Date: Wed, 7 Mar 2007 15:00:39 -0800 From: Chuck Swiger <cswiger@mac.com> To: Justin Robertson <justin@sk1llz.net> Cc: freebsd-ipfw@freebsd.org Subject: Re: IPFW SACK options Message-ID: <35795E3A-98DC-4354-A77E-BFB95E33913A@mac.com> In-Reply-To: <45EF3E33.2040507@sk1llz.net> 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>
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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
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