How can I use Apple Caching Service on a Network with Multiple Public IPs?

Question

Hello!
I help manage a network of ~4000 clients for a small liberal arts college in Michigan. I'm looking into the possibility of implimenting Apple Caching Server for our network.
We have one 400mbit pipe out to the internet, and all of our clients are given public-facing IPs to the internet. A caching server would be great, especially on update days. All wireless clients are on the same subnet, which is where I'd like the server to be serving the cached copies.
I have installed Mavericks on a fresh machine, downloaded OS X Server 3.0.3, and attempted to start the caching service. This is what I get.
Unable to start service.
Caching cannot be run on a public network. Consult documentation.
How can I get this up and running?

Answer 1

The way the Caching server works is that the server will be accessing the Internet and when doing so traffic will be coming from it via a particular public IP address. Usually this will not be the address of the server itelf but your router as for most networks NAT is used. In this by far more common scenario the client Macs (and likely iOS devices) will be going through the same router and hence show up via the same public IP address.
If the client request is the same as the address registered via the Caching server then Apple redirect the request via the Caching server.
The setup would look something like this -
           Internet
                |
            Router (with NAT)
                |
  (LAN)     +------Caching Server-----Client devices
With this setup because everything is using the same public IP address Apple can reasonably assume everything is on the same network and trigger a redirection to your Caching server.
If you try a setup like the following with the Caching server having its own public IP it will not work because the Caching server and client devices will have different public IP addresses
           Internet
               |
           Router (no NAT)-------------------+
               |                                      |
            Firewall (with NAT)       Caching Server
               |                                      |
               |                                      |
(LAN)     +-----Client devices-----------+-----------
Your configuration as described is more like the following
           Internet
               |
           Router (no NAT)
               |
(LAN)     +------Caching Server-----Client devices
With yours not having NAT each device has its own public IP address including the Caching server and Apple cannot redirect traffic as it thinks they are on different networks.