URL Rewriting Middleware in ASP.NET Core

By Kirk Larkin and Rick Anderson

This article introduces URL rewriting with instructions on how to use URL Rewriting Middleware in ASP.NET Core apps.

URL rewriting is the act of modifying request URLs based on one or more predefined rules. URL rewriting creates an abstraction between resource locations and their addresses so that the locations and addresses aren't tightly linked. URL rewriting is valuable in several scenarios to:

• Move or replace server resources temporarily or permanently and maintain stable locators for those resources.
• Split request processing across different apps or across areas of one app.
• Remove, add, or reorganize URL segments on incoming requests.
• Optimize public URLs for Search Engine Optimization (SEO).
• Permit the use of friendly public URLs to help visitors predict the content returned by requesting a resource.
• Redirect insecure requests to secure endpoints.
• Prevent hotlinking, where an external site uses a hosted static asset on another site by linking the asset into its own content.

URL rewriting can reduce the performance of an app. Limit the number and complexity of rules.

URL redirect and URL rewrite

The difference in wording between URL redirect and URL rewrite is subtle but has important implications for providing resources to clients. ASP.NET Core's URL Rewriting Middleware is capable of meeting the need for both.

A URL redirect involves a client-side operation, where the client is instructed to access a resource at a different address than the client originally requested. This requires a round trip to the server. The redirect URL returned to the client appears in the browser's address bar when the client makes a new request for the resource.

If /resource is redirected to /different-resource, the server responds that the client should obtain the resource at /different-resource with a status code indicating that the redirect is either temporary or permanent.

When redirecting requests to a different URL, indicate whether the redirect is permanent or temporary by specifying the status code with the response:

• The 301 - Moved Permanently status code is used where the resource has a new, permanent URL and that all future requests for the resource should use the new URL. The client may cache and reuse the response when a 301 status code is received.

• The 302 - Found status code is used where the redirection is temporary or generally subject to change. The 302 status code indicates to the client not to store the URL and use it in the future.

For more information on status codes, see RFC 9110: Status Code Definitions.

A URL rewrite is a server-side operation that provides a resource from a different resource address than the client requested. Rewriting a URL doesn't require a round trip to the server. The rewritten URL isn't returned to the client and doesn't appear in the browser's address bar.

If /resource is rewritten to /different-resource, the server internally fetches and returns the resource at /different-resource.

Although the client might be able to retrieve the resource at the rewritten URL, the client isn't informed that the resource exists at the rewritten URL when it makes its request and receives the response.

URL rewriting sample app

Explore the features of the URL Rewriting Middleware with the sample app. The app applies redirect and rewrite rules and shows the redirected or rewritten URL for several scenarios.

When to use URL rewriting middleware

Use URL Rewriting Middleware when the following approaches aren't satisfactory:

• URL Rewrite module with IIS on Windows Server
• Apache mod_rewrite module on Apache Server
• URL rewriting on Nginx

Use the URL rewriting middleware when the app is hosted on HTTP.sys server.

The main reasons to use the server-based URL rewriting technologies in IIS, Apache, and Nginx are:

• The middleware doesn't support the full features of these modules.

  Some of the features of the server modules don't work with ASP.NET Core projects, such as the IsFile and IsDirectory constraints of the IIS Rewrite module. In these scenarios, use the middleware instead.

• The performance of the middleware probably doesn't match that of the modules.

  Benchmarking is the only way to know with certainty which approach degrades performance the most or if degraded performance is negligible.

Extension and options

Establish URL rewrite and redirect rules by creating an instance of the RewriteOptions class with extension methods for each of the rewrite rules. Chain multiple rules in the order that they should be processed. The RewriteOptions are passed into the URL Rewriting Middleware as it's added to the request pipeline with UseRewriter:

using Microsoft.AspNetCore.Rewrite;
using RewriteRules;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

using (StreamReader apacheModRewriteStreamReader =
    File.OpenText("ApacheModRewrite.txt"))
using (StreamReader iisUrlRewriteStreamReader =
    File.OpenText("IISUrlRewrite.xml"))
{
    var options = new RewriteOptions()
        .AddRedirect("redirect-rule/(.*)", "redirected/$1")
        .AddRewrite(@"^rewrite-rule/(\d+)/(\d+)", "rewritten?var1=$1&var2=$2",
            skipRemainingRules: true)
        .AddApacheModRewrite(apacheModRewriteStreamReader)
        .AddIISUrlRewrite(iisUrlRewriteStreamReader)
        .Add(MethodRules.RedirectXmlFileRequests)
        .Add(MethodRules.RewriteTextFileRequests)
        .Add(new RedirectImageRequests(".png", "/png-images"))
        .Add(new RedirectImageRequests(".jpg", "/jpg-images"));

    app.UseRewriter(options);
}

app.UseStaticFiles();

app.Run(context => context.Response.WriteAsync(
    $"Rewritten or Redirected Url: " +
    $"{context.Request.Path + context.Request.QueryString}"));

app.Run();

In the preceding code, MethodRules is a user defined class. See RewriteRules.cs in this article for more information.

