ForwardedHeadersOptions from appsettings.json

appsettings.json

{
    "ForwardedForKnownNetworks": [
    {
      "Prefix": "1.2.3.4",
      "PrefixLength": 123
    },
    {
      "Prefix": "5.6.7.8",
      "PrefixLength": 111
    }
  ]
}

ForwardedForKnownNetworkOptions.cs

public class ForwardedForKnownNetworkOptions
{
    public string Prefix { get; set; }
    public int PrefixLength { get; set; }
}

startup.cs

// Register the Forwarded-For trusted networks.            
var ForwardedForKnownNetworkOptions = Configuration.GetSection("ForwardedForKnownNetworks").Get<List<ForwardedForKnownNetworkOptions>>();
services.Configure<ForwardedHeadersOptions>(options => {
    options.ForwardedHeaders = ForwardedHeaders.XForwardedFor | ForwardedHeaders.XForwardedProto;
    foreach (var knownNetwork in ForwardedForKnownNetworkOptions)
    {
        options.KnownNetworks.Add(new IPNetwork(IPAddress.Parse(knownNetwork.Prefix), knownNetwork.PrefixLength));
    }
});