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From: Michael on 12 Jul 2010 12:27
Trying to FTP to a system running - Linux lhi-ftp3 2.6.32-21-server
#32-Ubuntu SMP Fri Apr 16 09:17:34 UTC 2010 x86_64 GNU/Linux

The TCP connection is established and the ftp banner from the server
is sent, ack'ed, and received by the client but any TCP data sent from
the client is not ack'ed by the server. Here tcpdump output from the
server side -


15:18:36.826619 IP (tos 0x0, ttl 55, id 51642, offset 0, flags [DF],
proto TCP (6), length 60)
client.3714 > server.ftp: Flags [S], cksum 0x4c6b (correct), seq
1816442934, win 6144, options [mss 1280,nop,wscale 0,nop,nop,TS val
3654955 ecr 0], length 0
15:18:36.826682 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto
TCP (6), length 60)
server.ftp > client.3714: Flags [S.], cksum 0xf126 (correct), seq
2220034682, ack 1816442935, win 5792, options [mss 1460,nop,nop,TS val
26004352 ecr 3654955,nop,wscale 5], length 0
15:18:37.838401 IP (tos 0x0, ttl 55, id 51648, offset 0, flags [DF],
proto TCP (6), length 60)
client.3714 > server.ftp: Flags [S], cksum 0x4c69 (correct), seq
1816442934, win 6144, options [mss 1280,nop,wscale 0,nop,nop,TS val
3654957 ecr 0], length 0
15:18:37.838421 IP (tos 0x0, ttl 64, id 0, offset 0, flags [DF], proto
TCP (6), length 60)
server.ftp > client.3714: Flags [S.], cksum 0xf0c0 (correct), seq
2220034682, ack 1816442935, win 5792, options [mss 1460,nop,nop,TS val
26004454 ecr 3654955,nop,wscale 5], length 0
15:18:38.862747 IP (tos 0x0, ttl 55, id 51649, offset 0, flags [DF],
proto TCP (6), length 52)
client.3714 > server.ftp: Flags [.], cksum 0x1708 (correct), seq
1, ack 1, win 7300, options [nop,nop,TS val 3654959 ecr 26004352],
length 0
15:18:38.865315 IP (tos 0x0, ttl 64, id 50743, offset 0, flags [DF],
proto TCP (6), length 237)
server.ftp > client.3714: Flags [P.], seq 1:186, ack 1, win 181,
options [nop,nop,TS val 26004556 ecr 3654959], length 185
15:18:39.886297 IP (tos 0x0, ttl 55, id 0, offset 0, flags [DF], proto
TCP (6), length 60)
client.3714 > server.ftp: Flags [.], cksum 0xf87d (correct), seq
1, ack 1, win 5792, options [nop,nop,nop,nop,nop,nop,TS val 26004454
ecr 3654955,nop,nop,nop,nop], length 0
15:18:40.914816 IP (tos 0x0, ttl 55, id 51652, offset 0, flags [DF],
proto TCP (6), length 64)
client.3714 > server.ftp: Flags [P.], cksum 0xd4be (correct), seq
1:13, ack 186, win 7300, options [nop,nop,TS val 3654963 ecr
26004556], length 12
15:18:40.914834 IP (tos 0x0, ttl 64, id 50744, offset 0, flags [DF],
proto TCP (6), length 52)
server.ftp > client.3714: Flags [.], cksum 0x2879 (correct), seq
186, ack 1, win 181, options [nop,nop,TS val 26004761 ecr 26004454],
length 0
15:18:42.450306 IP (tos 0x0, ttl 55, id 51653, offset 0, flags [DF],
proto TCP (6), length 64)
client.3714 > server.ftp: Flags [P.], cksum 0xd4bb (correct), seq
1:13, ack 186, win 7300, options [nop,nop,TS val 3654966 ecr
26004556], length 12
15:18:42.450331 IP (tos 0x0, ttl 64, id 50745, offset 0, flags [DF],
proto TCP (6), length 52)
server.ftp > client.3714: Flags [.], cksum 0x27df (correct), seq
186, ack 1, win 181, options [nop,nop,TS val 26004915 ecr 26004454],
length 0
15:18:44.978241 IP (tos 0x0, ttl 64, id 50746, offset 0, flags [DF],
proto TCP (6), length 237)
server.ftp > client.3714: Flags [P.], seq 1:186, ack 1, win 181,
options [nop,nop,TS val 26005168 ecr 26004454], length 185
15:18:46.546165 IP (tos 0x0, ttl 55, id 50746, offset 0, flags [DF],
proto TCP (6), length 40)
client.3714 > server.ftp: Flags [P.], cksum 0xe1c5 (correct), seq
13, ack 186, win 7300, length 0
15:18:46.546279 IP (tos 0x0, ttl 55, id 51654, offset 0, flags [DF],
proto TCP (6), length 64)
client.3714 > server.ftp: Flags [P.], cksum 0xd4b3 (correct), seq
