How does Cisco Umbrella respond to the client if the FQDN in the DNS query is one of the grey-listed domains?

Returning the IP address of the content provider would bypass Umbrella's inspection for grey-listed domains, which require further scrutiny.

When a domain is grey-listed, Cisco Umbrella doesn't block it outright but instead redirects the client's DNS query to the IP address of its intelligent proxy. The intelligent proxy then performs a deeper inspection of the HTTP/HTTPS traffic in real-time to determine if the content is malicious before allowing or blocking access.

Returning the IP address of a blocked landing page is typically reserved for domains that are definitively identified as malicious or explicitly blocked, not for grey-listed domains requiring inspection.

Not sending a response or blocking traffic outright is for definitively blocked domains, whereas grey-listed domains are subject to conditional access via the intelligent proxy.