Dropping TCP options from retransmitted SYNs considered harmful

[Jason Wolfe]

On Fri, Oct 12, 2012 at 9:13 AM, John Baldwin <jhb at freebsd.org> wrote:
> Back in 2001 FreeBSD added a hack to strip TCP options from retransmitted SYNs
> starting with the 3rd SYN in this block in tcp_timer.c:
>
>         /*
>          * Disable rfc1323 if we haven't got any response to
>          * our third SYN to work-around some broken terminal servers
>          * (most of which have hopefully been retired) that have bad VJ
>          * header compression code which trashes TCP segments containing
>          * unknown-to-them TCP options.
>          */
>         if ((tp->t_state == TCPS_SYN_SENT) && (tp->t_rxtshift == 3))
>                 tp->t_flags &= ~(TF_REQ_SCALE|TF_REQ_TSTMP);
>
> There is even a PR for the original bug report: kern/1689
>
> [..snip..]
>
> The original motivation of this change is to work around broken terminal
> servers that were old when this change was added in 2001.  Over 10 years later
> I think we should at least have an option to turn this work-around off, and
> possibly disable it by default.
>
> Thoughts?
>
> --
> John Baldwin

Not that it alone merits keeping the code in, but there are some cases
where this comes in handy.  I ran into an issue with heavily
trafficked Linux <-> FBSD boxes here -
http://lists.freebsd.org/pipermail/freebsd-net/2012-March/031881.html.

Linux would deny the connection because in FBSD ithe n and outbound
timestamp randomization isn't sync'd to the same base, so when FBSD
would hit a 2MSL connection Linux would simply ignore the SYN.  After
the 3rd SYN FBSD would drop support, and Linux would finally honor the
request.  I doubt this is too widespread, but it would probably break
things for a few folks.

Jason