How can response headers be customized in Envoy?

Response headers in Envoy can be customized through HTTP header manipulation filters in its configuration. This method allows users to modify existing headers, add new ones, or remove headers altogether based on specific rules defined in the Envoy configuration. This flexibility is crucial for various use cases, such as adding security headers, modifying content types, implementing CORS policies, or modifying response caching behavior.

Using header manipulation filters can be employed dynamically and can be applied to specific routes or services within Envoy. This means that instead of making code-level changes in the application, which can be more complex and time-consuming, network and service behavior can be adjusted directly at the proxy layer, providing a clear separation between the application logic and network configurations.

The other options do not facilitate the desired manipulation of response headers in the effective manner that Envoy's filters provide. Modifying the code at the application level is often more cumbersome and less efficient for this purpose. Relying on external applications alone may introduce unnecessary complexity and is not the built-in functionality of Envoy. Lastly, keeping headers as default would not achieve any customization and would limit the ability to tailor responses to specific needs or requirements.