We are facing a problem with user auth and transparent proxy. Right now our users are redirected to the proxy via WCCP protocol when they try to open connections to ports 80/443. Then the proxy checks its auth session cache. If the user has a valid session, then the connection is allowed. If not, the request is redirected to the internal Authentication Server. The user's browser authenticates transparently and then the connection is allowed. Also the user's source IP is saved for 24 hours on this cache so no more auth prompt is done during the rest of the day.
The problem is the redirect. Instead of redirecting to the proxy IP its redirected to a weird uri: half destination IP, half proxy own uri. Its taking the IP of the original destination host and appends the internal auth url.