Update .NET 6 known-issues.md

[Rick-Anderson]

Fixes dotnet/aspnetcore#42072

@Ogglas can you help me document this problem. Please review what I have and suggest new text.

[Ogglas]

SPA template issues with Individual authentication when running in production

Applications based on SPA template with Individual authentication that requires login for every page and is hosted as an Azure App Service on your own domain like https://MyDomain.com can receive the following error:

WWW-Authenticate: Bearer error="invalid_token", error_description="The issuer 'https://MyDomain.com' is invalid"

Probably related to Azure DNS (MyDomain.azurewebsites.net) but this has not been verified. It will probably occur more frequently with Always on set to true for the Azure App Service but it can happen with Always on set to false as well.

To prevent this problem without having to stop and restart the app when the error occurs:

1. Add a new app setting which contains the target DNS address. For example, create IdentityServer:IssuerUri with value https://MyDomain.com/
2. Add the following code to the app (settings is a class mapped to app setting):

if (!string.IsNullOrEmpty(settings.IdentityServer.IssuerUri))
{
    builder.Services.Configure<JwtBearerOptions>(IdentityServerJwtConstants.IdentityServerJwtBearerScheme, o => o.Authority = settings.IdentityServer.IssuerUri);
}

below this code:

builder.Services.AddAuthentication()
    .AddIdentityServerJwt();

Then modify AddIdentityServer like this:

builder.Services.AddIdentityServer(options =>
                {
                    //Used until https://github.com/dotnet/aspnetcore/issues/42072 is fixed
                    if (!string.IsNullOrEmpty(settings.IdentityServer.IssuerUri))
                    {
                        options.IssuerUri = settings.IdentityServer.IssuerUri;
                    }
                })
    .AddApiAuthorization<ApplicationUser, ApplicationDbContext>();

For more information, see this GitHub issue