Squid Transparent Proxy and 3rd Party Software
We have a computer on our network that runs a piece of 3rd party software that is essentially a tool for transferring files, we send them Audio which they transcribe and send us back the text documents. When the Transparent Proxy is off, this works fine, when the Transparent Proxy is on, the software can't connect. We have exempted the computer from the restrictions in the proxy, and have whitelisted the IP addresses and web addresses that the software is attempting to connect to, but still get the same behavior, proxy off, everything fine, proxy on, won't connect even though it has full access to the web. Their tech support has been less than helpful, any suggestions are appreciated.
If I recall correctly we saw a problem like this once before too. It was some time ago, so this could be completely off - I remember playing with something along the lines of the safeports config in the squid.inc file. Seems like something was getting blocked even though it wasn't HTTP…I can't be quite sure. Might also try adding the destinations to bypass squid altogether. Search the forum as I have posted instructions on how to do that via squid.inc Good luck, let us know if you make any progress.
I've had problems with FTP when the transparent proxy is turned on.
See if their software can be configured to use an FTP proxy (yours is obviously on firewall:3128) and - if this is the problem - it should work.
Along the same lines as Bern suggested, try tinkering with the FTP helper application. Mine is checked on wan for Disable the userland FTP-Proxy application and unchecked on LAN. The setting is on the interfaces config page.