1:13, ack 186, win 7300, options [nop,nop,TS val 3654974 ecr
26004556], length 12
15:18:46.546291 IP (tos 0x0, ttl 64, id 50747, offset 0, flags [DF],
proto TCP (6), length 52)
server.ftp > client.3714: Flags [.], cksum 0x2646 (correct), seq
186, ack 1, win 181, options [nop,nop,TS val 26005324 ecr 26004454],
length 0
15:18:54.737767 IP (tos 0x0, ttl 55, id 51659, offset 0, flags [DF],
proto TCP (6), length 64)
client.3714 > server.ftp: Flags [P.], cksum 0xd4a3 (correct), seq
1:13, ack 186, win 7300, options [nop,nop,TS val 3654990 ecr
26004556], length 12
15:18:54.737787 IP (tos 0x0, ttl 64, id 50748, offset 0, flags [DF],
proto TCP (6), length 52)
server.ftp > client.3714: Flags [.], cksum 0x2313 (correct), seq
186, ack 1, win 181, options [nop,nop,TS val 26006143 ecr 26004454],
length 0
15:19:10.609658 IP (tos 0x0, ttl 55, id 51683, offset 0, flags [DF],
proto TCP (6), length 64)
client.3714 > server.ftp: Flags [P.], cksum 0xd483 (correct), seq
1:13, ack 186, win 7300, options [nop,nop,TS val 3655022 ecr
26004556], length 12
15:19:10.609681 IP (tos 0x0, ttl 64, id 50749, offset 0, flags [DF],
proto TCP (6), length 52)
server.ftp > client.3714: Flags [.], cksum 0x1cdf (correct), seq
186, ack 1, win 181, options [nop,nop,TS val 26007731 ecr 26004454],
length 0
15:19:42.376153 IP (tos 0x0, ttl 55, id 51707, offset 0, flags [DF],
proto TCP (6), length 64)
client.3714 > server.ftp: Flags [P.], cksum 0xd443 (correct), seq
1:13, ack 186, win 7300, options [nop,nop,TS val 3655086 ecr
26004556], length 12
15:19:42.376176 IP (tos 0x0, ttl 64, id 50750, offset 0, flags [DF],
proto TCP (6), length 52)
server.ftp > client.3714: Flags [.], cksum 0x1077 (correct), seq
186, ack 1, win 181, options [nop,nop,TS val 26010907 ecr 26004454],
length 0
15:20:14.627024 IP (tos 0x0, ttl 55, id 51721, offset 0, flags [DF],
proto TCP (6), length 64)
client.3714 > server.ftp: Flags [P.], cksum 0xd403 (correct), seq
1:13, ack 186, win 7300, options [nop,nop,TS val 3655150 ecr
26004556], length 12
15:20:14.627047 IP (tos 0x0, ttl 64, id 50751, offset 0, flags [DF],
proto TCP (6), length 52)
server.ftp > client.3714: Flags [.], cksum 0x03de (correct), seq
186, ack 1, win 181, options [nop,nop,TS val 26014132 ecr 26004454],
length 0
15:20:46.882937 IP (tos 0x0, ttl 55, id 51727, offset 0, flags [DF],
proto TCP (6), length 64)
client.3714 > server.ftp: Flags [P.], cksum 0xd3c3 (correct), seq
1:13, ack 186, win 7300, options [nop,nop,TS val 3655214 ecr
26004556], length 12
15:20:46.882959 IP (tos 0x0, ttl 64, id 50752, offset 0, flags [DF],
proto TCP (6), length 52)
server.ftp > client.3714: Flags [.], cksum 0xf743 (correct), seq
186, ack 1, win 181, options [nop,nop,TS val 26017358 ecr 26004454],
length 0
15:21:18.628372 IP (tos 0x0, ttl 55, id 51782, offset 0, flags [DF],
proto TCP (6), length 64)
client.3714 > server.ftp: Flags [P.], cksum 0xd383 (correct), seq
1:13, ack 186, win 7300, options [nop,nop,TS val 3655278 ecr
26004556], length 12
15:21:18.628395 IP (tos 0x0, ttl 64, id 50753, offset 0, flags [DF],
proto TCP (6), length 52)
server.ftp > client.3714: Flags [.], cksum 0xeadc (correct), seq
186, ack 1, win 181, options [nop,nop,TS val 26020533 ecr 26004454],
length 0
15:21:50.920519 IP (tos 0x0, ttl 55, id 51786, offset 0, flags [DF],
proto TCP (6), length 64)
client.3714 > server.ftp: Flags [P.], cksum 0xd343 (correct), seq
1:13, ack 186, win 7300, options [nop,nop,TS val 3655342 ecr
26004556], length 12
15:21:50.920547 IP (tos 0x0, ttl 64, id 50754, offset 0, flags [DF],
proto TCP (6), length 52)
server.ftp > client.3714: Flags [.], cksum 0xde3f (correct), seq
186, ack 1, win 181, options [nop,nop,TS val 26023762 ecr 26004454],
length 0

