Date: Mon, 1 Jan 2007 13:36:58 -0800 From: steve@Watt.COM (Steve Watt) To: hackers@freebsd.org Subject: Interesting TCP issue Message-ID: <200701012136.l01Lawp7017696@wattres.watt.com>
next in thread | raw e-mail | index | archive | help
One of my users is having trouble receiving mail from Skype. So,
after some sniffing, I discovered this:
# tcpdump -vv -s 1500 -i dc0 -X net 213.244.128.0/18
tcpdump: lestening on dc0, link-type EN10MB (Ethernet), capture size 1500 bytes
13:18:13.607493 IP (tos 0x20, ttl 58, id 12896, offset 0, flags [DF], proto: TCP (6), length: 74) share.skype.net.50406 > wattres.watt.com.smtp: P, cksum 0x9297 (correct), 4072464914:4072464936(22) ack 1248591103 win 46 <nop,nop,timestamp 2511885672 520058954>
0x0000: 4520 004a 3260 4000 3a06 c609 d5f4 aa50 E..J2`@.:......P
0x0010: 425d 8582 c4e6 0019 f2bc e212 4a6b fcff B]..........Jk..
0x0020: 8018 002e 9297 0000 0101 080a 95b8 5568 ..............Uh
0x0030: 1eff 784a 4548 4c4f 2073 6861 7265 2e73 ..xJEHLO.share.s
0x0040: 6b79 7065 2e6e 6574 0d0a kype.net..
0x0000: 4520 004a 3261 4000 3a06 c608 d5f4 aa50 E..J2a@.:......P
0x0010: 425d 8582 c4e6 0019 f2bc e212 4a6b fcff B]..........Jk..
0x0020: 8018 002e 1d67 0000 0101 080a 95b8 ca98 .....g..........
0x0030: 1eff 784a 4548 4c4f 2073 6861 7265 2e73 ..xJEHLO.share.s
0x0040: 6b79 7065 2e6e 6574 0d0a kype.net..
And no responses from my system.
Interesting. I presume it has something to do with the
idiotically small window the remote server is advertising. So I
set net.inet.tcp.minmss down to 46, and that resulted in a RST
being spit back to skype's server when its retransmit happened.
13:20:13.610975 IP (tos 0x20, ttl 58, id 12897, offset 0, flags [DF], proto: TCP (6), length: 74) share.skype.net.50406 > wattres.watt.com.smtp: P, cksum 0x1d67 (correct), 0:22(22) ack 1 win 46 <nop,nop,timestamp 2511915672 520058954>
13:20:13.611036 IP (tos 0x20, ttl 64, id 39789, offset 0, flags [DF], proto: TCP (6), length: 40) wattres.watt.com.smtp > share.skype.net.50406: R, cksum 0x5b51 (correct), 1248591103:1248591103(0) win 0
0x0000: 4520 0028 9b6d 4000 4006 571e 425d 8582 E..(.m@.@.W.B]..
0x0010: d5f4 aa50 0019 c4e6 4a6b fcff 0000 0000 ...P....Jk......
0x0020: 5004 0000 5b51 0000 P...[Q..
The packets shown here are the complete capture.
I can hand-type stuff to the skype server and it appears to get
through just fine, but when it connects to my box, no segments make
their way back to skype. I can see them sitting in the Send-Q:
# netstat -f inet
Active Internet connections
Proto Recv-Q Send-Q Local Address Foreign Address (state)
[ ... ]
tcp4 0 213 wattres.smtp share.skype.net.50406 ESTABLISHED
[ ... ]
Very strange. Any opinions on possible workarounds, since I'm not
expecting Skype to fix their (broken) system?
--
Steve Watt KD6GGD PP-ASEL-IA ICBM: 121W 56' 57.5" / 37N 20' 15.3"
Internet: steve @ Watt.COM Whois: SW32-ARIN
Free time? There's no such thing. It just comes in varying prices...
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?200701012136.l01Lawp7017696>