Mentioned in my other comment ( https://news.ycombinator.com/item?id=24447846 https://news.ycombinator.com/item?id=24447846 ) but at most portreserve makes it a

Mentioned in my other comment ( https://news.ycombinator.com/item?id=24447846 https://news.ycombinator.com/item?id=24447846 ) but at most portreserve makes it a race. It cannot guarantee that an unprivileged user cannot bind to that port.