What does N * * * mean in traceroute output?

Question

Wanted to perform simple trace for learning purposes:

traceroute google.com

It gave me among another lines some lines that look like the following

...
15  * * *
16  * * *
17  * * *
... few similar lines

What does it mean?

Answer 1

It shows that waiting for the ICMP "Time exceeded" (TTL expired) message timed out.

This can happen at the end of the line, if routing was ultimately unsuccessful (perhaps the destination machine is turned off), or at any point along the route, if the router at that hop does not generate ICMP "Time exceeded" notifications. Honoring the TTL is mandatory, but notifying the originator that a packet was discarded due to TTL reaching zero is optional.

Answer 2

It means that the packet timed out, by default waittime is 5 seconds, and if response is not received in that time it shows asterisk, and jumps to a next hop. It could time out because that router on that hop is blocking traffic, or because it can't find the route to the next hop IP.

You can use -w to increase the timeout period.

-w waittime
              Set the time (in seconds) to wait for a response to a probe (default 5.0 sec).

You can also try to use different protocols, to switch between UDP, ICMP, and TCP, as sometimes routers will block one, but allow some other type of protocol.

   -I     Use ICMP ECHO for probes

   -T     Use TCP SYN for probes

   -U     Use UDP datagrams for probes (it is default). Only UDP method is allowed for unprivileged users.

Answer 3

All implementations of traceroute rely on ICMP packets being sent to the originator.

This program attempts trace route by launching UDP probe packets with a small ttl then listening for an ICMP "time exceeded" reply from a gateway.