Does anyone have Squid3 working to successfully cache FTP requests coming in transparently? Should the proxy still listen on 3128, and I redirect incoming reuests on 21 to 3128?
Squid doesn't support FTP transparanet proxying. Squid is an HTTP proxy and only supports HTTP connection in its listening port. SO HTTP tranparent proxy only works for Squid. If you really need a transparent FTP proxy, use Frox
Squid support FTP proxying when you configure it manually in the browsers because browser request FTP object using HTTP messages in that case.
As far as I know there is no Proxy that will support FTP transparently.
This is partly because FTP in some ways is a more complex protocol the HTTP. HTTP uses a single TCP connection for everything. FTP uses a separate TCP connection for the control channel, and for data transfers. The additional connection details for the additional TCP connections are negotiated between the client and server within the control channel.
An ftp transparent proxy would be far more complicated for FTP then HTTP.