Redirect non-www to www

Three options permit the app to redirect non-www requests to www:

• AddRedirectToWwwPermanent: Permanently redirect the request to the www subdomain if the request is non-www. Redirects with a Status308PermanentRedirect status code.

• AddRedirectToWww: Redirect the request to the www subdomain if the incoming request is non-www. Redirects with a Status307TemporaryRedirect status code. An overload permits providing the status code for the response. Use a field of the StatusCodes class for a status code assignment.

URL redirect

Use AddRedirect to redirect requests. The first parameter contains the .NET regular expression (Regex) for matching on the path of the incoming URL. The second parameter is the replacement string. The third parameter, if present, specifies the status code. If the status code isn't specified, the status code defaults to 302 - Found, which indicates that the resource is temporarily moved or replaced.

using Microsoft.AspNetCore.Rewrite;
using RewriteRules;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

using (StreamReader apacheModRewriteStreamReader =
    File.OpenText("ApacheModRewrite.txt"))
using (StreamReader iisUrlRewriteStreamReader =
    File.OpenText("IISUrlRewrite.xml"))
{
    var options = new RewriteOptions()
        .AddRedirect("redirect-rule/(.*)", "redirected/$1")
        .AddRewrite(@"^rewrite-rule/(\d+)/(\d+)", "rewritten?var1=$1&var2=$2",
            skipRemainingRules: true)
        .AddApacheModRewrite(apacheModRewriteStreamReader)
        .AddIISUrlRewrite(iisUrlRewriteStreamReader)
        .Add(MethodRules.RedirectXmlFileRequests)
        .Add(MethodRules.RewriteTextFileRequests)
        .Add(new RedirectImageRequests(".png", "/png-images"))
        .Add(new RedirectImageRequests(".jpg", "/jpg-images"));

    app.UseRewriter(options);
}

app.UseStaticFiles();

app.Run(context => context.Response.WriteAsync(
    $"Rewritten or Redirected Url: " +
    $"{context.Request.Path + context.Request.QueryString}"));

app.Run();

In a browser with developer tools enabled, make a request to the sample app with the path /redirect-rule/1234/5678. The regular expression matches the request path on redirect-rule/(.*), and the path is replaced with /redirected/1234/5678. The redirect URL is sent back to the client with a 302 - Found status code. The browser makes a new request at the redirect URL, which appears in the browser's address bar. Since no rules in the sample app match on the redirect URL:

• The second request receives a 200 - OK response from the app.
• The body of the response shows the redirect URL.

A round trip is made to the server when a URL is redirected.

Warning

Be cautious when establishing redirect rules. Redirect rules are evaluated on every request to the app, including after a redirect. It's easy to accidentally create a loop of infinite redirects.

The part of the expression contained within parentheses is called a capture group. The dot (.) of the expression means match any character. The asterisk (*) indicates match the preceding character zero or more times. Therefore, the last two path segments of the URL, 1234/5678, are captured by capture group (.*). Any value provided in the request URL after redirect-rule/ is captured by this single capture group.

In the replacement string, captured groups are injected into the string with the dollar sign ($) followed by the sequence number of the capture. The first capture group value is obtained with $1, the second with $2, and they continue in sequence for the capture groups in the regular expression. There's only one captured group in the redirect rule regular expression in redirect-rule/(.*), so there's only one injected group in the replacement string, which is $1. When the rule is applied, the URL becomes /redirected/1234/5678.

Try /redirect-rule/1234/5678 with the browser tools on the network tab.

URL redirect to a secure endpoint

Use AddRedirectToHttps to redirect HTTP requests to the same host and path using the HTTPS protocol. If the status code isn't supplied, the middleware defaults to 302 - Found. If the port isn't supplied:

• The middleware defaults to null.
• The scheme changes to https (HTTPS protocol), and the client accesses the resource on port 443.

