TL;DR

Disable the portal, and display an "under maintenance" page instead.

Use

Configure a commons to server a maintenance page like this:

gen3 kube-setup-dashboard

deploy an "under maintenance" page to the dashboard
- install manifest-folder/dashboard/Public/maintenance-page/index.html via the normal cdis-manifest PR process
- gen3 dashboard gitops-sync to deploy files to the dashboard
- note: there is a sample maintenance page under cloud-automation/files/dashboard/maintenance-page/, and you may customize it with different logo images
enable the the maintenance page by setting global.maintenance_mode to on in the manifest.json
- you can make and revoke this change locally on the admin vm
- gen3 kube-setup-revproxy to update the proxy
when maintenance mode is enabled, then web pages normally served by the portal are redirected to the maintenance page - ex: https://commons.org/ redirects to https://commons.org/dashboard/Public/maintenance-page/index.html
a developer can bypass the maintenance page, and access the portal for testing by setting the devmode cookie in the browser console - ex: document.cookie = 'devmode=on;path=/'