regex - Redirect Non WWW to WWW using Asp.Net Core Middleware

Question

On an ASP.Net Core application startup I have:

RewriteOptions rewriteOptions = new RewriteOptions(); 

rewriteOptions.AddRedirectToHttps();

applicationBuilder.UseRewriter(rewriteOptions);

When in Production I need to redirect all Non WWW to WWW Urls

For example:

domain.com/about > www.domain.com/about

How can I do this using Rewrite Middleware?

I think this can be done using AddRedirect and Regex:

Github - ASP.NET Core Redirect Docs

But not sure how to do it ...

Answer 1

A reusable alternative would be to create a custom rewrite rule and a corresponsing extension method to add the rule to the rewrite options. This would be very similar to how AddRedirectToHttps works.

Custom Rule:

public class RedirectToWwwRule : IRule
{
    public virtual void ApplyRule(RewriteContext context)
    {
        var req = context.HttpContext.Request;
        if (req.Host.Host.Equals("localhost", StringComparison.OrdinalIgnoreCase))
        {
            context.Result = RuleResult.ContinueRules;
            return;
        }

        if (req.Host.Value.StartsWith("www.", StringComparison.OrdinalIgnoreCase))
        {
            context.Result = RuleResult.ContinueRules;
            return;
        }

        var wwwHost = new HostString($"www.{req.Host.Value}");
        var newUrl = UriHelper.BuildAbsolute(req.Scheme, wwwHost, req.PathBase, req.Path, req.QueryString);
        var response = context.HttpContext.Response;
        response.StatusCode = 301;
        response.Headers[HeaderNames.Location] = newUrl;
        context.Result = RuleResult.EndResponse;
    }
}

Extension Method:

public static class RewriteOptionsExtensions
{
    public static RewriteOptions AddRedirectToWww(this RewriteOptions options)
    {
        options.Rules.Add(new RedirectToWwwRule());
        return options;
    }
}

Usage:

var options = new RewriteOptions();
options.AddRedirectToWww();
options.AddRedirectToHttps();
app.UseRewriter(options);

Future versions of the rewrite middleware will contain the rule and the corresponding extension method. See this pull request