I'm trying to figure out why the server never ack's receiving the 12
bytes sent by the client. Everything appears fine to me.

thanks for any help or ideas
Mike
From: Michael on 12 Jul 2010 14:05
Here is a stripped down version of the output that shows the problem.
Not that despite seq 1:13 being sent the server always ack's 1.

server.ftp > client.3714: [S.], cksum 0xf126 (ok), seq 2220034682, ack
1816442935, win 5792, options length 0
server.ftp > client.3714: [S.], cksum 0xf0c0 (ok), seq 2220034682, ack
1816442935, win 5792, options length 0
client.3714 > server.ftp: [.], cksum 0x1708 (ok), seq 1, ack 1, win
7300, length 0
server.ftp > client.3714: [P.], seq 1:186, ack 1, win 181, length 185
client.3714 > server.ftp: [.], cksum 0xf87d (ok), seq 1, ack 1, win
5792, length 0
client.3714 > server.ftp: [P.], cksum 0xd4be (ok), seq 1:13, ack 186,
win 7300, length 12
server.ftp > client.3714: [.], cksum 0x2879 (ok), seq 186, ack 1, win
181, length 0
client.3714 > server.ftp: [P.], cksum 0xd4bb (ok), seq 1:13, ack 186,
win 7300, length 12
server.ftp > client.3714: [.], cksum 0x27df (ok), seq 186, ack 1, win
181, length 0
server.ftp > client.3714: [P.], seq 1:186, ack 1, win 181, length 185
client.3714 > server.ftp: [P.], cksum 0xe1c5 (ok), seq 13, ack 186,
win 7300, length 0
client.3714 > server.ftp: [P.], cksum 0xd4b3 (ok), seq 1:13, ack 186,
win 7300, length 12
server.ftp > client.3714: [.], cksum 0x2646 (ok), seq 186, ack 1, win
181, length 0
client.3714 > server.ftp: [P.], cksum 0xd4a3 (ok), seq 1:13, ack 186,
win 7300, length 12
server.ftp > client.3714: [.], cksum 0x2313 (ok), seq 186, ack 1, win
181, length 0
client.3714 > server.ftp: [P.], cksum 0xd483 (ok), seq 1:13, ack 186,
win 7300, length 12
server.ftp > client.3714: [.], cksum 0x1cdf (ok), seq 186, ack 1, win
181, length 0
client.3714 > server.ftp: [P.], cksum 0xd443 (ok), seq 1:13, ack 186,
win 7300, length 12
server.ftp > client.3714: [.], cksum 0x1077 (ok), seq 186, ack 1, win
181, length 0
client.3714 > server.ftp: [P.], cksum 0xd403 (ok), seq 1:13, ack 186,
win 7300, length 12
server.ftp > client.3714: [.], cksum 0x03de (ok), seq 186, ack 1, win
181, length 0
client.3714 > server.ftp: [P.], cksum 0xd3c3 (ok), seq 1:13, ack 186,
win 7300, length 12
server.ftp > client.3714: [.], cksum 0xf743 (ok), seq 186, ack 1, win
181, length 0

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