[NTLUG:Discuss] Stealth drop box

[Michael Barnes]

I need to set up a drop box type operation.  I have a 
server/firewall/router (CentOS5) that sits between two networks.  On the 
admin network, the server is essentially invisible, no open ports, no 
ping response, etc.  Nothing comes in, it only goes out.  However, I 
need to have users input files to that machine.

I think what I want to do is have a public folder on a Linux server in 
the admin net that users can drop files into, either through a file 
manager (konqueror, Windows Exploder, etc.) or possibly dropping onto a 
web page.  The stealth server would need to very frequently poll that 
folder or otherwise find out there are files there, transfer them to 
itself, then delete them from the public folder.  Security for the 
internal stealth network is extremely tight.  I really don't want 
something like a mounted folder.  Moving the files would likely be via 
rsync or scp or similar.  But nothing can be pushed, it all has to be 
pulled in.

On thing I don't know about for sure.  Is there a way to listen for some 
type of broadcast packet, yet not be visible?  So if a file were dropped 
into the public folder, that server could send out some type of UDP 
packet or something that the stealth server could listen for, but no one 
doing a port scan or anything would see the open port?

I am not looking for extensive programming or anything, and it doesn't 
have to be like National Security Agency stuff.  But I do need it to be 
protected.

Thanks for any ideas, suggestions, comments, alternate methods, etc.

Michael