We've just pushed some new code to the 'next' branch for better control over ControlPort. Part of this was to make using configuration/preference files more easily and widely used in GNU Radio. While we've had a bunch of configuration files installed, we only really used them for some audio and wxgui parameters. I think we might start using them more heavily in the future for better control over how GNU Radio works, especially now that we're adding a lot of new features that need a finer touch to handle.
First, a word on the preference files. These get installed by default into ${prefix}/etc/gnuradio/conf.d. Each component has its own, but we actually treat it as a flattened list of sections and options (and so we have to be careful that we don't duplicate section and option names). These config files would be system-wide defaults. However, any option can be overwritten for a user by making a [HOME]/.gnuradio/config.conf file. Just duplicate the section name and the option name in this file, and it will take precedence over the system file.
For example, gnuradio-core.conf has a section [ControlPort] and an option "on". This toggles ControlPort on/off. Say by default we want to keep ControlPort off (which is how GNU Radio is installed be default now). So our /opt/etc/gnuradio/conf.d/gnuradio-core.conf file has the "on" option set to "Off" (or False, or 0). But a particular user wants to use ControlPort. So he would create a file /home/me/.gnuradio/config.conf and put in the following:
[ControlPort] on = True
And then next time GNU Radio is launched, this preference is read and ControlPort is turned on.
As a side note, we can also control any preference with the use of environmental variables. Just take the section name and option name and convert them into all caps with underscores between them. Then append "GR_CONF_" to it and you have your variable name. Set this to whatever you'd like. So our user me above could use "export GR_CONF_CONTROLPORT_ON=True" to manipulate that setting.
But back to the main point here. ControlPort can now be easily configured based on any configuration parameters available through ICE. We have a section:option in gnuradio-core.conf called "ControlPort:config" that points to the ICE configuration file location. I've made a really simple example that you can find installed at ${prefix}/etc/gnuradio/ctrlport.conf.example. This just shows you the format of creating a specific endpoint, like:
ControlPort.Endpoints = tcp -t 300 -h 127.0.0.1 -p 23456
Why would this be important? Well, by default ControlPort doesn't know which interface or ports you want to use in your setup, so it defaults to opening a random port on all possible interfaces. This is good because it makes it easy and is the right behavior when no one knows any better, but it is also bad for two reasons. First, it's a security risk to open a random port on all of your machines network interfaces. Second, you probably want to know which port you are using so that you can connect remotely, possibly through a firewall. Setting specific endpoints allows us to control where ControlPort is exposed and on what port. And, frankly, it's about time.
By the way, most of what I've discussed here is also available in the Doxygen manual that you can build off the code in master/next (and will be made available online with the next release).