For security, it is often be desirable to enable client isolation, particularly on a guest wifi, to prevent untrusted devices from communicating with each other.
Unfortunately this is an all or nothing approach – unless using ACLs as a pseudo client isolation technique, there is no way on most access points to make exceptions to client isolation. Under the hood, isolation is usually achieved using private VLANs i.e. each client is dropped into its own VLAN, and can only communicate with the base station which refuses to forward traffic across the client VLANs, only upstream out of the AP ethernet port from the explicitly defined VLAN of that SSID (if VLANs are being used).
I resolved this issue very simply by using my AppleTV’s ethernet connection. Wifi clients may not communicate with each other due to client isolation on the AP, but from the AP’s point of view, anything on its ethernet interface in the same VLAN & subnet is fair game (i.e. the AppleTV). This also has the added benefit of offloading the AppleTV to AP traffic to ethernet, freeing up Wifi bandwidth for the client to AP link. As I run a segmented network, I simply configured the switch port that the AppleTV is connected to as an untagged access port on my Wifi VLAN.
It turns out this arrangement can also be replicated on the Chromecast, despite it not having a built in ethernet port. The secret is a largely unheard of feature – the Chromecast micro USB port can actually act as a host interface (despite being a type B connector) and allows the use of a USB ethernet adaptor. Such an adaptor needs to have a chipset that is supported by the Chromecast plus a power feedthrough connection, or be used in conjunction with a special power tap USB cable to still power the Chromecast. It turns out such a product exists and is cheaply available on Amazon or Ebay. It’s called the UGREEN Ethernet Adapter for Chromecast Micro USB to RJ45 Port 10/100Mbps USB 2.0.
This is what it looks like:
Problem solved! I’ve also seen cheaper USB ethernet adaptors that claim to work with Chromecast, as well as power tap USB cables for these (micro USB for Chromecast, USB B for ethernet adaptor and a USB A for power). I have not tested this combination, instead going for the complete solution from Ugreen which is working a treat.