The Google Cast extension for Chrome and related browsers (such as the open source Chromium) allows, like the similarly named Android app, arbitrary desktop content to be mirrored to a HDMI TV or monitor with a connected Chromecast device.
In recent versions of the official Google Chrome browser (version 51 and newer), the Cast functionality has been integrated into the browser such that no extension is necessary, through what is called the Media Router feature.
In a total coincidence, many users of the open source Chromium browser have had problems recently with the Google Cast extension no longer functioning. While initially it appeared to be a deliberate policy change thanks to a Google support channel miscommunication, it turns out that there are two separate issues that could be the cause of the Google Cast extension no longer working in the Chromium browser.
Conflict with new “Media Router” functionality
While the Chromium browser does not seem to have the required functionality to use the Chromecast via the Media Router feature built in, strangely it ships with the associated feature control flag enabled by default, which then prevents the traditional Google Cast extension from working. Simply follow the instructions posted by users in reviews on the Chrome Web Store:
What resolved the issue for me was going to chrome://flags/#media-router and disabling the Media Router from the drop down menu. Then relaunch the browser.
A bug introduced circa Chrome 53-54
A bug was introduced that blocked the Google Cast extension via an error in the content security policy. That bug was fixed on Sept. 20, 2016 and is not present in newer builds of Chrome 54 and onward.
So if you’ve found yourself suddenly unable to use your Chromecast via the Chromium browser: check your configuration flags, upgrade your browser if necessary, and you should be Chromecasting again in no time!
Addendum
The multicast network protocol that Chromecast clients use to communicate with the device is described well in this Cisco manual.
If you are wondering whether your Google Cast extension is working at a basic level (that it is attempting to communicate with the Chromecast(s) on your network), you can do:
# tcpdump -A -i wlan0 -n udp dst 239.255.255.250You should see DIAL protocol packets like the following originating from the IP address where the browser with the Cast extension is running:
16:08:30.400846 IP 172.16.2.24.58274 > 239.255.255.250.1900: UDP, length 172
E.....@...H............l....M-SEARCH * HTTP/1.1
HOST: 239.255.255.250:1900
MAN: "ssdp:discover"
MX: 1
ST: urn:dial-multiscreen-org:service:dial:1
USER-AGENT: Google Chrome/53.0.2785.143 LinuxIf you see these packets, the cast functionality in your browser is working and you’ve likely got a different problem on your network.