Re: [9fans] telnet vs. godaddy whois

[Bakul Shah <bakul+plan9@bitblocks.com>]

On Thu, 17 Apr 2008 15:29:42 EDT erik quanstrom <quanstro@quanstro.net>  wrote:
> > On Thu, 17 Apr 2008 09:18:31 BST Charles Forsyth <forsyth@terzarima.net>  w
> rote:
> >> > having said that, i now suspect that sending one byte into a zero-window
>  is
> >>  not the problem.
> >>
> >> because the one-byte probe can only be done if there is data to send, and
> i
> >> already knew that a plain connection (dial only) to that port also failed:
> >
> > Not setting the PSH bit on a pure ACK (== no data is being
> > sent) seems to fix this (see ip/tcp.c around line 2530).  May
> > be it tickles a bug on the receiver (0 byte read?).
>
> this does work for me.  is there some subtile reason *to* set the psh bit
> on a pure ack?  in certain circumstances?

While I wouldn't call it "wrong" it seems silly to set the
PSH bit when no data is being sent.  See rfc1122 section
4.2.2.2. Among other things it says:

    At the receiver, the PSH bit forces buffered data to be
    delivered to the application (even if less than a full
    buffer has been received).

Because of this what is likely happening is that on receiving
the PSH bit read() completes and returns to the caller app
with a count = 0 which the app must think indicates EOF!

May be this behavior in plan9 is due to a misplaced right
brace?  Move the closing brace on line 2530 to below the PSH
bit logic and all is fine.