TCP options order changed in FreeBSD 7, incompatible with some routers

[Kip Macy]

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 at 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
>
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