Rewrite HTTP headers and URL with Application Gateway

Application Gateway allows you to rewrite selected content of requests and responses. With this feature, you can translate URLs, query string parameters and 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.

Rewrite types supported

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 backend pools.

To learn how to rewrite request and response headers with Application Gateway using Azure portal, see here.

Supported headers

You can rewrite all headers in requests and responses, except for the Connection, and Upgrade headers. You can also use the application gateway to create custom headers and add them to the requests and responses being routed through it.

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 that match one or more of the conditions you set. These conditions are based on the request properties (request header and server variables).

• Choose to route the request (select the backend pool) based on either the original URL or the rewritten URL

To learn how to rewrite URL with Application Gateway using Azure portal, see here.

Rewrite actions

Rewrite actions are used to specify the URL. Request headers or response headers that you want to rewrite and the new URL destination value. The value of a URL or a new or existing header can be set to the following types of values:

• Text
• Request header. To specify a request header, you need to use the syntax {http_req_headerName}
• Response header. To specify a response header, you need to use the syntax {http_resp_headerName}
• 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 to evaluate the content of HTTP(S) requests and responses. This optional configuration enables you to 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.

Pattern Matching

Application Gateway uses regular expressions for pattern matching in the condition. You should use Regular Expression 2 (RE2) compatible expressions when writing your conditions. If you're running an Application Gateway Web Application Firewall (WAF) with Core Rule Set 3.1 or earlier, you might have issues when using Perl Compatible Regular Expressions (PCRE). Issues can happen when using lookahead and lookbehind (negative or positive) assertions.

Capturing

To capture a substring for later use, put parentheses around the subpattern that matches it in the condition regex definition. The first pair of parentheses stores its substring in 1, the second pair in 2, and so on. You may use as many parentheses as you like; Perl just keeps defining more numbered variables for you to represent these captured strings. Some examples from ref:

• (\d)(\d) # Match two digits, capturing them into groups 1 and 2

• (\d+) # Match one or more digits, capturing them all into group 1

• (\d)+ # Match a digit one or more times, capturing the last into group 1

Once captured, you can reference them in the action set using the following format:

• For a request header capture, you must use {http_req_headerName_groupNumber}. For example, {http_req_User-Agent_1} or {http_req_User-Agent_2}
• For a response header capture, you must use {http_resp_headerName_groupNumber}. For example, {http_resp_Location_1} or {http_resp_Location_2}
• For a server variable, you must use {var_serverVariableName_groupNumber}. For example, {var_uri_path_1} or {var_uri_path_2}

If you want to use the whole value, you shouldn't mention the number. Simply use the format {http_req_headerName}, etc. without the groupNumber.

Server variables

Application Gateway uses server variables to store useful information about the server, the connection with the client, and the current request on the connection. Examples of information stored include the client's IP address and the web browser type. Server variables change dynamically, for example, when a new page loads or when a form is posted. You can use these variables to evaluate rewrite conditions and rewrite headers. In order to use the value of server variables to rewrite headers, you need to specify these variables in the syntax {var_serverVariableName}

Application gateway supports the following server variables:

Variable name	Description
add_x_forwarded_for_proxy	The X-Forwarded-For client request header field with the client_ip variable (see explanation later in this table) appended to it in the format IP1, IP2, IP3, and so on. If the X-Forwarded-For field isn't in the client request header, the add_x_forwarded_for_proxy variable is equal to the $client_ip variable. This variable is useful when you want to rewrite the X-Forwarded-For header set by Application Gateway so that the header contains only the IP address without the port information.
ciphers_supported	A list of the ciphers supported by the client.
ciphers_used	The string of ciphers used for an established TLS connection.
client_ip	The IP address of the client from which the application gateway received the request. If there's a reverse proxy before the application gateway and the originating client, client_ip returns the IP address of the reverse proxy.
client_port	The client port.
client_tcp_rtt	Information about the client TCP connection. Available on systems that support the TCP_INFO socket option.
client_user	When HTTP authentication is used, the user name supplied for authentication.
host	In this order of precedence: the host name from the request line, the host name from the Host request header field, or the server name matching a request. Example: In the request http://contoso.com:8080/article.aspx?id=123&title=fabrikam, the host value is contoso.com
cookie_name	The name cookie.
http_method	The method used to make the URL request. For example, GET or POST.
http_status	The session status. For example, 200, 400, or 403.
http_version	The request protocol. Usually HTTP/1.0, HTTP/1.1, or HTTP/2.0.
query_string	The list of variable/value pairs that follows the "?" in the requested URL. Example: In the request http://contoso.com:8080/article.aspx?id=123&title=fabrikam, query_string value is id=123&title=fabrikam
received_bytes	The length of the request (including the request line, header, and request body).
request_query	The arguments in the request line.
request_scheme	The request scheme: http or https.
request_uri	The full original request URI (with arguments). Example: in the request http://contoso.com:8080/article.aspx?id=123&title=fabrikam*, request_uri value is /article.aspx?id=123&title=fabrikam
sent_bytes	The number of bytes sent to a client.
server_port	The port of the server that accepted a request.
ssl_connection_protocol	The protocol of an established TLS connection.
ssl_enabled	"On" if the connection operates in TLS mode. Otherwise, an empty string.
uri_path	Identifies the specific resource in the host that the web client wants to access. The variable refers to the original URL path, prior to any manipulation. This is the part of the request URI without the arguments. For example, in the request http://contoso.com:8080/article.aspx?id=123&title=fabrikam, the uri_path value is /article.aspx.

