RE: Info on "Can't open display"

["Christopher Landrieu" <landrieu at hotmail dot com>]

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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