What is the most likely reason why a user is not presented with a login prompt when accessing an external website through FortiGate?

When a user attempts to access an external website through FortiGate and does not receive a login prompt, it is most often due to the requirement of DNS services in the firewall policy. If the firewall policy does not include the proper configuration for DNS services, users may experience issues connecting to external websites, which can prevent any subsequent login prompts from appearing.

In this situation, allowing DNS resolution is crucial as it ensures that the FortiGate device can resolve the domain names of external websites to their corresponding IP addresses. Without proper DNS service configured in the firewall policy, the FortiGate may not establish the appropriate connections needed to display the login prompt or even reach the website itself.

Therefore, it's essential to ensure that the firewall policy explicitly allows DNS traffic, facilitating the name resolution process and allowing users to interact with external sites as intended.