traceroute原理
ICMP
ICMP全称为Internet Control Message Protocol，即，网络控制报文协议。
当一个IP数据报发送失败时，最后一个路由器会向发送发传递一条ICMP报文以报告错误。
traceroute利用此特性来探测路由。
原理如下，构造一份非法的数据，通过IP协议发送。第一份报文设置TTL为1，这样，第一个路由收到之后，就会给发送主机报告一个错误，这样，发送主机就知道此路由的第一跳。接着，设置TTL为2再次发送，第二个路由器也会报告错误。以此类推。
traceroute在windows上的名称为tracert。
C:\Users\Administrator>tracert www.baidu.com
通过最多 30 个跃点跟踪
到 www.a.shifen.com [180.97.33.108] 的路由:
1 1 ms 1 ms 1 ms 192.168.1.1
2 14 ms 22 ms 13 ms 100.64.0.1
3 5 ms 6 ms 8 ms 218.2.131.217
4 5 ms 7 ms 6 ms 202.102.69.26
5 * * * 请求超时。
6 10 ms 6 ms 5 ms 180.97.32.86
7 5 ms 5 ms 3 ms 10.203.195.2
8 6 ms 3 ms 3 ms 180.97.33.108
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