The following example shows how to set the status code to 301 - Moved Permanently and change the port to the HTTPS port used by Kestrel on localhost. In production, the HTTPS port is set to null:

using Microsoft.AspNetCore.Rewrite;
using RewriteRules;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

int? localhostHTTPSport = null;
if (app.Environment.IsDevelopment())
{
    localhostHTTPSport = Int32.Parse(Environment.GetEnvironmentVariable(
                   "ASPNETCORE_URLS")!.Split(new Char[] { ':', ';' })[2]);
}

using (StreamReader apacheModRewriteStreamReader =
    File.OpenText("ApacheModRewrite.txt"))
using (StreamReader iisUrlRewriteStreamReader =
    File.OpenText("IISUrlRewrite.xml"))
{
    var options = new RewriteOptions()
        // localhostHTTPport not needed for production, used only with localhost.
        .AddRedirectToHttps(301, localhostHTTPSport)
        .AddRedirect("redirect-rule/(.*)", "redirected/$1")
        .AddRewrite(@"^rewrite-rule/(\d+)/(\d+)", "rewritten?var1=$1&var2=$2",
            skipRemainingRules: true)
        .AddApacheModRewrite(apacheModRewriteStreamReader)
        .AddIISUrlRewrite(iisUrlRewriteStreamReader)
        .Add(MethodRules.RedirectXmlFileRequests)
        .Add(MethodRules.RewriteTextFileRequests)
        .Add(new RedirectImageRequests(".png", "/png-images"))
        .Add(new RedirectImageRequests(".jpg", "/jpg-images"));

    app.UseRewriter(options);
}

app.UseStaticFiles();

app.Run(context => context.Response.WriteAsync(
    $"Rewritten or Redirected Url: " +
    $"{context.Request.Path + context.Request.QueryString}"));

app.Run();

Use AddRedirectToHttpsPermanent to redirect insecure requests to the same host and path with secure HTTPS protocol on port 443. The middleware sets the status code to 301 - Moved Permanently.

using Microsoft.AspNetCore.Rewrite;
using RewriteRules;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

using (StreamReader apacheModRewriteStreamReader =
    File.OpenText("ApacheModRewrite.txt"))
using (StreamReader iisUrlRewriteStreamReader =
    File.OpenText("IISUrlRewrite.xml"))
{
    var options = new RewriteOptions()
        .AddRedirectToHttpsPermanent()
        .AddRedirect("redirect-rule/(.*)", "redirected/$1")
        .AddRewrite(@"^rewrite-rule/(\d+)/(\d+)", "rewritten?var1=$1&var2=$2",
            skipRemainingRules: true)
        .AddApacheModRewrite(apacheModRewriteStreamReader)
        .AddIISUrlRewrite(iisUrlRewriteStreamReader)
        .Add(MethodRules.RedirectXmlFileRequests)
        .Add(MethodRules.RewriteTextFileRequests)
        .Add(new RedirectImageRequests(".png", "/png-images"))
        .Add(new RedirectImageRequests(".jpg", "/jpg-images"));

    app.UseRewriter(options);
}

app.UseStaticFiles();

app.Run(context => context.Response.WriteAsync(
    $"Rewritten or Redirected Url: " +
    $"{context.Request.Path + context.Request.QueryString}"));

app.Run();

Note

When redirecting to a secure endpoint without the requirement for additional redirect rules, we recommend using HTTPS Redirection Middleware. For more information, see Enforce HTTPS.

The sample app demonstrates how to use AddRedirectToHttps or AddRedirectToHttpsPermanent. Make an insecure HTTP request to the app at http://redirect6.azurewebsites.net/iis-rules-rewrite/xyz. When testing HTTP to HTTPS redirection with localhost:

• Use the HTTP URL, which has a different port than the HTTPS URL. The HTTP URL is in the Properties/launchSettings.json file.
• Removing the s from https://localhost/{port} fails because localhost doesn't respond on HTTP to the HTTPS port.

