Problem

rest.RealIP extracts client IP using the rightmost IP in the X-Forwarded-For chain (RFC 7239 security model). This doesn't work with Cloudflare and similar CDNs.

With Cloudflare, the X-Forwarded-For header looks like:

X-Forwarded-For: <original-client>, <cloudflare-edge-server>

The rightmost IP is a Cloudflare edge server which:

1. Varies per request - Cloudflare routes through different edges
2. Is not the actual client IP

Example

Same client making requests gets different "client IPs":

172.71.203.9    (Cloudflare edge)
172.70.231.89   (different edge)
104.23.190.60   (another edge)

Should have been: 203.0.113.195 (actual client) for all requests.

Proposed Solution

Add RealIPFromCDN middleware with CDN-friendly header priority:

1. X-Real-IP - trusted proxy (reproxy/nginx) sets this to actual client
2. CF-Connecting-IP - Cloudflare's dedicated header for original client
3. X-Forwarded-For (leftmost public IP) - original client in CDN chain
4. RemoteAddr - fallback for direct connections

Key: filter out private/loopback IPs from headers before accepting.

API

// new middleware
func RealIPFromCDN(next http.Handler) http.Handler

// new function in realip package  
func GetFromCDN(r *http.Request) (string, error)