ASP.NET Core url rewrite only domain

Have you considered URL Rewriting Middleware?

It's pretty simple.

1. Drop a IISUrlRewrite.xml file in the root of your application folder. Mark it as "Content" and "Copy to output directory" set to true, looks like this in your csproj

  <ItemGroup>
    <Content Include="IISUrlRewrite.xml">
      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
    </Content>
  </ItemGroup>

2. Add following content in the file

<rewrite>
  <rules>    
    <rule name="Host replace - Old to new" stopProcessing="true">
      <match url=".*" />
      <conditions>
        <add input="{HTTP_HOST}" pattern="www\.myolddomain\.net" ignoreCase="true" />
      </conditions>
      <action type="Redirect" url="https://www.mynewdomain.com{REQUEST_URI}" redirectType="Permanent" appendQueryString="true" />
    </rule>
   </rules>
</rewrite>

3. Register the URL rewrite module in the Configure method of your Startup.cs file

public void Configure(IApplicationBuilder app, IHostingEnvironment env)
{   
    // Irrelevant code omitted

    using (var iisUrlRewriteStreamReader = File.OpenText(Path.Combine(env.ContentRootPath, "IISUrlRewrite.xml")))
    {
        var options = new RewriteOptions().AddIISUrlRewrite(iisUrlRewriteStreamReader);
        app.UseRewriter(options);
    }

    // Irrelevant code omitted
}