如何在ASP.Net Core生产环境中通过Rewrite Middleware将非WWW重定向到WWW

Hey there! Let's break down how to implement that non-WWW to WWW redirect using ASP.NET Core's Rewrite Middleware, and make sure it only activates in production. Here are two reliable approaches:

Option 1: Use the Built-in Canonical Host Rule (Simplest) #

ASP.NET Core has a ready-made rule for canonical host redirection, which is perfect for this scenario—it handles all the heavy lifting for you.

In your Program.cs (for .NET 6+ with top-level statements), add this logic after building the app but before other core middleware like routing or static files:

var builder = WebApplication.CreateBuilder(args);

// Add your services here (e.g., AddControllers, AddRazorPages, etc.)
builder.Services.AddControllersWithViews();

var app = builder.Build();

// Configure error handling first (standard practice)
if (!app.Environment.IsDevelopment())
{
    app.UseExceptionHandler("/Home/Error");
    app.UseHsts();
}

// 👇 Add the rewrite rule ONLY in production
if (app.Environment.IsProduction())
{
    var rewriteOptions = new RewriteOptions()
        // Replace "www.yourdomain.com" with your actual WWW domain
        .AddCanonicalHostRewriteRule("www.yourdomain.com", permanent: true);

    app.UseRewriter(rewriteOptions);
}

// Continue with other middleware setup
app.UseHttpsRedirection();
app.UseStaticFiles();
app.UseRouting();
app.UseAuthorization();

app.MapControllerRoute(
    name: "default",
    pattern: "{controller=Home}/{action=Index}/{id?}");

app.Run();

Key Notes for This Option: #

• permanent: true sends a 301 Permanent Redirect, which is ideal for SEO (tells search engines to update their indexes to the WWW domain).
• The rule automatically preserves the request path, query strings, and scheme (HTTP/HTTPS) when redirecting.
• Make sure UseRewriter comes before middleware that processes the request (like UseStaticFiles or UseRouting) so the redirect happens early.

Option 2: Custom Rewrite Rule (For Flexibility) #

If you need more control (e.g., handling multiple non-WWW domains, port-specific logic), you can create a custom rule instead:

if (app.Environment.IsProduction())
{
    app.UseRewriter(new RewriteOptions().Add(context =>
    {
        var request = context.HttpContext.Request;
        var currentHost = request.Host;

        // Skip if the host already starts with www.
        if (currentHost.Host.StartsWith("www.", StringComparison.OrdinalIgnoreCase))
        {
            return;
        }

        // Build the new WWW host (preserve port if needed)
        var newHost = new HostString($"www.{currentHost.Host}", currentHost.Port);
        // Construct the full redirect URL
        var redirectUrl = UriHelper.BuildAbsolute(
            request.Scheme,
            newHost,
            request.PathBase,
            request.Path,
            request.QueryString);

        // Send 301 redirect and stop processing further middleware
        context.HttpContext.Response.Redirect(redirectUrl, permanent: true);
        context.Result = RuleResult.EndResponse;
    }));
}

Why This Might Be Useful: #

• You can add additional checks (e.g., exclude specific subdomains if needed).
• Full control over how the redirect URL is constructed.

Critical Reminders: #

• Environment Check: Always wrap the rule in app.Environment.IsProduction() so it doesn't interfere with local development (you don't want to redirect localhost to www.localhost!).
• HTTPS: If your production site uses HTTPS, ensure UseHttpsRedirection is in place (it should be by default in new projects) to redirect HTTP to HTTPS first, then handle the WWW redirect.
• Testing: Before deploying, verify the redirect works using tools like curl or browser dev tools to confirm the 301 status code and correct target URL.

内容的提问来源于stack exchange，提问作者Miguel Moura