[management, reverse proxy] Add reverse proxy feature (#5291)

* implement reverse proxy


---------

Co-authored-by: Alisdair MacLeod <git@alisdairmacleod.co.uk>
Co-authored-by: mlsmaycon <mlsmaycon@gmail.com>
Co-authored-by: Eduard Gert <kontakt@eduardgert.de>
Co-authored-by: Viktor Liu <viktor@netbird.io>
Co-authored-by: Diego Noguês <diego.sure@gmail.com>
Co-authored-by: Diego Noguês <49420+diegocn@users.noreply.github.com>
Co-authored-by: Bethuel Mmbaga <bethuelmbaga12@gmail.com>
Co-authored-by: Zoltan Papp <zoltan.pmail@gmail.com>
Co-authored-by: Ashley Mensah <ashleyamo982@gmail.com>

proxy/internal/proxy/trustedproxy.go

@@ -0,0 +1,60 @@
+package proxy
+
+import (
+	"net/netip"
+	"strings"
+)
+
+// IsTrustedProxy checks if the given IP string falls within any of the trusted prefixes.
+func IsTrustedProxy(ipStr string, trusted []netip.Prefix) bool {
+	if len(trusted) == 0 {
+		return false
+	}
+
+	addr, err := netip.ParseAddr(ipStr)
+	if err != nil {
+		return false
+	}
+
+	for _, prefix := range trusted {
+		if prefix.Contains(addr) {
+			return true
+		}
+	}
+	return false
+}
+
+// ResolveClientIP extracts the real client IP from X-Forwarded-For using the trusted proxy list.
+// It walks the XFF chain right-to-left, skipping IPs that match trusted prefixes.
+// The first untrusted IP is the real client.
+//
+// If the trusted list is empty or remoteAddr is not trusted, it returns the
+// remoteAddr IP directly (ignoring any forwarding headers).
+func ResolveClientIP(remoteAddr, xff string, trusted []netip.Prefix) string {
+	remoteIP := extractClientIP(remoteAddr)
+
+	if len(trusted) == 0 || !IsTrustedProxy(remoteIP, trusted) {
+		return remoteIP
+	}
+
+	if xff == "" {
+		return remoteIP
+	}
+
+	parts := strings.Split(xff, ",")
+	for i := len(parts) - 1; i >= 0; i-- {
+		ip := strings.TrimSpace(parts[i])
+		if ip == "" {
+			continue
+		}
+		if !IsTrustedProxy(ip, trusted) {
+			return ip
+		}
+	}
+
+	// All IPs in XFF are trusted; return the leftmost as best guess.
+	if first := strings.TrimSpace(parts[0]); first != "" {
+		return first
+	}
+	return remoteIP
+}