Date: Wed, 12 Mar 2008 08:49:12 +0000 From: "Jake Rizzo" <rizzojake@gmail.com> To: "d.s. al coda" <coda.trigger@gmail.com> Cc: Kip Macy <kip.macy@gmail.com>, freebsd-net@freebsd.org Subject: Re: TCP options order changed in FreeBSD 7, incompatible with some routers Message-ID: <b07c271a0803120149l677c5263o73cfa20cf604f02c@mail.gmail.com> In-Reply-To: <f90b44e40803111948n435e3901hc6a3f80f20673041@mail.gmail.com> References: <f90b44e40803111756h517b373ala8afdff9395b7fac@mail.gmail.com> <b1fa29170803111853p1989cadey4507a3c8468de2e5@mail.gmail.com> <f90b44e40803111948n435e3901hc6a3f80f20673041@mail.gmail.com>
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Exact same problem that i'm having. I confirmed it exists in 7.0 only since downgrading one of our servers back to 6.3 stable allowed the same clients to connect again. This seems to work for us as a workaround: sysctl net.inet.tcp.sack.enable=0 On 3/12/08, d.s. al coda <coda.trigger@gmail.com> wrote: > > On 3/11/08, Kip Macy <kip.macy@gmail.com> wrote: > > > Are you running 7.0-RELEASE? What I believe was this issue was a > > showstopper for it, so I'm surprised to hear of it now. > > > > -Kip > > > > > Yes, we are running 7.0-RELEASE. > > -coda > > > On Tue, Mar 11, 2008 at 5:56 PM, d.s. al coda <coda.trigger@gmail.com> > > wrote: > > > Hi, > > > We recently upgraded one of our webservers to FreeBSD 7, and we > started > > > receiving complaints from some users not able to connect to that > server > > > anymore. On top of that, users were saying that the problem only > > occurred on > > > Windows (at least, the ones who had more than on OS to try it out). > > > > > > After managing to get a user who had the problem running windump, > > running > > > tcpdump on the new server, and comparing that to the windump & > tcpdump > > > output for a "control" user (me) that could connect, we managed to > > figure > > > out the following: > > > - For the user with this problem, ping works fine, but all TCP > > connections > > > to the server fail. > > > - The user, trying to connect, sends out a SYN packet, receives no > > response, > > > and retries a few times until timing out. > > > - The server sees a bunch of SYN packets and responds with SYN-ACK > each > > > time. > > > - The issue only seems to arise if the sender has RFC1323 disabled. > > > > > > So, the SYN-ACK is getting lost somewhere. > > > > > > - For the control user (who can connect via TCP just fine), we set > the > > TCP > > > window size and RFC1323 options the same as the user with the > problem. > > > - The control user sees the SYN-ACK packet. > > > - We send a connection attempt to one of our other servers, running > > FreeBSD > > > 5.5, and one to the server running FreeBSD 7. > > > - There is only one notable difference between the responses: the > order > > of > > > the options. > > > - FreeBSD 5.5 has <mss 1412, nop, nop, sackOK> > > > - FreeBSD 7 has <mss 1412, sackOK, eol> (there is of course an > aligning > > nop > > > after the eol, which tcpdump skips) > > > - These options don't appear in this exact configuration when using > > RFC1323 > > > options. > > > > > > I get a hunch that the users with the problem have a router that > > erroneously > > > thinks that these options are invalid, or thinks that the some part > of > > byte > > > sequence (e.g. 0204 05b4 0101 0402) is an attack. > > > > > > Just to try it out, I patched tcp_output.c so that the SACK permitted > > option > > > was aligned on a 4-byte boundary, preventing the "sackOK, eol" > pattern > > from > > > ever occuring. Looking through previous versions, I found where the > > tcp > > > option code had changed, and there used to be a comment about putting > > SACK > > > permitted last, but I can't tell if it's relevant. > > > > > > http://www.freebsd.org/cgi/cvsweb.cgi/src/sys/netinet/tcp_output.c.diff?r1=1.125;r2=1.126 > > > > > > The one-line patch to tcp_output.c is attached. > > > > > > Sure enough, it fixed the problem. Afterwards, we collected some > > information > > > about the routers the users who had the problem were using, and while > > they > > > didn't all have the same manufacturer, several mentioned that their > > router > > > had a built-in firewall, which, when they disabled it, allowed them > to > > > access the server. > > > > > > Does all of this sound reasonable? And if so, would it be worth > > submitting > > > this patch? I don't know if this particular change in options order > was > > > intentional, or just a side-effect of the new code, but it certainly > > works > > > around an extremely hard-to-diagnose problem. > > > > > > -coda > > > > > > _______________________________________________ > > > freebsd-net@freebsd.org mailing list > > > http://lists.freebsd.org/mailman/listinfo/freebsd-net > > > To unsubscribe, send any mail to "freebsd-net-unsubscribe@freebsd.org > " > > > > > > _______________________________________________ > freebsd-net@freebsd.org mailing list > http://lists.freebsd.org/mailman/listinfo/freebsd-net > To unsubscribe, send any mail to "freebsd-net-unsubscribe@freebsd.org" >
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