFORMNUD,AUTO_LINKLOCAL>
        Opened by PID 1345

tun0 don't computed checksum.