feat(xff): add AddTrustedCIDR for custom proxies

- Add AddTrustedCIDR() method to support non-Fastly proxies
- Enable trusting custom CIDR ranges (e.g., 10.0.0.0/8)
- Validate CIDR format before adding to trusted list
- Maintain backward compatibility with Fastly-only usage

Allows mixed proxy environments where requests pass through
both Fastly CDN and custom internal proxies/load balancers.
Uses precise CIDR terminology instead of generic "range".

xff/fastlyxff/xff_test.go

@@ -213,3 +213,144 @@ func TestGetRealIPWithoutMiddleware(t *testing.T) {
 		t.Errorf("GetRealIP() = %s, expected %s", realIP, expected)
 	}
 }
+
+func TestAddTrustedCIDR(t *testing.T) {
+	xff := &FastlyXFF{
+		IPv4: []string{"192.0.2.0/24"},
+		IPv6: []string{"2001:db8::/32"},
+	}
+
+	tests := []struct {
+		name    string
+		cidr    string
+		wantErr bool
+	}{
+		{"valid IPv4 range", "10.0.0.0/8", false},
+		{"valid IPv6 range", "fc00::/7", false},
+		{"valid single IP", "203.0.113.1/32", false},
+		{"invalid CIDR", "not-a-cidr", true},
+		{"invalid format", "10.0.0.0/99", true},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			err := xff.AddTrustedCIDR(tt.cidr)
+			if (err != nil) != tt.wantErr {
+				t.Errorf("AddTrustedCIDR(%s) error = %v, wantErr %v", tt.cidr, err, tt.wantErr)
+			}
+		})
+	}
+}
+
+func TestCustomTrustedCIDRs(t *testing.T) {
+	xff := &FastlyXFF{
+		IPv4: []string{"192.0.2.0/24"},
+		IPv6: []string{"2001:db8::/32"},
+	}
+
+	// Add custom trusted CIDRs
+	err := xff.AddTrustedCIDR("10.0.0.0/8")
+	if err != nil {
+		t.Fatalf("Failed to add trusted CIDR: %v", err)
+	}
+
+	err = xff.AddTrustedCIDR("172.16.0.0/12")
+	if err != nil {
+		t.Fatalf("Failed to add trusted CIDR: %v", err)
+	}
+
+	tests := []struct {
+		ip       string
+		expected bool
+	}{
+		// Original Fastly ranges
+		{"192.0.2.1", true},
+		{"2001:db8::1", true},
+		// Custom CIDRs
+		{"10.1.2.3", true},
+		{"172.16.1.1", true},
+		// Not trusted
+		{"198.51.100.1", false},
+		{"172.15.1.1", false},
+		{"10.0.0.0", true}, // Network address should still match
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.ip, func(t *testing.T) {
+			result := xff.isTrustedProxy(tt.ip)
+			if result != tt.expected {
+				t.Errorf("isTrustedProxy(%s) = %v, expected %v", tt.ip, result, tt.expected)
+			}
+		})
+	}
+}
+
+func TestHTTPMiddlewareWithCustomCIDRs(t *testing.T) {
+	xff := &FastlyXFF{
+		IPv4: []string{"192.0.2.0/24"},
+		IPv6: []string{"2001:db8::/32"},
+	}
+
+	// Add custom trusted CIDR for internal proxies
+	err := xff.AddTrustedCIDR("10.0.0.0/8")
+	if err != nil {
+		t.Fatalf("Failed to add trusted CIDR: %v", err)
+	}
+
+	middleware := xff.HTTPMiddleware()
+
+	tests := []struct {
+		name           string
+		remoteAddr     string
+		xForwardedFor  string
+		expectedRealIP string
+	}{
+		{
+			name:           "custom trusted proxy with XFF",
+			remoteAddr:     "10.1.2.3:80",
+			xForwardedFor:  "198.51.100.1",
+			expectedRealIP: "198.51.100.1",
+		},
+		{
+			name:           "fastly proxy with XFF",
+			remoteAddr:     "192.0.2.1:80",
+			xForwardedFor:  "198.51.100.1",
+			expectedRealIP: "198.51.100.1",
+		},
+		{
+			name:           "untrusted proxy ignored",
+			remoteAddr:     "172.16.1.1:80",
+			xForwardedFor:  "198.51.100.1",
+			expectedRealIP: "172.16.1.1",
+		},
+		{
+			name:           "chain through custom and fastly",
+			remoteAddr:     "192.0.2.1:80",
+			xForwardedFor:  "198.51.100.1, 10.1.2.3",
+			expectedRealIP: "198.51.100.1",
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			var capturedIP string
+			handler := http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				capturedIP = GetRealIP(r)
+				w.WriteHeader(http.StatusOK)
+			})
+
+			req := httptest.NewRequest("GET", "/", nil)
+			req.RemoteAddr = tt.remoteAddr
+			if tt.xForwardedFor != "" {
+				req.Header.Set("X-Forwarded-For", tt.xForwardedFor)
+			}
+
+			rr := httptest.NewRecorder()
+			middleware(handler).ServeHTTP(rr, req)
+
+			if capturedIP != tt.expectedRealIP {
+				t.Errorf("expected real IP %s, got %s", tt.expectedRealIP, capturedIP)
+			}
+		})
+	}
+}