Web Application Firewall:Retrieve real client IP from an HTTP header

How it works

After you use CNAME access to connect your domain to WAF, client requests are first routed through WAF and possibly other intermediate proxies (such as CDN or Anti-DDoS Pro) before they reach the origin server. At this point, the source IP address of the TCP connection received by the origin server is the IP address of the last-hop proxy, not the actual client IP address.

When WAF forwards HTTP requests, it automatically adds the X-Forwarded-For header field. The initial value of this field is the client IP address. Each time the request passes through a proxy node, the IP address of that node is appended to the end of the field in the following format:

X-Forwarded-For: client IP, proxy 1 IP, proxy 2 IP, …

Therefore, you must configure the backend web server to extract the leftmost IP address from the X-Forwarded-For header, which is the original client IP address. The following sections describe the configuration methods for different platforms.

Step 1: Obtain the WAF back-to-origin IP ranges

Log on to the Web Application Firewall console. In the top navigation bar, select the resource group and region of your WAF instance (Chinese Mainland or Outside Chinese Mainland). Then, click Onboarding > CNAME Record > View Back-to-origin CIDR Blocks and record all WAF back-to-origin IP ranges for later configuration.

Configure Nginx

Nginx uses the http_realip_module module to parse X-Forwarded-For. Perform the following steps to modify the Nginx configuration.

nginx -V 2>&1 | grep -o with-http_realip_module

# For Alibaba Cloud Linux / CentOS / RHEL
sudo yum reinstall nginx -y
# For Ubuntu / Debian
sudo apt install --reinstall nginx-core nginx-full

Configure IIS 7 (applicable to IIS 7 and later)

When WAF acts as a reverse proxy, the c-ip field in the default IIS logs records the WAF back-to-origin IP instead of the actual client IP. You can use the W3C custom log field feature in IIS to write the X-Forwarded-For request header to the logs for auditing and troubleshooting purposes.

1. Open IIS Manager. In the left-side Connections tree, expand the server node and select the target website (for example, Default Web Site).

2. In the Features View pane on the right, double-click the Logging icon.

3. In the Log Files section, make sure that the Format is set to W3C (the default value). If a different format is selected, change it to W3C.

4. In the Log Files section, click Select Fields....

5. At the bottom of the dialog box that appears, click Add Field....

6. In the Add Custom Field dialog box, enter the following information and click OK.

   Parameter	Description
   Field Name	Enter X-FORWARDED-FOR
   Source Type	Select Request Header
   Source	Enter X-Forwarded-For

7. Click OK to save the custom field.

8. Click OK again to close the logging fields window, and then click Apply in the Actions pane to save the changes.

9. Restart IIS for the changes to take effect.

10. Access the website to verify the configuration, and view the access logs (default path: C:\inetpub\logs\LogFiles\W3SVC1\). The real client IP address should be displayed.

Configure Apache

Apache 2.4 and later includes the remoteip_module (mod_remoteip.so), which can be used to obtain the real client IP address.

Configure Tomcat

Tomcat can obtain the real client IP address by enabling the X-Forwarded-For feature.

1. Go to the Tomcat installation directory and open the tomcat/conf/server.xml configuration file.

2. Modify the AccessLogValve log configuration to the following:

   <Valve className="org.apache.catalina.valves.AccessLogValve" directory="logs"
   prefix="localhost_access_log." suffix=".txt"
   pattern="%{X-FORWARDED-FOR}i %l %u %t %r %s %b %D %q %{User-Agent}i %T" resolveHosts="false"/>

3. Restart Tomcat.

   sudo systemctl restart tomcat

4. Access the website to verify the configuration, and view the access logs. The real client IP address should be displayed.

   # Replace the following command with the actual Tomcat installation directory path and the actual log file
   tail -f /opt/tomcat/logs/localhost_access_log.2026-01-01.txt 

Configure a Kubernetes cluster

When a Kubernetes cluster that uses the Nginx Ingress Controller is connected to WAF, the real client IP address is added by WAF to the X-Forwarded-For request header. By default, the Nginx Ingress Controller does not trust this field, and business containers cannot obtain the real client IP address.

Perform the following steps to configure the Nginx Ingress Controller.

1. Edit the ConfigMap of the Ingress Controller:

   kubectl -n kube-system edit cm nginx-configuration

2. Add the following configuration items to the data field:

   compute-full-forwarded-for: "true"
   forwarded-for-header: "X-Forwarded-For"
   use-forwarded-headers: "true"

3. Save and exit. The configuration automatically takes effect without the need to restart pods.

   After the configuration is complete, the Ingress Controller identifies the first IP address in X-Forwarded-For as the real client IP address and retains this value when forwarding requests.

