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A10 X-forwarded-for May 2026

A10 provides a configuration option to prevent this. Instead of appending, you can configure the ADC to or replace the XFF header.

If your backend server reads only the first IP (leftmost) as the client, it will believe the request is coming from 127.0.0.1 (localhost)—bypassing all ACLs. a10 x-forwarded-for

If your A10 is configured to append the client IP (the default), the header becomes: X-Forwarded-For: 127.0.0.1, 203.0.113.5 A10 provides a configuration option to prevent this

Unlike XFF, which is HTTP-specific, PROXY Protocol prepends a binary header at the transport layer. It preserves the original client IP for any protocol—HTTP, HTTPS, SMTP, or raw TCP. If your backend server supports PROXY Protocol (e.g., HAProxy, Nginx, Apache 2.4.30+), this is a more robust solution than XFF. X-Forwarded-For on A10 Networks devices is a powerful but subtle tool. When configured correctly—preferably with replace mode to block spoofing—it restores end-to-end visibility. However, it shifts responsibility to the backend developer to parse headers securely. If your A10 is configured to append the

However, by inserting itself between the client and the server, an ADC creates a classic networking paradox:

When a client connects to an A10 VIP (Virtual IP), the A10 establishes a separate TCP connection to the backend server. From the server’s perspective, the source IP of every single packet is the A10’s own LAN IP—not the remote user. This breaks logging, geo-location, rate-limiting, and security rules.

If a backend server receives requests from multiple clients over the same persistent connection from the A10, the XFF header will change per request . Your backend application code must be designed to parse the XFF header on every HTTP request, not just at the TCP connection establishment. Java HttpServletRequest.getRemoteAddr() will still return the A10’s IP; you must explicitly call getHeader("X-Forwarded-For") . Blindly trusting the first XFF value you see is a common and dangerous anti-pattern.