RemoteAddr はほとんど役に立たない

type LocalNetworkConfig struct {
	Skipper middleware.Skipper
}

func localNetworkWithConfig(cfg LocalNetworkConfig) echo.MiddlewareFunc {
	return func(next echo.HandlerFunc) echo.HandlerFunc {
		return func(c echo.Context) error {
			if cfg.Skipper(c) {
				return next(c)
			}

			req := c.Request()
			addr, _, err := net.SplitHostPort(req.RemoteAddr)
			if err != nil {
				slog.Error("failed to get remote address",
					"err", err, "addr", addr)
				return echo.ErrForbidden
			}
			ip := net.ParseIP(addr)
			if ip.IsPrivate() || ip.IsLoopback() {
				return next(c)
			}

			slog.Error("requested from an external network", "ip", ip.String())
			return echo.ErrForbidden
		}
	}
}

func NewLocalNetwork() echo.MiddlewareFunc {
	return localNetworkWithConfig(LocalNetworkConfig{
		Skipper: func(c echo.Context) bool { return false },
	})
}