Adding support for CORS rules to dotnet-monitor app

[SachiraChin]

Reasonning

I wanted to develop a simple client side web app which works with REST APIs provided by dotnet-monitor tool. My reasoning behind developing this client application was that, system admins could get rich UI experience for APIs provided by dotnet-monitor without need of other tools. The client side app would be a static site which can be configured to use one or more REST endpoint of dotnet-monitor tool and to be navigated via UI without manually providing URLs in the browser. Static app like this can be even embedded inside the tool itself to provide user experience for admins (and I agree that this scenario will not require CORS management in app itself).

Issue

When I started development, I found that, direct queries cannot be sent by browser application to the APIs provided by the tool because API does not support CORS rules.

Solution

I have worked on proof-of-concept for this which will allow user to configure CORS rules.

CORS rules for dotnet-monitor can be provided in two ways,

• Command line parameter: dotnet monitor collect --corsAllowedOrigins {origin}. This only allows user to enter allowed origins for running instance of dotnet-monitor. User can enter one or more origins using this parameter, values provided here will be merged with origins provided in appsettings.json file.

• Using static configuration using appsettings.json section. This allows user to configure all CorsPolicy properties defined in Microsoft.AspNetCore.Cors.Infrastructure.CorsPolicy class. appsettings.json section can be configured as below,

{
  "CorsPolicy": {
    "AllowAnyHeader": false,
    "AllowAnyMethod": false,
    "AllowAnyOrigin": false,
    "IsOriginAllowed": null,
    "ExposedHeaders": null,
    "Headers": null,
    "Methods": null,
    "Origins": [ "http://localhost:4200/" ],
    "PreflightMaxAge": null,
    "SupportsCredentials": false
  }
}

Implementation

Proof-of-concept implementation changes can be found in this commit in my fork.

As a highlight, changes are

Program.cs

private static Option CorsRules() =>
    new Option(
        aliases: new[] { "--corsAllowedOrigins" },
        description: "CORS allowed origins for the REST api.")
    {
        Argument = new Argument<string[]>(name: "corsAllowedOrigins", defaultValue: Array.Empty<string>())
    };

Above rule is added as an argument for CommandHandler.Create

DiagnosticsMonitorCommandHandler.cs

var corsPolicySection = context.Configuration.GetSection("CorsPolicy");
if (corsPolicySection.Exists())
{
    var corsPolicy = new CorsPolicy();
    corsPolicySection.Bind(corsPolicy);
    UpdateCorsPolicy(corsAllowedOrigins, corsPolicy);
    services.AddCors(options =>
    {
        options.AddPolicy(Startup.CorsPolicyName, corsPolicy);
    });
}
else if (corsAllowedOrigins?.Length > 0)
{
    services.AddCors(options =>
    {
        var corsPolicy = new CorsPolicy();
        UpdateCorsPolicy(corsAllowedOrigins, corsPolicy);
        options.AddPolicy(Startup.CorsPolicyName, corsPolicy);
    });
}

Above logic has added inside .ConfigureServices((WebHostBuilderContext context, IServiceCollection services) => block to create the CORS policy.

Startup.cs

app.UseCors(CorsPolicyName);

Finally added UseCors to Startup class so runtime can use the configured CORS policy.

Repository with the updated code can be found here: https://github.com/SachiraChin/diagnostics/

Conclusion

I really would appreciate if you could take this request to consideration and add it to the tool. I can make pull requests to the repository to make this change available if required.

[hoyosjs]

cc: @shirhatti @jander-msft @wiktork

[wiktork]

@SachiraChin Thank you for the detailed issue! The PR for the fix is at #1377.

[SachiraChin]

@wiktork Thanks a lot for the quick PR. :) I will use this for development of client UI I'm working on.

[noahfalk]

@SachiraChin - A bunch of us devs at MS working on .NET and VS saw the UI you were doing and its very cool!

[SachiraChin]

@noahfalk That's really really nice to hear! 😊 I'm adding few more features to the app. You will see new updates in the repo soon. Here's small glance of it,

If you or anyone had chance to look at the code and see any issue with how it written, please don't hesitate to ping back at me. I'm gladly take any criticism you have. 😊