Traceroute times out for all hops after 24th, up to 70th

networking

I'm running traceroute from my PC and from a few sources in the US to www.hku.hk for a homework assignment.

Here's the output from my PC for 30 hops:

Tracing route to www.hku.hk [147.8.2.58]
over a maximum of 30 hops:

   1    
   2     *        *        *     Request timed out.
   3    30 ms    31 ms    31 ms  10.0.1.26
   4    31 ms    31 ms    31 ms  10.9.8.134
   5    30 ms    33 ms    38 ms  212.36.211.162
   6    38 ms    34 ms    35 ms  172.16.44.6
   7    30 ms    31 ms    31 ms  172.16.44.5
   8    71 ms    68 ms    67 ms  ix-1-1-1-0.tcore1.WYN-Marseille.as6453.net [80.231.217.33]
   9    94 ms    92 ms     *     if-2-2.tcore2.WYN-Marseille.as6453.net [80.231.217.2]
   10   119 ms    89 ms    90 ms  if-9-2.tcore2.L78-London.as6453.net [80.231.200.14]
   11    92 ms    90 ms    90 ms  if-2-2.tcore1.L78-London.as6453.net [80.231.131.2]
   12    91 ms    92 ms    92 ms  if-17-2.tcore1.LDN-London.as6453.net [80.231.130.130]
   13    91 ms    92 ms    92 ms  195.219.83.186
   14   298 ms   235 ms    91 ms  ae-4.r23.londen03.uk.bb.gin.ntt.net [129.250.5.40]
   15   664 ms   369 ms   431 ms  as-0.r22.osakjp01.jp.bb.gin.ntt.net [129.250.5.35]
   16     *        *        *     Request timed out.
   17   399 ms   401 ms   404 ms  as-5.r20.newthk02.hk.bb.gin.ntt.net [129.250.6.115]
   18   397 ms   397 ms   394 ms  ae-1.r02.newthk02.hk.bb.gin.ntt.net [129.250.2.37]
   19   401 ms   399 ms   404 ms  203.131.246.154
   20   391 ms   386 ms   396 ms  115.160.187.102
   21   397 ms   391 ms   392 ms  175.45.11.98
   22   399 ms   392 ms   393 ms  203.188.117.2
   23   398 ms   387 ms   391 ms  147.8.239.15
   24     *        *        *     Request timed out.
   25     *        *        *     Request timed out.
   26     *        *        *     Request timed out.
   27     *        *        *     Request timed out.
   28     *        *        *     Request timed out.
   29     *        *        *     Request timed out.
   30     *        *        *     Request timed out.

When I increased the maximum number of hops to 70 (in case it's actually over 30 hops), I basically just got "Request timed out" from hop 24 til 70.

The address at hop 23 is actually in Hong Kong (using some IP tracker tool will show that) and so it's extremely unlikely that it takes 47+ additional hops to get from somewhere in HK to HKU.

Why does this happen? The most logical thing I could think of was that perhaps the destination itself (hku.hk) has some firewall and doesn't respond to traceroute requests, but why would this fool traceroute into practically not stopping?

Best Answer

The most logical thing I could think of was that perhaps the destination itself (hku.hk) has some firewall and doesn't respond to traceroute requests

This is the most common cause you will see on the net. Traceroute either uses UDP or ICMP packets with a reduced TTL value so packets would expire before they get to the destination. At the routers where they expire, it might or might not be allowed to respond with TTL-exceeded messages to the originator. Additionally, ICMP might be filtered altogether somewhere along the route so the response would be generated but simply filtered before it gets back to you.

why would this fool traceroute into practically not stopping?

Because traceroute is designed not to stop unless it receives a response from the destination.

Further reading: http://en.wikipedia.org/wiki/Traceroute#Implementation

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