I am a part of a team that is building an ASP.NET web system. We need to guarantee, that when the main hosting server with IIS is being updated, web site visitors get the special message that the server is being updated and will be back on track shortly. For that, clearly, we need to have another server in place, that is available when the main host is not. But what mechanism is best to automatically feed a "will be back soon" page to a requesting client? Is is a smart DNS that checks if one local IP is getting pinged, and if so, returns it's server response, or else asks a backup server with a maintenance page to server a response to a user? Or is it done with some other technology in production? Thanks.
A load balancer or other application-layer proxy software/device at the front end can be configured to direct clients to a "busy" page when none of the servers are available, either through testing or manual removal of servers from the pool. I think that's what you mean when you say "smart DNS", because DNS is not smart, in any way, ever.