IIS Access-Control-Allow-Origin

When building web applications, we often need to make requests to resources on different domains. However, browsers have implemented a security feature called Same-Origin Policy, which prevents web pages from making cross-domain requests. This prevents malicious code from hijacking a user's session and accessing sensitive data on other domains.

To allow cross-domain requests, we need to configure our web server to send the Access-Control-Allow-Origin header. This header tells the browser that the requested resource is allowed to be accessed from a different domain.

Configuring IIS to send Access-Control-Allow-Origin header

To configure IIS to send the Access-Control-Allow-Origin header, we need to modify the web.config file of our website. We can do this using any text editor or IDE. Here is an example web.config file that allows all cross-domain requests:

<?xml version="1.0" encoding="utf-8"?>
<configuration>
  <system.webServer>
    <httpProtocol>
      <customHeaders>
        <add name="Access-Control-Allow-Headers" value="*" />
        <add name="Access-Control-Allow-Methods" value="GET, POST, PUT, DELETE, OPTIONS" />
        <add name="Access-Control-Allow-Origin" value="*" />
      </customHeaders>
    </httpProtocol>
  </system.webServer>
</configuration>

Understanding the Access-Control-Allow-Origin header

The Access-Control-Allow-Origin header specifies the domains that are allowed to access the requested resource. The value of the header can be a single domain, a list of domains separated by commas, or the special value "". If "" is used, any domain is allowed to access the resource.

Conclusion

The Access-Control-Allow-Origin header is a powerful tool that allows web developers to make cross-domain requests without violating the Same-Origin Policy. By configuring IIS to send this header, we can open up our web applications to a whole new world of resources and possibilities.