I have set up a 6in4 tunnel, namely "sit1", but I have discovered that if I traceroute non exist IP within the subnet, it will loop. What should I do to solve this?

The routing table:

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  • I'm confused about a 6to4 tunnel, do you mean you are trying to tunnel IPv6 over IPv4 to a tunnel host? Also what IPv6 addresses are on this box?
    – cpt_fink
    Dec 5 '14 at 6:39
  • Yes. My IP address on the tunnel interface is 2001:460:18:c7b::2. And gateway address is 2001:460:18:c7b::1 Dec 5 '14 at 6:40
  • As I understand, it's bad idea to assign IP from one subnet to bridge-local and to sit1
    – mmv-ru
    Dec 5 '14 at 12:59
  • What would be a better solution? Dec 5 '14 at 13:09
  • Were you assigned another /64 (or a /48 to use internally) by the tunnel broker?
    – cpt_fink
    Dec 6 '14 at 5:47

The tunnel is a point-to-point interface, all packets sent over the tunnel are received and processed by the remote device. When the remote device realizes the packet is destined for a subnet-local address it sends it back across the tunnel to your Microtik. You are hitting a the ping-pong vulnerability in IPv6.


The simplest solution would be to refrain from assigning a prefix to the point-to-point link, and to use a link-local address as the next hop.

  • 1
    We don't always have this option. With this tunnel, in particular, the global address is always assigned, and the link-local address of the other endpoint may not be available. Dec 30 '14 at 5:22
  • Then you can hack around the issue by adding some firewall rules to drop the offending packets. But I strongly recommend that you use link-local addresses as next hops whenever possible.
    – jch
    Dec 30 '14 at 16:17

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