On my Linux box I need to use an ISA proxy that requires authentication to reach the internet. I therefore installed CNTLM and configured it to point to the proxy address and listen on port 4321.
I then configured my GNOME distribution to use localhost:4321 as global proxy for HTTP and HTTPS.
The result: I can connect to the internet. I can ping intranet IPs, I do receive name resolution for intranet sites, yet I cannot ping them or open any intranet site in a browser (configured to use the distributions proxy) unless I use the site's IP address.
I tried blocking the intranet IP range in the CNTLM config file without luck.