Date: Tue, 11 Mar 2008 18:53:16 -0700 From: "Kip Macy" <kip.macy@gmail.com> To: "d.s. al coda" <coda.trigger@gmail.com> Cc: freebsd-net@freebsd.org Subject: Re: TCP options order changed in FreeBSD 7, incompatible with some routers Message-ID: <b1fa29170803111853p1989cadey4507a3c8468de2e5@mail.gmail.com> In-Reply-To: <f90b44e40803111756h517b373ala8afdff9395b7fac@mail.gmail.com> References: <f90b44e40803111756h517b373ala8afdff9395b7fac@mail.gmail.com>
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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 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" >
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