[tor-dev] Torsocks development status

[David Goulet]

Matthew Finkel:
> On Thu, Jun 27, 2013 at 03:11:23PM -0400, David Goulet wrote:
>> Ian Goldberg:
>>> Are non-blocking sockets, select/poll/etc. (especially at connect()
>>> time), and optimistic data on the to-do list?
>>
>> Yes! Good point I should have put the todo list. So yes, non block socket support.
>>
>> For optimistic data, it is kind of tricky.
> 
> It definitely is tricky. You just need to find the best way to have
> torsocks return the least untrue response that's allowed by the OS. I'm
> not going to reiterate what Ian said, but I'll just make some points
> about what I did.
> 
>> I can use it for DNS resolution
>> without a problem because torsocks control the complete flow of data from
>> opening a SOCKS5 connection to closing it after the DNS response is received
>> however for actual real data (sendmsg, send, ...) a connect is needed before so
>> it would means that a connect() call will return "yes OK socket connected" but
>> where in fact it is not really true. So, when the first data are sent, there is
>> a possibility that the Tor connections failed or even we block for an unknown
>> amount of time during the send*/write() call.
> 
> Yup, this is exactly the case (in addition to SOCKS4/A also).

Actually, I did not see any reasons why this rewrite should support SOCKS4. Is
there ?

> 
>>
>> Now the question is, is this the kind of behavior that would be acceptable
>> meaning basically lying to the caller at connect() and possibly blocking I/O
>> calls and returning something like ECONNRESET or ENOTCONN if the Tor socks5
>> connection fails.
>>
> 
> The main problem I foresee with this is when torsocks wraps a program that
> does not fully implement error handling or does not implement it
> correctly. And, to be honest, I don't think you can let potentially
> faulty programs influence the features of *your* program (too much).
> 
> For what it's worth, I returned ENOTCONN and EBADF, but I think ENOTCONN
> is the most descriptive, I'm just not sure most programs check for it
> after a send()/write().
> 
>> This is *real* tricky especially with non blocking socket, if torsocks needs to
>> do some possible blocking call for the SOCKS5 replies during an I/O call from
>> the caller that is not suppose to block. Furthermore, having pending data that
>> *might* come at any time on the connection from the SOCKS5 negotiation, the
>> caller could put the file descriptor in poll() mode, wake up and try to receive
>> the data but where in fact it's the socks5 reply... it's possible to handle that
>> but it seems here a VERY intrusive behavior. Does optimistic data worth it here
>> vis-a-vis the complexity of handling that it and high intrusiveness ?
> 
> Well, you actually have more guarantees than you may think (unless I
> misunderstand you). You know torsocks will send the SOCKS5/4{,A) request
> and you know that before torsocks returns anything to the client
> application, torsocks *must* receive a response from Tor regarding the
> success or failure of establishing the proxy connection. As such, if you
> receive optdata from the client app and pass it to Tor which then will
> pass it to the endpoint (if possible), you know Tor *must* return a
> SOCKS reply *before* you receive any client data, so you simply read
> that off the buffer and then handle the connection in an appropriate
> manner. Simple. :)

Yup agree. Actually, the question here was more about if supporting optdata
worth that non trivial effort but 33% is quite a big factor to consider for
performance :).

> 
> Regarding poll(), torsocks really needs to wrap the multiplexing I/O
> syscalls ({p,}poll, {p,}select, epoll, kqueue, etc) or else you will
> run into some major problems (select() and poll() being much more
> important than the others). This is intrusive, but it's only a single
> write request (for all values of "write").

Can you detail why it's very important? You did some hacking in the old code
base and I would like to know your experience on that. What possible major
problems? What happens if it's not hijacked?

> 
> Personally, I think the most important feature of the optdata
> implementation is that you make it configurable. 

By configurable you mean disabled or not ? What else is there to configure?

Thanks!
David

> 
>>
>> Cheers!
>> David
>>
>>>
>>> Thanks,
>>>
>>>    - Ian
>>
> 
> - Matt
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