Dealing with Application Base URLs and Razor link generation while hosting ASP.NET web apps behind Reverse Proxies

Since the hanselman.com app will point to hanselman.azurewebsites.net (or one of its staging slots) there's no issue with URL generation. If I say / I mean /, the root of the site. If I generate a URL like "~/about" I'll get hanselman.com/about, right?

But with http://hanselmanblog.azurewebsites.net it's different.

I want hanselman.com/blog/ to point to hanselmanblog.azurewebsites.net.

That means that the Azure Front Door will be receiving traffic, then forward it on to the Azure Web App. That means:

• hanselman.com/blog/foo -> hanselmanblog.azurewebsites.net/foo
• hanselman.com/blog/bar -> hanselmanblog.azurewebsites.net/foo
• hanselman.com/blog/foo/bar/baz -> hanselmanblog.azurewebsites.net/foo/bar/baz

There's a few things to consider when dealing with reverse proxies like this.

Is part of the /path being removed or is a path being added?

In the case of DasBlog, we have a configuration setting so that the app knows where it LOOKS like it is, from the Browser URL's perspective.

My blog is at /blog so I add that in some middleware in my Startup.cs. Certainly YOU don't need to have this in config - do whatever works for you as long as context.Request.PathBase is set as the app should see it. I set this very early in my pipeline.

That if statement is there because most folks don't install their blog at /blog, so it doesn't add the middleware.

//if you've configured it at /blog or /whatever, set that pathbase so ~ will generate correctly

Uri rootUri = new Uri(dasBlogSettings.SiteConfiguration.Root);

string path = rootUri.AbsolutePath;


//Deal with path base and proxies that change the request path

if (path != "/")

{

    app.Use((context, next) =>

    {

        context.Request.PathBase = new PathString(path);

        return next.Invoke();

    });

}

Sometimes you want the OPPOSITE of this. That would mean that I wanted, perhaps hanselman.com to point to hanselman.azurewebsites.net/blog/. In that case I'd do this in my Startup.cs's ConfigureServices:

app.UsePathBase("/blog");

Be aware that If you're hosting ASP.NET Core apps behind Nginx or Apache or really anything, you'll also want ASP.NET Core to respect  X-Forwarded-For and other X-Forwarded standard headers. You'll also likely want the app to refuse to speak to anyone who isn't a certain list of proxies or configured URLs.

I configure these in Startup.cs's ConfigureServices from a semicolon delimited list in my config, but you can do this in a number of ways.

services.Configure<ForwardedHeadersOptions>(options =>

{

    options.ForwardedHeaders = ForwardedHeaders.All;

    options.AllowedHosts = Configuration.GetValue<string>("AllowedHosts")?.Split(';').ToList<string>();

});

Since Azure Front Door adds these headers as it forwards traffic, from my app's point of view it "just works" once I've added that above and then this in Configure()

app.UseForwardedHeaders();

There seems to be some confusion on hosting behind a reverse proxy in a few GitHub Issues. I'd like to see my scenario ( /foo -> / ) be a single line of code, as we see that the other scenario ( / -> /foo ) is a single line.

Have you had any issues with URL generation when hosting your Apps behind a reverse proxy?

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