Sockets -> WSAAsyncSelect has no equivilant in linux ... wondering how bad this is.
Berkley sockets have no concept of an 'application window'. Pick up Unix Network Programming by W. Richard Stevens.
MFC -> wxWindows !!! no idea of a difference`s ???? anybody
I'll bet it's easier and better designed than MFC :P
You could also do away with the MFC gui code and write one in something like Python Tkinter. If that ends up not performing well enough you can then invest your time in writing the gui in wxWindows or some other cross platform toolkit.
(besides byte alignment and ordering etc).
You can hope that whoever wrote the original app wrote portable code then you won't have to worry about this much.