Home-Made TCP Session Cutter problem

Experts,

I'm trying to code a program to close a tcp session between my local
machine and a remote one.
I'm using the idea (double RST) and some of the code from "TCP cutter"
(http://www.lowth.com/cutter)
My problem is that I am not BETWEEN the two machines, I'm at one end.
So I need to send a RST to myself and make me believe it comes from
the remote machine ...

My problem is as follow :

My local machine is A (192.168.1.3) and my distant machine is B
(xx.xx.xx.xx)
A is sending a RST to B that works fine
A is sending a RST to A ("from B") and this does not work...
as you will see, the sequence numbers look fine.
(sorry if the lines are wrapped, I can't change that)

15:11:59.369482 192.168.1.3.33254 > xx.xx.xx.xx.imaps: F [tcp sum ok]
0:0(0) win 0 (DF) (ttl 253, id 0, len 40)
15:11:59.372507 xx.xx.xx.xx.imaps > 192.168.1.3.33254: . [tcp sum ok]
1589:1589(0) ack 461 win 6432 <nop,nop,timestamp 163410425 1403391>
(DF) (ttl 62, id 40208, len 52)
15:11:59.373290 192.168.1.3.33254 > xx.xx.xx.xx.imaps: R [tcp sum ok]
2365895080:2365895080(0) win 0 (DF) (ttl 253, id 0, len 40)
15:11:59.373699 xx.xx.xx.xx.imaps > 192.168.1.3.33254: R [tcp sum ok]
3960647341:3960647341(0) win 0 (DF) (ttl 253, id 0, len 40)
15:12:06.073374 192.168.1.3.33254 > xx.xx.xx.xx.imaps: P [tcp sum ok]
461:490(29) ack 1589 win 8870 <nop,nop,timestamp 1406204 163410425>
(DF) (ttl 64, id 39495, len 81)
15:12:06.076633 xx.xx.xx.xx.imaps > 192.168.1.3.33254: R [tcp sum ok]
3960647341:3960647341(0) win 0 (DF) (ttl 253, id 0, len 40)

let's go deeper

1) A sends FIN to find the sequence number:
15:11:59.369482 192.168.1.3.33254 > xx.xx.xx.xx.imaps: F [tcp sum ok]
0:0(0) win 0 (DF) (ttl 253, id 0, len 40)

2) reply from B : ACK with a good seq number:
15:11:59.372507 xx.xx.xx.xx.imaps > 192.168.1.3.33254: . [tcp sum ok]
1589:1589(0) ack 461 win 6432 <nop,nop,timestamp 163410425 1403391>
(DF) (ttl 62, id 40208, len 52)

3) A sends rst to B (with good seq number)
consequence: B closes the socket, on its side
15:11:59.373290 192.168.1.3.33254 > xx.xx.xx.xx.imaps: R [tcp sum ok]
2365895080:2365895080(0) win 0 (DF) (ttl 253, id 0, len 40)

4) A sends "fake" rst to A ("from B" thx to raw sockets)
This should close the socket on A's side, but that's the pb : it does
not! :
15:11:59.373699 xx.xx.xx.xx.imaps > 192.168.1.3.33254: R [tcp sum ok]
3960647341:3960647341(0) win 0 (DF) (ttl 253, id 0, len 40)

5) the real owner of the socket on A's side sends some data into the
socket
(though packet 4 should have made understand the socket's dead) :
15:12:06.073374 192.168.1.3.33254 > xx.xx.xx.xx.imaps: P [tcp sum ok]
461:490(29) ack 1589 win 8870 <nop,nop,timestamp 1406204 163410425>
(DF) (ttl 64, id 39495, len 81)

6) B replies : the socket's closed --> RST --> this closes the socket
on A's side for good
15:12:06.076633 xx.xx.xx.xx.imaps > 192.168.1.3.33254: R [tcp sum ok]
3960647341:3960647341(0) win 0 (DF) (ttl 253, id 0, len 40)

packet 4 = packet 6 or is it ?
So why is the 6th packet closing the socket on A's side and the 4th
not ?


some tcpdump details :

15:16:03.029155 192.168.1.3.33264 > xx.xx.xx.xx.imaps: F [tcp sum ok]
0:0(0) win 0 (DF) (ttl 253, id 0, len 40)
0x0000 4510 0028 0000 4000 fd06 3f8f c0a8 0103
E..(..@...?.....
0x0010 xxxx xxxx 81f0 03e1 0000 0000 0000 0000
QP+E............
0x0020 5001 0000 ebd1 0000 P.......
15:16:03.032450 xx.xx.xx.xx.imaps > 192.168.1.3.33264: . [tcp sum ok]
1547:1547(0) ack 432 win 6432 <nop,nop,timestamp 163434791 1428906>
(DF) (ttl 62, id 47958, len 52)
0x0000 4500 0034 bb56 4000 3e06 432d xxxx xxxx
E..4.V@.>.C-QP+E
0x0010 c0a8 0103 03e1 81f0 04bb 7c74 a601 53be
...........|t..S.
0x0020 8010 1920 75f7 0000 0101 080a 09bd d127
.....u..........'
0x0030 0015 cdaa ....
15:16:03.032751 192.168.1.3.33264 > xx.xx.xx.xx.imaps: R [tcp sum ok]
2785104830:2785104830(0) win 0 (DF) (ttl 253, id 0, len 40)
0x0000 4510 0028 0000 4000 fd06 3f8f c0a8 0103
E..(..@...?.....
0x0010 xxxx xxxx 81f0 03e1 a601 53be 0000 0000
QP+E......S.....
0x0020 5004 0000 f20e 0000 P.......
15:16:03.032760 xx.xx.xx.xx.imaps > 192.168.1.3.33264: R [tcp sum ok]
79395956:79395956(0) win 0 (DF) (ttl 253, id 0, len 40)
0x0000 4510 0028 0000 4000 fd06 3f8f xxxx xxxx
E..(..@...?.QP+E
0x0010 c0a8 0103 03e1 81f0 04bb 7c74 0000 0000
...........|t....
0x0020 5004 0000 6a9f 0000 P...j...
15:16:06.658510 192.168.1.3.33264 > xx.xx.xx.xx.imaps: P [tcp sum ok]
432:461(29) ack 1547 win 8870 <nop,nop,timestamp 1430262 163434791>
(DF) (ttl 64, id 40850, len 81)
0x0000 4500 0051 9f92 4000 4006 5cd4 c0a8 0103
E..Q..@.@.\.....
0x0010 xxxx xxxx 81f0 03e1 a601 53be 04bb 7c74
QP+E......S...|t
0x0020 8018 22a6 9384 0000 0101 080a 0015 d2f6
...".............
0x0030 09bd d127 1703 0100 1890 5710 c450 b838
....'......W..P.8
0x0040 697e b447 cbaa 576b 5725 6f31 267d 2e9e
i~.G..WkW%o1&}..
0x0050 73 s
15:16:06.661649 xx.xx.xx.xx.imaps > 192.168.1.3.33264: R [tcp sum ok]
79395956:79395956(0) win 0 (DF) (ttl 253, id 0, len 40)
0x0000 4500 0028 0000 4000 fd06 3f8f xxxx xxxx
E..(..@...?.QP+E
0x0010 c0a8 0103 03e1 81f0 04bb 7c74 0000 0000
...........|t....
0x0020 5004 0000 6a9f 0000 0000 0000 0000 P...j.........

The packets really look like the same.
What I am thinking wrong ? any idea ?

Florent