Advanced optimization: Pass the real IP address by using a custom header

In complex network architectures, relying on the standard X-Forwarded-For (XFF) header carries risks such as spoofing and IP confusion across multiple proxy layers. You can use a custom HTTP header to pass the real client IP address to improve security and control.

The following example shows how to use a custom header to retrieve the real client IP.

1. Configure CDN

   By default, CDN carries the Ali-Cdn-Real-Ip HTTP request header in back-to-origin requests, which indicates the real IP address of the client when it connects to the CDN node. You can use Add a custom outbound request header to pass this IP address to WAF. The following table shows an example of the CDN console configuration.

   Parameter	Example
   Operation	Add
   Request Header	Custom back-to-origin request header
   Header Name	Enter a custom name, for example, TrueIP
   Header Value	$http_Ali_Cdn_Real_Ip
   Allow Duplicates	Not Allowed
   Rule Condition	Not used

2. Configure WAF

   Complete the following two configurations in the WAF console:

   • Is a Layer 7 proxy such as Anti-DDoS Proxy or CDN deployed in front of WAF: Enable this option so that WAF correctly identifies the real client IP address.

   • Enable Traffic Tagging: Enable this option so that the origin server can obtain the real client IP address.

   For more information about how to configure these options, see Real client information.

   Parameter	Example
   Is a Layer 7 proxy such as Anti-DDoS Proxy or CDN deployed in front of WAF	Select Yes, and then select [Recommended] Use the First IP Address in Specified Header Field as Actual IP Address of Client to Prevent X-Forwarded-For Forgery. Enter the Header Name that you specified in the CDN configuration, for example, TrueIP.
   Enable Traffic Tagging	Select Originating IP Address, and enter the Header Name that you specified in the CDN configuration, for example, TrueIP.

3. Configure the origin server

   Take Nginx as an example. Edit the main configuration file nginx.conf and add the following content to the http {} block. For more information, see Configure Nginx.

   http {
     # Trust request headers from the WAF back-to-origin IP addresses.
     # Replace the following placeholders with the WAF back-to-origin IP ranges you obtained from the console.
     # You must add all IP ranges. If any are missing, some requests might still log the WAF IP instead of the real client IP.
     set_real_ip_from    <ip_range1>;    
     set_real_ip_from    <ip_range2>;   
     ...
     set_real_ip_from    <ip_rangex>;
     # Tell Nginx to get the real IP from your custom header, TrueIP in this example.
     real_ip_header      TrueIP;
   }

   We recommend that you configure the security group of the origin server to only allow traffic from WAF back-to-origin IP ranges. This ensures that only WAF can communicate with the origin server, preventing attackers from bypassing WAF and directly accessing the public IP address of the origin server.

What is the difference between the real IP address configuration on the origin server (described in this topic) and the Is a Layer 7 proxy such as Anti-DDoS Proxy or CDN deployed in front of WAF option in the WAF console?

• Real IP address configuration on the origin server (described in this topic)

  • Purpose: After you use CNAME integration to connect your domain to WAF, this configuration enables the origin server (such as Nginx) to record the real client IP address in its access logs (such as access.log) instead of the WAF back-to-origin IP address.

  • Required: Only required if you need to view or process the real client IP address on the origin server.

• The Is a Layer 7 proxy such as Anti-DDoS Proxy or CDN deployed in front of WAF option in the WAF console

  • Applicable scenario: Other Layer 7 proxies (such as CDN or Anti-DDoS Pro) are deployed in front of WAF.

  • Purpose: Instructs WAF to extract the real client IP address from a specific HTTP request header (such as Ali-Cdn-Real-Ip or X-Forwarded-For) to ensure that security reports and attack identification are based on the real source IP address.

  • Required: Only required if a Layer 7 proxy is deployed in front of WAF. If no front-end proxy exists, keep the default value No.

What is X-Forwarded-For spoofing and how do I prevent it?

X-Forwarded-For (XFF) spoofing refers to an attack where an attacker constructs or tampers with the X-Forwarded-For request header in an HTTP request to inject a fake client IP address, with the goal of bypassing IP-based access controls, log auditing, or security policies. X-Forwarded-For is a non-standard request header that can be arbitrarily set by clients. If a backend service directly trusts this field without validation, security risks arise.

Example: XFF spoofing attack command
An attacker can use the curl command to forge an X-Forwarded-For header, spoofing the IP address as 1.2.3.4:

curl -H "X-Forwarded-For: 1.2.3.4" https://example.com/

To prevent XFF spoofing, use a custom trusted request header to pass the real IP address:
Configure CDN or WAF to use a predefined custom HTTP request header to pass the real client IP address when sending requests back to the origin server. The origin server then only parses this custom header. This request header is injected by trusted middleware (such as WAF) and cannot be directly controlled by clients, effectively preventing spoofing.