The following image shows the F12 browser tools image of a request to http://redirect6.azurewebsites.net/iis-rules-rewrite/xyz using the preceding code:

URL rewrite

Use AddRewrite to create a rule for rewriting URLs. The first parameter contains the regular expression for matching on the incoming URL path. The second parameter is the replacement string. The third parameter, skipRemainingRules: {true|false}, indicates to the middleware whether or not to skip additional rewrite rules if the current rule is applied.

using Microsoft.AspNetCore.Rewrite;
using RewriteRules;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

using (StreamReader apacheModRewriteStreamReader =
    File.OpenText("ApacheModRewrite.txt"))
using (StreamReader iisUrlRewriteStreamReader =
    File.OpenText("IISUrlRewrite.xml"))
{
    var options = new RewriteOptions()
        .AddRedirectToHttpsPermanent()
        .AddRedirect("redirect-rule/(.*)", "redirected/$1")
        .AddRewrite(@"^rewrite-rule/(\d+)/(\d+)", "rewritten?var1=$1&var2=$2",
            skipRemainingRules: true)
        .AddApacheModRewrite(apacheModRewriteStreamReader)
        .AddIISUrlRewrite(iisUrlRewriteStreamReader)
        .Add(MethodRules.RedirectXmlFileRequests)
        .Add(MethodRules.RewriteTextFileRequests)
        .Add(new RedirectImageRequests(".png", "/png-images"))
        .Add(new RedirectImageRequests(".jpg", "/jpg-images"));

    app.UseRewriter(options);
}

app.UseStaticFiles();

app.Run(context => context.Response.WriteAsync(
    $"Rewritten or Redirected Url: " +
    $"{context.Request.Path + context.Request.QueryString}"));

app.Run();

Try the request to https://redirect6.azurewebsites.net/rewrite-rule/1234/5678

The caret (^) at the beginning of the expression means that matching starts at the beginning of the URL path.

In the earlier example with the redirect rule, redirect-rule/(.*), there's no caret (^) at the start of the regular expression. Therefore, any characters may precede redirect-rule/ in the path for a successful match.

Path	Match
/redirect-rule/1234/5678	Yes
/my-cool-redirect-rule/1234/5678	Yes
/anotherredirect-rule/1234/5678	Yes

The rewrite rule, ^rewrite-rule/(\d+)/(\d+), only matches paths if they start with rewrite-rule/. In the following table, note the difference in matching.

Path	Match
/rewrite-rule/1234/5678	Yes
/my-cool-rewrite-rule/1234/5678	No
/anotherrewrite-rule/1234/5678	No

Following the ^rewrite-rule/ portion of the expression, there are two capture groups, (\d+)/(\d+). The \d signifies match a digit (number). The plus sign (+) means match one or more of the preceding character. Therefore, the URL must contain a number followed by a forward-slash followed by another number. These capture groups are injected into the rewritten URL as $1 and $2. The rewrite rule replacement string places the captured groups into the query string. The requested path /rewrite-rule/1234/5678 is rewritten to return the resource at /rewritten?var1=1234&var2=5678. If a query string is present on the original request, it's preserved when the URL is rewritten.

There's no round trip to the server to return the resource. If the resource exists, it's fetched and returned to the client with a 200 - OK status code. Because the client isn't redirected, the URL in the browser's address bar doesn't change. Clients can't detect that a URL rewrite operation occurred on the server.

Performance tips for URL rewrite and redirect

For the fastest response:

• Order rewrite rules from the most frequently matched rule to the least frequently matched rule.
• Use skipRemainingRules: true whenever possible because matching rules is computationally expensive and increases app response time. Skip the processing of the remaining rules when a match occurs and no additional rule processing is required.

Warning

A malicious user can provide expensive to process input to RegularExpressions causing a Denial-of-Service attack. ASP.NET Core framework APIs that use RegularExpressions pass a timeout. For example, the RedirectRule and RewriteRule classes both pass in a one second timeout.

Apache mod_rewrite

Apply Apache mod_rewrite rules with AddApacheModRewrite. Make sure that the rules file is deployed with the app. For more information and examples of mod_rewrite rules, see Apache mod_rewrite.

A StreamReader is used to read the rules from the ApacheModRewrite.txt rules file:

using Microsoft.AspNetCore.Rewrite;
using RewriteRules;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

using (StreamReader apacheModRewriteStreamReader =
    File.OpenText("ApacheModRewrite.txt"))
using (StreamReader iisUrlRewriteStreamReader =
    File.OpenText("IISUrlRewrite.xml"))
{
    var options = new RewriteOptions()
        .AddRedirectToHttpsPermanent()
        .AddRedirect("redirect-rule/(.*)", "redirected/$1")
        .AddRewrite(@"^rewrite-rule/(\d+)/(\d+)", "rewritten?var1=$1&var2=$2",
            skipRemainingRules: true)
        .AddApacheModRewrite(apacheModRewriteStreamReader)
        .AddIISUrlRewrite(iisUrlRewriteStreamReader)
        .Add(MethodRules.RedirectXmlFileRequests)
        .Add(MethodRules.RewriteTextFileRequests)
        .Add(new RedirectImageRequests(".png", "/png-images"))
        .Add(new RedirectImageRequests(".jpg", "/jpg-images"));

    app.UseRewriter(options);
}

app.UseStaticFiles();

app.Run(context => context.Response.WriteAsync(
    $"Rewritten or Redirected Url: " +
    $"{context.Request.Path + context.Request.QueryString}"));

app.Run();

