Date: Fri, 20 Jan 2017 12:59:33 -0800 From: Bakul Shah <bakul@bitblocks.com> 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 Message-ID: <20170120205933.8948A124AEA3@mail.bitblocks.com> In-Reply-To: Your message of "Fri, 20 Jan 2017 21:43:33 %2B0100." <FB01B6F5-5269-4FE4-9B22-51A6AA60705E@FreeBSD.org> References: <20170120083555.ACCF9124AEA4@mail.bitblocks.com> <7C29D00C-94C0-4550-B1B2-CE307482B544@FreeBSD.org> <CAOtMX2hTcEkw_WzgtcEEipGY391zB=skrk7O=dknRMMG%2BDa%2BBA@mail.gmail.com> <20170120203106.CD2C8124AEA4@mail.bitblocks.com> <FB01B6F5-5269-4FE4-9B22-51A6AA60705E@FreeBSD.org>
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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.
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