From owner-freebsd-net@freebsd.org  Fri Jan 20 20:59:34 2017
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To: "Kristof Provost" <kp@FreeBSD.org>
cc: "Bakul Shah" <bakul@bitblocks.com>, "Alan Somers" <asomers@freebsd.org>,
 "FreeBSD Net" <freebsd-net@freebsd.org>
Subject: Re: pf & NAT issue
In-reply-to: Your message of "Fri, 20 Jan 2017 21:43:33 +0100."
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Comments: In-reply-to "Kristof Provost" <kp@FreeBSD.org>
 message dated "Fri, 20 Jan 2017 21:43:33 +0100."
Date: Fri, 20 Jan 2017 12:59:33 -0800
From: Bakul Shah <bakul@bitblocks.com>
Message-Id: <20170120205933.8948A124AEA3@mail.bitblocks.com>
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On Fri, 20 Jan 2017 21:43:33 +0100 "Kristof Provost" <kp@FreeBSD.org> wrote:
> On 20 Jan 2017, at 21:31, Bakul Shah wrote:
> >> 11:56:28.168693 IP 192.168.125.7.65042 > 149.20.1.200.21: Flags [P.], 
> >> seq 1:10, ack 55, win 1026, options [nop,nop,TS val 198426 ecr 
> >> 1468113725], length 9
> > < 11:56:28.168712 IP 173.228.5.8.52015 > 149.20.1.200.21: Flags [P.], 
> > seq 3080825147:3080825156, ack 3912707414, win 1026, options 
> > [nop,nop,TS val 198426 ecr 1468113725], length 9
> >
> > 	Right here we see the problem. NAT mapping for the
> > 	port changed from 63716 to 52015.
> >
> Changing source ports is an entirely normal NAT behaviour.
> 
> The best explanation is this: imagine that you have two clients A and B, 
> both connect to X on port 80 via the NAT gateway G.
> Both use port 1000 as their source port.
> A connects, and the gateway maps A:1000 -> X:80 to G:1000 -> X:80.
> B connects, and now the gateway has to map B:1000 -> X:80 onto G:1000 -> 
> X:80, but then it wouldn't be able to tell the two connections apart.
> That't can remap it onto G:1001 -> X:80 instead.

It is the same connection!  As a tcp connection is identified
by <src ip, src port, dst ip, dst port>, If the port number
changes on the same connection, the remote side would see this
as a separate connection.