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Re: EOF, close(), shutdown(), on POSIX sockets

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Kaz Kylheku <kaz@kylheku.com> writes:

> On 2012-01-05, David Dyer-Bennet <dd-b@dd-b.net> wrote:
>> If I receive EOF when I read from A, I think I should do shutdown(B,
>> SHUT_WR), and if I receive EOF when I read from B, I should similarly do
>> shutdown (A, SHUT_WR).
>
> Indeed, if you want the proxy in the middle to be transparent to half-closes,
> which is a wortwhile requirement to implement if you want it to be as
> transparent as possible.

Thanks for the confirmation.  My concept is to make the backend of the
proxying, after the initial load-balancing and routing, as transparent
as possible.

>> 17:58:48.764575 shutdown(3, 0 /* receive */) = 0
>>
>> nc promptly shuts down the socket it just received EOF on.  Should I be
>> doing this in addition to the other?
>
> This is not necessary, and SHUT_RD can basically be even regarded
> as useless in TCP communication.
>
> Look in net/ipv4/tcp_ipv4.c in the tcp_shutdown function. It bails out
> if it's not in the write direction:
>
>         if (!(how & SEND_SHUTDOWN))
>                 return;
>
> Shutting down for reading is basically nonsensical; don't waste cycles
> tripping into the kernel for this.

I really should get more used to digging into the source on these,
thanks.  I'm not used to looking at Linux kernel source enough.  

>> Another confusing factor: immediately after this, nc reads 1024 bytes
>> from stdin (leaving it in the middle of the test file, about 56%; and
>> that's only 1/8 of the amount of data requested), writes it to 3, then
>> closes and exits -- meaning it aborts reading and transmitting the input
>> file.  
>
> I've just looked at four different variants of netcat source code. (From this I can gather that you're running the one from the netcat-openbsd package.)

What I know is that I'm using the package nc-1.84-10.fc6 for Centos 5.
Took me a while to figure oout thta the name "netcat" wasn't what was
going to get me anything in their repository.

> I looked at the repository for GNU netcat, openbsd-netcat some original
> netcat sources from "hobbit@avian.org", as well as the NMAP project's
> netcat-like program.
>
> All of them bail their loop on EOF from the socket.
>
> This is probably because the typical use case for netcat is to do a
> one-directional transfer, in the absence of any kind of request/response
> transaction. One side dumps data into the socket, the other dumps it
> into a file. When EOF is detected from the network, of course all previously
> received data has been written, and the sender is not expecting anything
> in return.
>
> Half-close logic is required for correctly implementing two-way transactions,
> whereby the client indicates "I have no more requests for the server",
> but not all the responses have been received.
>
> So these netcat utilities do not seem to provide a complete test case for a
> proxy's transparent handling of half close. Try proxying some true
> client/server protocols.  (HTTP? RDBMS's? ...)

Excellent.  So I don't have to worry about that.  It's the one glitch I
seem to have in my behavior (that my current tests find).

I've run other tests using some simple protocols I wrote myself.  I
could certainly arrange to use some real ones as well, I should do that.

Thanks very much for your exceedingly clear and complete response!
-- 
David Dyer-Bennet, dd-b@dd-b.net; http://dd-b.net/
Snapshots: http://dd-b.net/dd-b/SnapshotAlbum/data/
Photos: http://dd-b.net/photography/gallery/
Dragaera: http://dragaera.info

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Thread

EOF, close(), shutdown(), on POSIX sockets David Dyer-Bennet <dd-b@dd-b.net> - 2012-01-05 17:17 -0600
  Re: EOF, close(), shutdown(), on POSIX sockets Kaz Kylheku <kaz@kylheku.com> - 2012-01-06 01:10 +0000
    Re: EOF, close(), shutdown(), on POSIX sockets David Dyer-Bennet <dd-b@dd-b.net> - 2012-01-06 09:59 -0600
  Re: EOF, close(), shutdown(), on POSIX sockets Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2012-01-05 21:20 -0700
    Re: EOF, close(), shutdown(), on POSIX sockets David Dyer-Bennet <dd-b@dd-b.net> - 2012-01-06 10:00 -0600
    Re: EOF, close(), shutdown(), on POSIX sockets Andrei Voropaev <avorop@mail.ru> - 2012-01-12 16:07 +0000
      Re: EOF, close(), shutdown(), on POSIX sockets Joe Pfeiffer <pfeiffer@cs.nmsu.edu> - 2012-01-12 12:57 -0700
        Re: EOF, close(), shutdown(), on POSIX sockets Kaz Kylheku <kaz@kylheku.com> - 2012-01-12 20:04 +0000

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