首先：
traceroute是如何工作的？
Traceroute transmits packets with small TTL (Time To Live) values.
The TTL is an IP header field that is used to prevent packets from running into endless loops.
When a router that handles the packet subtracts one from the packet’s TTL. The packet expires and it’s discarded when the TTL reaches zero.
Traceroute sends ICMP Time Exceeded messages, (
RFC 792
), back to the sender when this occurs.
By using small TTL values, the packets will quickly expire, so traceroute causes all routers along a packet’s path to generate the ICMP messages that identify the router.
For example, TTL = 1 should produce the message from the first router, TTL = 2 generates a message from the second router in