Mutual authentication server variables

Application Gateway supports the following server variables for mutual authentication scenarios. Use these server variables the same way as above with the other server variables.

Variable name	Description
client_certificate	The client certificate in PEM format for an established SSL connection.
client_certificate_end_date	The end date of the client certificate.
client_certificate_fingerprint	The SHA1 fingerprint of the client certificate for an established SSL connection.
client_certificate_issuer	The "issuer DN" string of the client certificate for an established SSL connection.
client_certificate_serial	The serial number of the client certificate for an established SSL connection.
client_certificate_start_date	The start date of the client certificate.
client_certificate_subject	The "subject DN" string of the client certificate for an established SSL connection.
client_certificate_verification	The result of the client certificate verification: SUCCESS, FAILED:<reason>, or NONE if a certificate was not present.

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: This configuration is optional. Rewrite conditions evaluate the content of the HTTP(S) requests and responses. The rewrite action occurs 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 3 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.
    • URL Query String: The value to which the query string is to be rewritten.
    • Reevaluate path map: Used to determine whether the URL path map is to be reevaluated or not. If kept unchecked, the original URL path is used to match the path-pattern in the URL path map. If set to true, the URL path map is reevaluated to check the match with the rewritten path. Enabling this switch helps in routing the request to a different backend pool post rewrite.

Rewrite configuration common pitfalls

• Enabling 'Reevaluate path map' isn't allowed for basic request routing rules. This is to prevent infinite evaluation loop for a basic routing rule.

• There needs to be at least 1 conditional rewrite rule or 1 rewrite rule which doesn't have 'Reevaluate path map' enabled for path-based routing rules to prevent infinite evaluation loop for a path-based routing rule.

• Incoming requests would be terminated with a 500 error code in case a loop is created dynamically based on client inputs. The Application Gateway continues to serve other requests without any degradation in such a scenario.

Using URL rewrite or Host header rewrite with Web Application Firewall (WAF_v2 SKU)

When you configure URL rewrite or host header rewrite, the WAF evaluation happens after the modification to the request header or URL parameters (post-rewrite). And when you remove the URL rewrite or host header rewrite configuration on your Application Gateway, the WAF evaluation is done before the header rewrite (pre-rewrite). This order ensures that WAF rules are applied to the final request that would be received by your backend pool.

For example, say you have the following header rewrite rule for the header "Accept" : "text/html" - if the value of header "Accept" is equal to "text/html", then rewrite the value to "image/png".

Here, with only header rewrite configured, the WAF evaluation is done on "Accept" : "text/html". But when you configure URL rewrite or host header rewrite, then the WAF evaluation is done on "Accept" : "image/png".

Common scenarios for header rewrite

Remove port information from the X-Forwarded-For header

Application Gateway inserts an X-Forwarded-For header into all requests before it forwards the requests to the backend. This header is a comma-separated list of IP ports. There might be scenarios in which the backend servers only need the headers to contain IP addresses. You can use header rewrite to remove the port information from the X-Forwarded-For header. One way to do this is to set the header to the add_x_forwarded_for_proxy server variable. Alternatively, you can also use the variable client_ip:

Modify a redirection URL

Modification of a redirect URL can be useful under certain circumstances. For example: clients were originally redirected to a path like "/blog" but now should be sent to "/updates" due to a change in content structure.

When the app service sends a redirection response, it uses the same hostname in the location header of its response as the one in the request it receives from the application gateway. So the client makes the request directly to contoso.chinacloudsites.cn/path2 instead of going through the application gateway (contoso.com/path2). Bypassing the application gateway isn't desirable.

You can resolve this issue by setting the hostname in the location header to the application gateway's domain name.

