The problem with the Centos system is the port number, I think -- if you use DNS to handle the LAN/WAN issue, the port number on the target host must be the same from both the LAN and the WAN.
Thus, if WAN access uses port 999, then you must modify the config on the CentOS box so that feng office works on port 999 from the LAN, too.
Early on in this discussion, Charlie suggested
ProxyPass - this, too, might solve your problem:
- Configure feng office to work from the LAN
- Configure proxypass on the SME to pass the desired domain (fengoffice.blah.com) or ibay (blah.com/fengoffice) back and forth to the CentOS box.
[offthewall idea]
You may be able to configure your web server (and feng office) to use the same database and data folders from two different URLs - so LAN users could use one URL, and outside users could use another URL.
[/offthewall idea]