The sample app redirects requests from /apache-mod-rules-redirect/(.\*) to /redirected?id=$1. The response status code is 302 - Found.

# Rewrite path with additional sub directory
RewriteRule ^/apache-mod-rules-redirect/(.*) /redirected?id=$1 [L,R=302]

Try the request to https://redirect6.azurewebsites.net/apache-mod-rules-redirect/1234

The Apache middleware supports the following Apache mod_rewrite server variables:

• CONN_REMOTE_ADDR
• HTTP_ACCEPT
• HTTP_CONNECTION
• HTTP_COOKIE
• HTTP_FORWARDED
• HTTP_HOST
• HTTP_REFERER
• HTTP_USER_AGENT
• HTTPS
• IPV6
• QUERY_STRING
• REMOTE_ADDR
• REMOTE_PORT
• REQUEST_FILENAME
• REQUEST_METHOD
• REQUEST_SCHEME
• REQUEST_URI
• SCRIPT_FILENAME
• SERVER_ADDR
• SERVER_PORT
• SERVER_PROTOCOL
• TIME
• TIME_DAY
• TIME_HOUR
• TIME_MIN
• TIME_MON
• TIME_SEC
• TIME_WDAY
• TIME_YEAR

IIS URL Rewrite Module rules

To use the same rule set that applies to the IIS URL Rewrite Module, use AddIISUrlRewrite. Make sure that the rules file is deployed with the app. Don't direct the middleware to use the app's web.config file when running on Windows Server IIS. With IIS, these rules should be stored outside of the app's web.config file in order to avoid conflicts with the IIS Rewrite module. For more information and examples of IIS URL Rewrite Module rules, see Using Url Rewrite Module 2.0 and URL Rewrite Module Configuration Reference.

A StreamReader is used to read the rules from the IISUrlRewrite.xml rules file:

using Microsoft.AspNetCore.Rewrite;
using RewriteRules;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

using (StreamReader apacheModRewriteStreamReader =
    File.OpenText("ApacheModRewrite.txt"))
using (StreamReader iisUrlRewriteStreamReader =
    File.OpenText("IISUrlRewrite.xml"))
{
    var options = new RewriteOptions()
        .AddRedirectToHttpsPermanent()
        .AddRedirect("redirect-rule/(.*)", "redirected/$1")
        .AddRewrite(@"^rewrite-rule/(\d+)/(\d+)", "rewritten?var1=$1&var2=$2",
            skipRemainingRules: true)
        .AddApacheModRewrite(apacheModRewriteStreamReader)
        .AddIISUrlRewrite(iisUrlRewriteStreamReader)
        .Add(MethodRules.RedirectXmlFileRequests)
        .Add(MethodRules.RewriteTextFileRequests)
        .Add(new RedirectImageRequests(".png", "/png-images"))
        .Add(new RedirectImageRequests(".jpg", "/jpg-images"));

    app.UseRewriter(options);
}

app.UseStaticFiles();

app.Run(context => context.Response.WriteAsync(
    $"Rewritten or Redirected Url: " +
    $"{context.Request.Path + context.Request.QueryString}"));

app.Run();

The sample app rewrites requests from /iis-rules-rewrite/(.*) to /rewritten?id=$1. The response is sent to the client with a 200 - OK status code.

<rewrite>
  <rules>
    <rule name="Rewrite segment to id querystring" stopProcessing="true">
      <match url="^iis-rules-rewrite/(.*)$" />
      <action type="Rewrite" url="rewritten?id={R:1}" appendQueryString="false"/>
    </rule>
  </rules>
</rewrite>

Try the request to https://redirect6.azurewebsites.net/iis-rules-rewrite/xyz

Apps that have an active IIS Rewrite Module with server-level rules configured that impacts the app in undesirable ways:

• Consider disabling the IIS Rewrite Module for the app.
• For more information, see Disabling IIS modules.

