From owner-freebsd-hackers@FreeBSD.ORG Mon Jan 1 21:54:06 2007 Return-Path: X-Original-To: hackers@freebsd.org Delivered-To: freebsd-hackers@FreeBSD.ORG Received: from mx1.freebsd.org (mx1.freebsd.org [69.147.83.52]) by hub.freebsd.org (Postfix) with ESMTP id 2DB3816A407 for ; Mon, 1 Jan 2007 21:54:06 +0000 (UTC) (envelope-from steve@Watt.COM) Received: from wattres.watt.com (wattres.watt.com [66.93.133.130]) by mx1.freebsd.org (Postfix) with ESMTP id EE4FF13C468 for ; Mon, 1 Jan 2007 21:54:05 +0000 (UTC) (envelope-from steve@Watt.COM) Received: from wattres.watt.com (localhost.watt.com [127.0.0.1]) by wattres.watt.com (8.13.8/8.13.8) with ESMTP id l01LawHF017697 for ; Mon, 1 Jan 2007 13:36:58 -0800 (PST) (envelope-from steve@wattres.watt.com) Received: (from steve@localhost) by wattres.watt.com (8.13.8/8.13.8/Submit) id l01Lawp7017696 for hackers@freebsd.org; Mon, 1 Jan 2007 13:36:58 -0800 (PST) (envelope-from steve) Message-Id: <200701012136.l01Lawp7017696@wattres.watt.com> From: steve@Watt.COM (Steve Watt) Date: Mon, 1 Jan 2007 13:36:58 -0800 X-Mailer: Mail User's Shell (7.2.6 beta(5) 10/07/98) To: hackers@freebsd.org X-Archived: 1167687418.194853807@wattres.Watt.COM Cc: Subject: Interesting TCP issue X-BeenThere: freebsd-hackers@freebsd.org X-Mailman-Version: 2.1.5 Precedence: list List-Id: Technical Discussions relating to FreeBSD List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Mon, 01 Jan 2007 21:54:06 -0000 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 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 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...