iptables 大佬来问了问题

tproxy

Transparent proxying often involves "intercepting" traffic on a router. This is usually done with the iptables REDIRECT target; however, there are serious limitations of that method. One of the major issues is that it actually modifies the packets to change the destination address -- which might not be acceptable in certain situations. (Think of proxying UDP for example: you won't be able to find out the original destination address. Even in case of TCP getting the original destination address is racy.)

在一些透明代理的资料，上面这段话中的加粗部分如何理解？ UDP 无法知道原始的 dst address ？