Firewalls

Since Windows machines are usually festering with viruses and other malware, it has become popular to install products called "firewalls" that prevent certain forms of network connections. A typical method to do this is stopping programs from initiating socket connections, often before the destination of the connection becomes known. The user then may or may not be presented with a popup window warning that a program has attempted to "open an Internet connection".

mental ray, like many parallel programs, depends on socket connections, both for internal thread communication and for external connections to other programs such as external image viewers like imf_disp, or slave hosts that contribute to rendering, or the license server on the local network. Those "firewalls" stop those as well, often claiming that "an Internet connection" was attempted, preventing mental ray (or the application that it is built into) from starting up. mental ray is not given a chance to prevent this or even warn the user about what has just happened.

mental ray attempts to work around a few of these problems by avoiding certain types of internal communication to utilize sockets, but the underlying problem caused by those "firewalls" cannot be fixed. This workaround mode is enabled in some integrations only.

This is a Windows-specific problem. Unix, Linux or Mac OS X do not suffer from this.