RE: Info on "Can't open display"

["Doug Reiland" <dreiland at hotmail dot com>]

PLease keep us posted. I have the same problem, but no VPN software 
installed that I know of (win98 se).

I have put this on the back-burner after I had no luck building a debug 
version of the server.


>From: "Christopher Landrieu" <landrieu@hotmail.com>
>To: cygwin-xfree@cygwin.com
>Subject: RE: Info on "Can't open display"
>Date: Thu, 14 Jun 2001 05:19:19
>
>Ok... I've done some testing and have some more info on this problem.  I
>would have more info, and would have posted it sooner, except my win2k
>installation crashed really hard (completely trashed registry) over the
>weekend (not in any way due to Cygwin or xfree86).
>
>First, as has been discussed previously on this list, and as many of you
>kindly suggested in response to my original post, this problem IS related 
>to
>a VPN package that I have installed.  I have Aventail Connect 3.21 
>installed
>on both machines.  I found that I can get local xclients to work by 
>tweaking
>the Aventail Connect configuration file to exclude those programs
>explicitly.  It is _alot_ of work, however, to put all of the xfree86 
>client
>apps in this list.
>
>The interesting thing that I noticed with this is that it is not enough to
>simply shutdown the Aventail Connect software.  This usually disables all
>redirections of network traffic.  In this case, I found that tweaking the
>config file is the only solution.
>
>Now, I would be satisfied to assume that this is an Aventail Connect bug,
>BUT... I am not convinced that that is the case.  Here is my reasoning:
>
>   - Aventail Connect does not have any affect on non-xfree86 local 
>xclients
>(TeraTermSSH for example).  Therefore, it IS possible to write local
>xclients that are not impaired by the VPN software.
>
>   - Aventail Connect does not screw up local TCP/IP traffic to server
>processes running on localhost.  I tested this by running Apache Jakarta's
>Tomcat 4.0 Beta 1 JSP/Servlet engine on localhost port 8080.  This should 
>be
>equivalent to xfree86 running on localhost at port 6000.  I am able to
>access JSPs, servlets, and HTML pages at http://localhost:8080 with a local
>web browser with no problems at all.
>
>   - Aventail Connect prevents local xfree86 xclients from displaying on
>_remote_ X servers.  Yet, no other client TCP/IP programs are affected
>(telnet, ssh, ftp, etc. - to the same remote host... so networking is 
>fine).
>
>   - No other applications other than xfree86 xclients are affected by the
>Aventail Connect software.  Even other cygwin programs, which I expect to
>use the same networking APIs and features, are _not_ impared by the VPN
>software.
>
>   - The TCP/IP part of it is working.  I found this by sniffing packets
>between my win2k machine and a linux box using ethereal.  I ran
>Cygwin/xfree86's xterm on win2k with the display set to the linux boxes'
>display (with "xhost +" on the linux box).  I did this with and with out 
>the
>Aventail Connect config file tweak (so it was successful for one try and
>failed on the other).  What I found was that the TCP socket is successfully
>establed in both cases.  But, in the case where the the tweak was not
>applied (the failed X connect), the win2k machine closed the connection
>immediately after openning it.  In fact it sent two acknowledgements to the
>linux boxes' SYN/ACK packet.  The first acknowledgement was simply just an
>acknowledgement (no other TCP flags set).  The second packet had both ACK
>and FIN set.  Both packets sent the exact same TCP sequence numbers and
>acknowledgement numbers (without a "next sequence number").
>
>   - I have experienced no other networking problems with either test
>machine.
>
>Now I at least have a work around for the problem.  However, I still 
>believe
>that the problem should be fixed and _is_ fixable within Cygwin and/or
>xfree86.  It also is not necessarily the VPN software's problem.  Clearly
>there is some kind of bad interaction between the xclients and the VPN
>software, but since no other networking software (that I have used) is
>broken by the VPN software and since the other programs generally do 
>similar
>networking stuff to that of the xclients,  I don't see any reason why the
>xclients can't be made to work okay with the VPN software (without tweaking
>the config file).  Unless the X client libs are using obscure networking
>features that are broken by the VPN software but are not used by any of the
>other software, it seems to me that, maybe, they are doing things that:
>   - they shouldn't be doing (things that have unpredictable results in the
>APIs), or
>   - aren't fully or properly supported in the Cygwin socket libs, or
>   - are unnecessarily complex or obscure.
>
>Questions:
>   - Where is a good place in the source code of the Cygwin and xfree86
>libraries for me to start tracing these problems down?
>   - Where are all the socket calls made when establishing an X client
>connection?
>   - Is it safe for me to assume that xfree86 is strictly doing
>Cygwin/POSIX-type socket calls (therefore no direct calls to the Win32
>networking APIs)?
>
>Thank you all for your generous hints, suggestions, and helpful responses.
>
>Chris Landrieu
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