Unsupported features

The middleware doesn't support the following IIS URL Rewrite Module features:

• Outbound Rules
• Custom Server Variables
• Wildcards
• LogRewrittenUrl

Supported server variables

The middleware supports the following IIS URL Rewrite Module server variables:

• CONTENT_LENGTH
• CONTENT_TYPE
• HTTP_ACCEPT
• HTTP_CONNECTION
• HTTP_COOKIE
• HTTP_HOST
• HTTP_REFERER
• HTTP_URL
• HTTP_USER_AGENT
• HTTPS
• LOCAL_ADDR
• QUERY_STRING
• REMOTE_ADDR
• REMOTE_PORT
• REQUEST_FILENAME
• REQUEST_URI

IFileProvider can be obtained via a PhysicalFileProvider. This approach may provide greater flexibility for the location of rewrite rules files. Make sure that the rewrite rules files are deployed to the server at the path provided.

var fileProvider = new PhysicalFileProvider(Directory.GetCurrentDirectory());

Method-based rule

Use Add to implement custom rule logic in a method. Add exposes the RewriteContext, which makes available the HttpContext for use in redirect methods. The RewriteContext.Result property determines how additional pipeline processing is handled. Set the value to one of the RuleResult fields described in the following table.

Rewrite context result	Action
RuleResult.ContinueRules (default)	Continue applying rules.
RuleResult.EndResponse	Stop applying rules and send the response.
RuleResult.SkipRemainingRules	Stop applying rules and send the context to the next middleware.

using Microsoft.AspNetCore.Rewrite;
using RewriteRules;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

using (StreamReader apacheModRewriteStreamReader =
    File.OpenText("ApacheModRewrite.txt"))
using (StreamReader iisUrlRewriteStreamReader =
    File.OpenText("IISUrlRewrite.xml"))
{
    var options = new RewriteOptions()
        .AddRedirectToHttpsPermanent()
        .AddRedirect("redirect-rule/(.*)", "redirected/$1")
        .AddRewrite(@"^rewrite-rule/(\d+)/(\d+)", "rewritten?var1=$1&var2=$2",
            skipRemainingRules: true)
        .AddApacheModRewrite(apacheModRewriteStreamReader)
        .AddIISUrlRewrite(iisUrlRewriteStreamReader)
        .Add(MethodRules.RedirectXmlFileRequests)
        .Add(MethodRules.RewriteTextFileRequests)
        .Add(new RedirectImageRequests(".png", "/png-images"))
        .Add(new RedirectImageRequests(".jpg", "/jpg-images"));

    app.UseRewriter(options);
}

app.UseStaticFiles();

app.Run(context => context.Response.WriteAsync(
    $"Rewritten or Redirected Url: " +
    $"{context.Request.Path + context.Request.QueryString}"));

app.Run();

The sample app demonstrates a method that redirects requests for paths that end with .xml. When a request is made for /file.xml:

• The request is redirected to /xmlfiles/file.xml
• The status code is set to 301 - Moved Permanently. When the browser makes a new request for /xmlfiles/file.xml, Static File Middleware serves the file to the client from the wwwroot/xmlfiles folder. For a redirect, explicitly set the status code of the response. Otherwise, a 200 - OK status code is returned, and the redirect doesn't occur on the client.

RewriteRules.cs:

public static void RedirectXmlFileRequests(RewriteContext context)
{
    var request = context.HttpContext.Request;

    // Because the client is redirecting back to the same app, stop 
    // processing if the request has already been redirected.
    if (request.Path.StartsWithSegments(new PathString("/xmlfiles")) ||
        request.Path.Value==null)
    {
        return;
    }

    if (request.Path.Value.EndsWith(".xml", StringComparison.OrdinalIgnoreCase))
    {
        var response = context.HttpContext.Response;
        response.StatusCode = (int) HttpStatusCode.MovedPermanently;
        context.Result = RuleResult.EndResponse;
        response.Headers[HeaderNames.Location] = 
            "/xmlfiles" + request.Path + request.QueryString;
    }
}

