From owner-freebsd-net@freebsd.org  Fri Jan 20 20:43:37 2017
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From: "Kristof Provost" <kp@FreeBSD.org>
To: "Bakul Shah" <bakul@bitblocks.com>
Cc: "Alan Somers" <asomers@freebsd.org>,
 "FreeBSD Net" <freebsd-net@freebsd.org>
Subject: Re: pf & NAT issue
Date: Fri, 20 Jan 2017 21:43:33 +0100
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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’s why it can remap it onto G:1001 -> X:80 instead.

Regards,
Kristof