I managed to adjust the GUI and to open a socket through the firewall with the SSH header arriving from the server.
The socket can be passed to the SSH session with ssh_options_set( my_ssh_session, SSH_OPTIONS_FD, &proxysocket);
This about where I am stuck, because it is unclear to me how to signal to the libssl library (in sshmasterconnection.cpp) that the socket is already connected.
The changed coding is available at http://www.xypron.de/viewvc/svn/x2go/x2goclient/branches/proxy/ svn co http://www.xypron.de/svn/x2go/x2goclient/branches/proxy/
Best regards
Heinrich