This approach can also rewrite requests. The sample app demonstrates rewriting the path for any text file request to serve the file.txt text file from the wwwroot folder. Static File Middleware serves the file based on the updated request path:

using Microsoft.AspNetCore.Rewrite;
using RewriteRules;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

using (StreamReader apacheModRewriteStreamReader =
    File.OpenText("ApacheModRewrite.txt"))
using (StreamReader iisUrlRewriteStreamReader =
    File.OpenText("IISUrlRewrite.xml"))
{
    var options = new RewriteOptions()
        .AddRedirectToHttpsPermanent()
        .AddRedirect("redirect-rule/(.*)", "redirected/$1")
        .AddRewrite(@"^rewrite-rule/(\d+)/(\d+)", "rewritten?var1=$1&var2=$2",
            skipRemainingRules: true)
        .AddApacheModRewrite(apacheModRewriteStreamReader)
        .AddIISUrlRewrite(iisUrlRewriteStreamReader)
        .Add(MethodRules.RedirectXmlFileRequests)
        .Add(MethodRules.RewriteTextFileRequests)
        .Add(new RedirectImageRequests(".png", "/png-images"))
        .Add(new RedirectImageRequests(".jpg", "/jpg-images"));

    app.UseRewriter(options);
}

app.UseStaticFiles();

app.Run(context => context.Response.WriteAsync(
    $"Rewritten or Redirected Url: " +
    $"{context.Request.Path + context.Request.QueryString}"));

app.Run();

RewriteRules.cs:

public static void RewriteTextFileRequests(RewriteContext context)
{
    var request = context.HttpContext.Request;

    if (request.Path.Value != null &&
        request.Path.Value.EndsWith(".txt", StringComparison.OrdinalIgnoreCase))
    {
        context.Result = RuleResult.SkipRemainingRules;
        request.Path = "/file.txt";
    }
}

IRule-based rule

Use Add to use rule logic in a class that implements the IRule interface. IRule provides greater flexibility over using the method-based rule approach. The implementation class may include a constructor that allows passing in parameters for the ApplyRule method.

using Microsoft.AspNetCore.Rewrite;
using RewriteRules;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

using (StreamReader apacheModRewriteStreamReader =
    File.OpenText("ApacheModRewrite.txt"))
using (StreamReader iisUrlRewriteStreamReader =
    File.OpenText("IISUrlRewrite.xml"))
{
    var options = new RewriteOptions()
        .AddRedirectToHttpsPermanent()
        .AddRedirect("redirect-rule/(.*)", "redirected/$1")
        .AddRewrite(@"^rewrite-rule/(\d+)/(\d+)", "rewritten?var1=$1&var2=$2",
            skipRemainingRules: true)
        .AddApacheModRewrite(apacheModRewriteStreamReader)
        .AddIISUrlRewrite(iisUrlRewriteStreamReader)
        .Add(MethodRules.RedirectXmlFileRequests)
        .Add(MethodRules.RewriteTextFileRequests)
        .Add(new RedirectImageRequests(".png", "/png-images"))
        .Add(new RedirectImageRequests(".jpg", "/jpg-images"));

    app.UseRewriter(options);
}

app.UseStaticFiles();

app.Run(context => context.Response.WriteAsync(
    $"Rewritten or Redirected Url: " +
    $"{context.Request.Path + context.Request.QueryString}"));

app.Run();

The values of the parameters in the sample app for the extension and the newPath are checked to meet several conditions. The extension must contain a value, and the value must be .png, .jpg, or .gif. If the newPath isn't valid, an ArgumentException is thrown. If a request is made for image.png, the request is redirected to /png-images/image.png. If a request is made for image.jpg, the request is redirected to /jpg-images/image.jpg. The status code is set to 301 - Moved Permanently, and the context.Result is set to stop processing rules and send the response.

public class RedirectImageRequests : IRule
{
    private readonly string _extension;
    private readonly PathString _newPath;

    public RedirectImageRequests(string extension, string newPath)
    {
        if (string.IsNullOrEmpty(extension))
        {
            throw new ArgumentException(nameof(extension));
        }

        if (!Regex.IsMatch(extension, @"^\.(png|jpg|gif)$"))
        {
            throw new ArgumentException("Invalid extension", nameof(extension));
        }

        if (!Regex.IsMatch(newPath, @"(/[A-Za-z0-9]+)+?"))
        {
            throw new ArgumentException("Invalid path", nameof(newPath));
        }

        _extension = extension;
        _newPath = new PathString(newPath);
    }

    public void ApplyRule(RewriteContext context)
    {
        var request = context.HttpContext.Request;

        // Because we're redirecting back to the same app, stop 
        // processing if the request has already been redirected
        if (request.Path.StartsWithSegments(new PathString(_newPath)) ||
            request.Path.Value == null)
        {
            return;
        }

        if (request.Path.Value.EndsWith(_extension, StringComparison.OrdinalIgnoreCase))
        {
            var response = context.HttpContext.Response;
            response.StatusCode = (int) HttpStatusCode.MovedPermanently;
            context.Result = RuleResult.EndResponse;
            response.Headers[HeaderNames.Location] = 
                _newPath + request.Path + request.QueryString;
        }
    }
}