Here are the steps for replacing the hostname:

1. Create a rewrite rule with a condition that evaluates if the location header in the response contains chinacloudsites.cn. Enter the pattern (https?):\/\/.*chinacloudsites\.cn(.*)$.

2. Perform an action to rewrite the location header so that it has the application gateway's hostname. Do this by entering {http_resp_Location_1}://contoso.com{http_resp_Location_2} as the header value. Alternatively, you can also use the server variable host to set the hostname to match the original request.

   

Implement security HTTP headers to prevent vulnerabilities

You can fix several security vulnerabilities by implementing necessary headers in the application response. These security headers include X-XSS-Protection, Strict-Transport-Security, and Content-Security-Policy. You can use Application Gateway to set these headers for all responses.

Delete unwanted headers

You might want to remove headers that reveal sensitive information from an HTTP response. For example, you might want to remove information like the backend server name, operating system, or library details. You can use the application gateway to remove these headers:

It isn't possible to create a rewrite rule to delete the host header. If you attempt to create a rewrite rule with the action type set to delete and the header set to host, it results in an error.

Check for the presence of a header

You can evaluate an HTTP request or response header for the presence of a header or server variable. This evaluation is useful when you want to perform a header rewrite only when a certain header is present.

Common scenarios for URL rewrite

Parameter based path selection

To accomplish scenarios where you want to choose the backend pool based on the value of a header, part of the URL, or query string in the request, you can use a combination of URL Rewrite capability and path-based routing.

To do this, create a rewrite set with a condition that checks for a specific parameter (query string, header, etc.) and then performs an action where it changes the URL path (ensure Reevaluate path map is enabled). The rewrite set must then be associated to a path based rule. The path based rule must contain the same URL paths specified in the rewrite set and their corresponding backend pool.

Thus, the rewrite set allows users to check for a specific parameter and assign it a new path, and the path based rule allows users to assign backend pools to those paths. As long as "Reevaluate path map" is enabled, traffic routs based on the path specified in the rewrite set.

For a use case example using query strings, see Route traffic using parameter based path selection in portal.

Rewrite query string parameters based on the URL

Consider a scenario of a shopping website where the user visible link should be simple and legible, but the backend server needs the query string parameters to show the right content.

In that case, Application Gateway can capture parameters from the URL and add query string key-value pairs from those from the URL. For example, let's say the user wants to rewrite, https://www.contoso.com/fashion/shirts to https://www.contoso.com/buy.aspx?category=fashion&product=shirts, it can be achieved through the following URL rewrite configuration.

Condition - If server variable uri_path equals to the pattern /(.+)/(.+)

Action - Set URL path to buy.aspx and query string to category={var_uri_path_1}&product={var_uri_path_2}

For a step-by-step guide to achieve the scenario described above, see Rewrite URL with Application Gateway using Azure portal

URL rewrite vs URL redirect

For a URL rewrite, Application Gateway rewrites the URL before the request is sent to the backend. This won't change what users see in the browser because the changes are hidden from the user.

For a URL redirect, Application Gateway sends a redirect response to the client with the new URL. That, in turn, requires the client to resend its request to the new URL provided in the redirect. The URL that the user sees in the browser updates to the new URL.

Limitations

• If a response has more than one header with the same name, rewriting the value of one of those headers results in dropping the other headers in the response. This can happen with Set-Cookie header since you can have more than one Set-Cookie header in a response. One such scenario is when you're using an app service with an application gateway and have configured cookie-based session affinity on the application gateway. In this case the response contains two Set-Cookie headers. For example: one used by the app service, Set-Cookie: ARRAffinity=ba127f1caf6ac822b2347cc18bba0364d699ca1ad44d20e0ec01ea80cda2a735;Path=/;HttpOnly;Domain=sitename.chinacloudsites.cn and another for application gateway affinity, Set-Cookie: ApplicationGatewayAffinity=c1a2bd51lfd396387f96bl9cc3d2c516; Path=/. Rewriting one of the Set-Cookie headers in this scenario can result in removing the other Set-Cookie header from the response.
• Rewrites aren't supported when the application gateway is configured to redirect the requests or to show a custom error page.
• Request header names can contain alphanumeric characters and hyphens. Header names containing other characters are discarded when a request is sent to the backend target.
• Response header names can contain any alphanumeric characters and specific symbols as defined in RFC 7230.
• Connection and upgrade headers cannot be rewritten
• Rewrites aren't supported for 4xx and 5xx responses generated directly from Application Gateway

Next steps

• Learn how to rewrite HTTP headers with Application Gateway using Azure portal
• Learn how to rewrite URL with Application Gateway using Azure portal