62

The below is a part of traceroute to my hosted server:

 9  ae-2-2.ebr2.dallas1.level3.net (4.69.132.106)  19.433 ms  19.599 ms  19.275 ms
10  ae-72-72.csw2.dallas1.level3.net (4.69.151.141)  19.496 ms
    ae-82-82.csw3.dallas1.level3.net (4.69.151.153)  19.630 ms
    ae-62-62.csw1.dallas1.level3.net (4.69.151.129)  19.518 ms
11  ae-3-80.edge4.dallas3.level3.net (4.69.145.141)  19.659 ms
    ae-2-70.edge4.dallas3.level3.net (4.69.145.77)  90.610 ms
    ae-4-90.edge4.dallas3.level3.net (4.69.145.205)  19.658 ms
12  the-planet.edge4.dallas3.level3.net (4.59.32.30)  19.905 ms  19.519 ms  19.688 ms
13  te9-2.dsr01.dllstx3.networklayer.com (70.87.253.14)  40.037 ms  24.063 ms
    te2-4.dsr02.dllstx3.networklayer.com (70.87.255.46)  28.605 ms
14  * * *
15  * * *
16  zyzzyva.site5.com (174.122.37.66)  20.414 ms  20.603 ms  20.467 ms

What's the meaning of lines 14 and 15? Information hidden?

42

If a packet is not acknowledged within the expected timeout, an asterisk is displayed.

From http://en.wikipedia.org/wiki/Traceroute

However zyzzyva.site5.com did eventually respond which is why you have line 16.

  • Thanks. Do you know why it takes two lines? – Chang Aug 4 '11 at 16:01
  • 2
    @Chang : it takes two lines because there were two hosts that didn't respond, yet they still passed responses back from other hosts. See Tim's response. – Joe Jan 15 '13 at 17:11
42

Traceroute sends packets to the destination with the field "time to live" (TTL) equal to the number of hops.

Every router decreases the value of TTL of an incoming packet and if it sees an incoming packet with TTL = 0 then drops it, otherwise it decreases the value and sends it further. At the same time it sends diagnosing information to the source about router's identity.

If router does not respond within a timeout then traceroute prints an asterisk. Lines 14 and 15 show that routers which drop packets with original TTLs 14 and 15 did not respond within timeout.

  • 2
    Voted up for reminding me how traceroute actually works. – pix Nov 26 '14 at 0:36
  • 3
    Just to clarify Tim's answer. Tracert is a series of PINGs. The first ping sets the TTL to 1. When the first router receives the PING, it decrements the TTL by 1 and because the new TTL is 0, returns an "error" with it's IP address. Tracert repeats the PING this time with a TTL of 2. Router 1 decrements the TTL by 1, and forwards to router 2. Router 2 decrements the TTL and because it's now 0, returns it's IP address. This process is repeated until either the final destination is reached or the max allowed TTL is reached. If no answer is heard from a PING, tracert displays the *. – Trebor Jan 3 at 1:29
16

Another reason is that typically 3 packets are sent to each machine, and some machines will only respond to the first packet no matter what.

If the problem is just a timeout issue, you can set the -w parameter to the number of seconds you want to wait; e.g., traceroute -w 10 google.com will wait 10 seconds instead of the default of 5 seconds.

Note: Some traceroute clients use milliseconds instead of seconds for the -w parameter.

1

Arp may have timed out. I set up a test network with packetracer and got a asterisk for a destination host. I then pinged it, got a response then tracerouted again and it worked. I then noticed that the machines that were asterisking back did not have arp entries in the local router

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