Try:

• PNG request: https://redirect6.azurewebsites.net/image.png
• JPG request: https://redirect6.azurewebsites.net/image.jpg

Regex examples

Goal	Regex String & Match Example	Replacement String & Output Example
Rewrite path into querystring	^path/(.*)/(.*) /path/abc/123	path?var1=$1&var2=$2 /path?var1=abc&var2=123
Strip trailing slash	^path2/(.*)/$ /path2/xyz/	$1 /path2/xyz
Enforce trailing slash	^path3/(.*[^/])$ /path3/xyz	$1/ /path3/xyz/
Avoid rewriting specific requests	^(.*)(?<!\.axd)$ or ^(?!.*\.axd$)(.*)$ Yes: /path4/resource.htm No: /path4/resource.axd	rewritten/$1 /rewritten/resource.htm /resource.axd
Rearrange URL segments	path5/(.*)/(.*)/(.*) path5/1/2/3	path5/$3/$2/$1 path5/3/2/1
Replace a URL segment	^path6/(.*)/segment2/(.*) ^path6/segment1/segment2/segment3	path6/$1/replaced/$2 /path6/segment1/replaced/segment3

The links in the preceding table use the following code deployed to Azure:

using Microsoft.AspNetCore.Rewrite;
using RewriteRules;

var builder = WebApplication.CreateBuilder(args);
var app = builder.Build();

using (StreamReader apacheModRewriteStreamReader =
    File.OpenText("ApacheModRewrite.txt"))
using (StreamReader iisUrlRewriteStreamReader =
    File.OpenText("IISUrlRewrite.xml"))
{
    var options = new RewriteOptions()
        .AddRedirectToHttpsPermanent()
        .AddRedirect("redirect-rule/(.*)", "redirected/$1")
        .AddRewrite(@"^rewrite-rule/(\d+)/(\d+)", "rewritten?var1=$1&var2=$2",
            skipRemainingRules: true)

        // Rewrite path to QS.
        .AddRewrite(@"^path/(.*)/(.*)", "path?var1=$1&var2=$2",
            skipRemainingRules: true)
        // Skip trailing slash.
        .AddRewrite(@"^path2/(.*)/$", "path2/$1",
            skipRemainingRules: true)
         // Enforce trailing slash.
         .AddRewrite(@"^path3/(.*[^/])$", "path3/$1/",
            skipRemainingRules: true)
         // Avoid rewriting specific requests.
         .AddRewrite(@"^path4/(.*)(?<!\.axd)$", "rewritten/$1",
            skipRemainingRules: true)
         // Rearrange URL segments
         .AddRewrite(@"^path5/(.*)/(.*)/(.*)", "path5/$3/$2/$1",
            skipRemainingRules: true)
          // Replace a URL segment
          .AddRewrite(@"^path6/(.*)/segment2/(.*)", "path6/$1/replaced/$2",
            skipRemainingRules: true)

        .AddApacheModRewrite(apacheModRewriteStreamReader)
        .AddIISUrlRewrite(iisUrlRewriteStreamReader)
        .Add(MethodRules.RedirectXmlFileRequests)
        .Add(MethodRules.RewriteTextFileRequests)
        .Add(new RedirectImageRequests(".png", "/png-images"))
        .Add(new RedirectImageRequests(".jpg", "/jpg-images"));

    app.UseRewriter(options);
}

app.UseStaticFiles();

app.Run(context => context.Response.WriteAsync(
    $"Rewritten or Redirected Url: " +
    $"{context.Request.Path + context.Request.QueryString}"));

app.Run();

In most of the preceding regular expression samples, the literal path is used to make unique testable rewrite rules for the deployed sample. Typically the regular expression wouldn't include path. For example, see these regular expression examples table.

Additional resources

• View or download sample code (how to download)
• RewriteMiddleware source on GitHub
• App startup in ASP.NET Core
• ASP.NET Core Middleware
• Regular expressions in .NET
• Regular expression language - quick reference
• Apache mod_rewrite
• Using Url Rewrite Module 2.0 (for IIS)
• URL Rewrite Module Configuration Reference
• Keep a simple URL structure
• 10 URL Rewriting Tips and Tricks
• To slash or not to slash