On Demand VPN is Apple's smart feature that allows you to initiate a VPN connection without having to go into settings or even leave the application you want to use. However, note that iOS only supports VPN On Demand for networks using certificate-based authentication (Rathe than username and password authentication). You specify which domains require a VPN connection using the configuration profile A VPN connection is made whenever a device tries to connect to specific domains. Specific applications do not need to do anything; as soon as they try to access such a server, a VPN connection is initiated.
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