Traceroute important tool to detect routing between host and destination hosts

Application of ICMP--traceroute

Traceroute is an important tool to detect the routing between host and destination hosts, and is also the most convenient tool. As mentioned earlier, although the Ping tool can also detect, however, because of the IP header limit, ping can not be fully recorded under the router. So the traceroute just fills this flaw.

The principle of traceroute is very very interesting, it receives the IP of the destination host, first sends a ttl=1 to the destination host (remember what TTL is?). UDP (the next to know what UDP is the packet), and after the first router receives the packet, the TTL is automatically reduced by 1, and the TTL becomes 0, the router will discard the packet, and at the same time produce a host unreachable ICMP datagram to the host. The host receives this datagram and sends a ttl=2 UDP datagram to the destination host, then stimulates the second router to send ICMP datagrams to the host. This is repeated until the destination host is reached. In this way, traceroute gets all the router IPs. This avoids the problem that the IP header can only record limited routed IP.

Someone asked, how do I know if UDP arrives at the destination host? This involves a trick question, the TCP and UDP protocols have a port number definition, while the normal network program only monitors a few small numbers of ports, such as 80, 23, and so on. And traceroute send is the port number >30000 (true abnormal) of the UDP, so when the destination host, the destination host can only send a port unreachable ICMP datagram to the host. Host received this report will know, the host to, so, said Traceroute is a liar is not a bit too:)

There are some useful options in the Traceroute program, including options for IP routing, see the man documentation to learn about these, and don't dwell on them.

Traceroute important tool to detect routing between host and destination hosts