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

[Andre Oppermann]

Kip Macy 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.

No, this is a different issue and not really the fault of TCP but
of certain cable modem vendors with broken code in their devices.
FreeBSD is fully compliant to the spec.  Sibly committed a workaround
for this issue to -current and I expect the MFC to RELENG_7 and
RELENG_7_0 soon.

-- 
Andre

>  -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
>>
>> _______________________________________________
>>  freebsd-net at freebsd.org mailing list
>>  http://lists.freebsd.org/mailman/listinfo/freebsd-net
>>  To unsubscribe, send any mail to "freebsd-net-unsubscribe at freebsd.org"
>>
> _______________________________________________
> freebsd-net at freebsd.org mailing list
> http://lists.freebsd.org/mailman/listinfo/freebsd-net
> To unsubscribe, send any mail to "freebsd-net-unsubscribe at freebsd.org"
> 
>