preserve client IP on layer 4 VIP

Manipulating the HTTP header is at layer 7 so without changing the virtual server type of having something in front of it that manipulates it and sends it on its way then you cannot insert an HTTP header.

Preserving the client IP address while using a Layer 4 VIP (Virtual IP) with auto map without inserting a standard VIP in front of it and enabling X-Forwarded-For headers can be challenging, as the nature of a Layer 4 load balancer is to forward traffic without inspecting or modifying the packet payload. However, depending on your specific setup and requirements, there might be some alternative approaches or workarounds you can consider:

1. Source NAT (SNAT): Configure your load balancer to use Source NAT (SNAT) instead of auto map. With SNAT, the load balancer replaces the source IP address of the packets with its own IP address before forwarding them to the backend servers. This allows the backend servers to see the source IP address as the load balancer's IP address. While this doesn't preserve the original client IP address, it ensures that all traffic appears to come from the load balancer.

2. Proxy Protocol: Some Layer 4 load balancers support the Proxy Protocol, which is a standardized protocol for carrying connection information (including client IP address) through proxies or load balancers. If your load balancer and backend servers support Proxy Protocol, you can enable it to pass the client IP address transparently to the backend servers.

3. Layer 7 Load Balancer: Consider using a Layer 7 (Application Layer) load balancer instead of a Layer 4 load balancer. Layer 7 load balancers can inspect the HTTP headers and insert X-Forwarded-For headers to preserve the client IP address. This approach might involve more overhead and complexity but provides more flexibility in preserving client IP addresses.

4. Custom Solution: Depending on your specific requirements and environment, you may need to develop a custom solution to preserve client IP addresses. This could involve using scripts, middleware, or custom configurations to manipulate traffic and maintain the original client IP address.

Since Fast L4 VIP does not have the luxury of using HTTP XFF header, the only way to me is to turn SNAT off and change backed servers' default gateway to point to f5 selfip.