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ASP.NET Core - ASPNETCORE_FORWARDEDHEADERS_ENABLED environment variable

Although you can directly use Kestrel to act as an “Edge” server, a typical approach is to host it behind a reverse proxy(NGINX, Azure Application Gateway, …). If you are running your ASP.NET Core application inside Kubernetes, this is typically handled through an ingress. This ingress defines the hostname and paths your application should be exposed at. An ingress controller will map this configuration to an implementation. This can be a direct mapping to some infrastructure configuration such as a load balancer(e.g. Azure Application Gateway) or a reverse proxy running inside the cluster.

When your app is running behind a reverse proxy, it is important to configure your application to use the “forwarded headers” added by the reverse proxy. This can be done through the Forwarded Headers middleware.

Starting from .NET Core 3.0 you can take a different approach through the usage of the ASPNETCORE_FORWARDEDHEADERS_ENABLED environment variable. . Turning on the Forwarded Headers Middleware is as simple as setting this environment variable to ‘true’.

Important to notice is that you should only take this approach when you’re:
A. Positive you're behind a trusted proxy
B. AND the deployment environment is unpredictable such that you can't specify KnownProxies or KnownNetworks. A cloud provider is such an environment.

Otherwise you should configure ForwardedHeadersOptions as shown above and set KnownProxies/Networks.

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