Website Owner Must Know: X-Forwarded-For

About Author

I'm Carrie, a cybersecurity engineer and writer, working for SafeLine WAF. SafeLine is a free and open source web application firewall, self-hosted, very easy to use.

What is X-Forwarded-For?

X-Forwarded-For (XFF) is an HTTP header field that identifies the originating IP address of a client connecting to a web server through an HTTP proxy or a load balancer. This header is a de facto standard for identifying the original IP address of a client in scenarios where intermediate proxies are involved.

How Does It Work?

1. Client Request: When a client makes a request to a web server, it typically includes the client's IP address.

2. Proxy Involvement: If the request passes through a proxy server or load balancer, the original client IP is replaced with the IP of the proxy.

3. X-Forwarded-For in Action: To preserve the original client IP, the proxy adds the X-Forwarded-For header to the request.

4. Header Format: The header typically looks like this:
   

   X-Forwarded-For: <client>, <proxy1>, <proxy2>

1. Multiple Proxies: If multiple proxies are involved, each appends the IP address of the preceding proxy to the header.

Importance and Use Cases

• Accurate Logging: Helps maintain accurate logs of client IP addresses.
• Security: Assists in identifying the true source of requests for security purposes.
• Geolocation: Enables more accurate geolocation services.
• Access Control: Facilitates IP-based access control in multi-tier architectures.

Considerations

• Trust: The X-Forwarded-For header can be spoofed, so it should only be trusted from known proxies.
• Privacy: Care must be taken to handle this information in compliance with privacy regulations.
• Configuration: Proper configuration of proxies and web servers is crucial for accurate XFF handling.

Conclusion

X-Forwarded-For is a crucial component in modern web infrastructure, especially in environments using reverse proxies or load balancers. It helps maintain transparency and traceability of client requests, balancing the need for efficient routing with the importance of preserving original client information.