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I have just gotten dual stack support on my CMTS and enabled IPv6 on my router. I have debian installed on my PC, and I am noticing the default traceroute (uses UDP on linux) is only making it to my router and no further.

traceroute ipv6.google.com  #only first hop is shown, the rest timeout

However, giving the -I option forcing ICMPv6 usage, the traceroute is successful.

traceroute -I ipv6.google.com #able to see all hops until the destination

Can anyone clarify why this is?

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1 Answer 1

The -I forces ICMP instead of UDP, ICMP being more of the universal traceroute option.

The best I can give you as to why would be that many ports are blocked for various reasons, and the higher range for default UDP traceroutes seem to be among them.

This might give more info too:

I certainly can not speak to why Microsoft did this, but I will observe that in today's networking environment which is much more security conscious, that access list filtering will sometimes deny UDP packets to various high port numbers (which is what traceroute does) but may permit ICMP packets. So I have seen quite a few times where an IOS traceroute will not work where a Microsoft tracert will work just fine.

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I understand that the protocols that each uses is different; however, I am just wondering why the first edge router from my ISP and there on would all block UDP traceroutes for IPv6 but not for IPv4. –  Fred Thomsen Apr 24 '13 at 11:53
    
Since this is a QA site I added those for those who might now, you did show you understood that in your question. –  AthomSfere Apr 24 '13 at 11:58

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