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Rewrite policies ​

Configure rewrite policies in Application Gateway ​

Application Gateway allows you to rewrite selected content of requests and responses. With this feature, you can translate URLs, query string parameters as well as modify request and response headers. It also allows you to add conditions to ensure that the URL or the specified headers are rewritten only when certain conditions are met. These conditions are based on the request and response information.

HTTP header and URL rewrite features are only available for the Application Gateway v2 SKU.

Supported rewrite types ​

Application Gateway supports multiple rewrite types.

Request and response headers ​

HTTP headers allow a client and server to pass additional information with a request or response. By rewriting these headers, you can accomplish important tasks, such as adding security-related header fields like HSTS/ X-XSS-Protection, removing response header fields that might reveal sensitive information, and removing port information from X-Forwarded-For headers.

Application Gateway allows you to add, remove, or update HTTP request and response headers while the request and response packets move between the client and back-end pools.

header-rewrite-overview

URL path and query string ​

With URL rewrite capability in Application Gateway, you can:

Rewrite the host name, path, and query string of the request URL Choose to rewrite the URL of all requests on a listener or only those requests which match one or more of the conditions you set. These conditions are based on the request and response properties (request, header, response header and server variables). Choose to route the request (select the backend pool) based on either the original URL or the rewritten URL.

url-rewrite-overview

Rewrite actions ​

You use rewrite actions to specify the URL, request headers or response headers that you want to rewrite and the new value to which you intend to rewrite them to. The value of a URL or a new or existing header can be set to these types of values:

  • Text
  • Request header. To specify a request header, you need to use the syntax
  • Response header. To specify a response header, you need to use the syntax
  • Server variable. To specify a server variable, you need to use the syntax {var_serverVariable}. See the list of supported server variables

A combination of text, a request header, a response header, and a server variable.

Rewrite conditions ​

You can use rewrite conditions, an optional configuration, to evaluate the content of HTTP(S) requests and responses and perform a rewrite only when one or more conditions are met. The application gateway uses these types of variables to evaluate the content of requests and responses:

  • HTTP headers in the request
  • HTTP headers in the response
  • Application Gateway server variables

You can use a condition to evaluate whether a specified variable is present, whether a specified variable matches a specific value, or whether a specified variable matches a specific pattern.

Rewrite configuration ​

To configure a rewrite rule, you need to create a rewrite rule set and add the rewrite rule configuration in it.

A rewrite rule set contains:

Request routing rule association: ​

The rewrite configuration is associated to the source listener via the routing rule. When you use a basic routing rule, the rewrite configuration is associated with a source listener and is a global header rewrite. When you use a path-based routing rule, the rewrite configuration is defined on the URL path map. In that case, it applies only to the specific path area of a site. You can create multiple rewrite sets and apply each rewrite set to multiple listeners. But you can apply only one rewrite set to a specific listener.

Rewrite Condition: ​

It is an optional configuration. Rewrite conditions evaluate the content of the HTTP(S) requests and responses. The rewrite action will occur if the HTTP(S) request or response matches the rewrite condition. If you associate more than one condition with an action, the action occurs only when all the conditions are met. In other words, the operation is a logical AND operation.

Rewrite type: ​

There are three types of rewrites available:

  • Rewriting request headers

  • Rewriting response headers

  • Rewriting URL components

    • URL path: The value to which the path is to be rewritten to.
    • URL Query String: The value to which the query string is to be rewritten to.
    • Re-evaluate path map: Used to determine whether the URL path map is to be re-evaluated or not. If kept unchecked, the original URL path will be used to match the path-pattern in the URL path map. If set to true, the URL path map will be re-evaluated to check the match with the rewritten path. Enabling this switch helps in routing the request to a different backend pool post rewrite.