Date: Wed, 07 Mar 2007 15:10:59 -0800 From: Justin Robertson <justin@sk1llz.net> To: Chuck Swiger <cswiger@mac.com>, freebsd-ipfw@freebsd.org Subject: Re: IPFW SACK options Message-ID: <45EF4683.7060508@sk1llz.net> In-Reply-To: <35795E3A-98DC-4354-A77E-BFB95E33913A@mac.com> References: <000301c760fa$df57eb40$9e07c1c0$@net> <CDAD9871-3C36-4225-AABC-749BC703058D@mac.com> <45EF32E2.8000807@sk1llz.net> <4C17F7D1-6763-41EC-970E-E88FF3782D22@mac.com> <45EF3B1E.1080706@sk1llz.net> <45EF3E33.2040507@sk1llz.net> <35795E3A-98DC-4354-A77E-BFB95E33913A@mac.com>
next in thread | previous in thread | raw e-mail | index | archive | help
Chuck Swiger wrote: > On Mar 7, 2007, at 2:35 PM, Justin Robertson wrote: >>> The issue here is that no windows PC is compliant, and continues to >>> try and send SYN SACK packets until giving up entirely on the >>> connection - I've already tried this. I can't tell you why the bsd >>> stack doesn't have an issue with bare SYNs, but does on SYNs with >>> SACK set in the tcpoptions, but it apparently does. >>> >> Example - windows PC trying to connect to POP mailserver, sacks >> blocked (being viewed from firewall) >> >> 14:31:54.632444 client.52985 > server.110: S [tcp sum ok] >> 3734795545:3734795545(0) win 8192 <mss 1460,nop,wscale >> 8,nop,nop,sackOK> (DF) (ttl 115, id 12030, len 52) >> 14:31:57.629151 client.52985 > server.110: S [tcp sum ok] >> 3734795545:3734795545(0) win 8192 <mss 1460,nop,wscale >> 8,nop,nop,sackOK> (DF) (ttl 115, id 12034, len 52) >> 14:32:03.635511 client.52985 > server.110: S [tcp sum ok] >> 3734795545:3734795545(0) win 8192 <mss 1460,nop,nop,sackOK> (DF) (ttl >> 115, id 12046, len 48) >> >> You get three sack attempted retransmissions, then failure. It's not >> sending selective acks, it's trying to establish that selective acks >> are allowed for the rest of the communication between client/server. > > Which flavor of Windows was the sender? > > I seem to recall that Win2000/XP/2003 was willing to resend the SYN > without enabling the SACK option after a few retries, but it's been a > while since I've done that sort of testing... > >> I just want to strip the sack options from the packet and pass it on. >> The "issue" is that BSD can't cope with a flood of syn/sack permitted >> packets. All IP lags, and on 6.x stops all-together. > > Is the machine you're testing against the destination of this traffic, > or are you wanting to change this on a firewall box which lies between > the sender and destination? > > If the latter, I would agree that you'd need to hack the firewall > code. But if the former, take a look into > /usr/src/sys/netinet/tcp_input.c for the tcp_dooptions() function, and > try to #ifdef out the cases for TCPOPT_SACK_PERMITTED & TCPOPT_SACK? > > ---Chuck > > All latest windows updated XP machines, as well as all new Vista machines have this issue. And this is being done on a firewall box between sender/destination. The firewall machine itself (even in a transparent bridge) lags when passing SYN w/ SACK - hence the need to hack up ipfw to strip TCP options. -- Justin
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?45EF4683.7060508>