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My ISP provides an IPv6 prefix delegation but with a dynamic prefix. I want a publicly routed static prefix for some local services, so I also have a Hurricane Electric tunnel set up, which means clients must operate in a multi-address multihoming mode. The HE tunnel also requires traffic sourced in the HE static prefix to be routed through the HE tunnel; response packets are dropped upstream of my router otherwise.

The tunnel is necessarily slower than native IPv6, so I want clients to prefer source IP addresses that fall in the ISP's dynamic prefix except when connecting directly to the local services. How do I configure clients (a mix of Ubuntu and Windows) to prefer the native (but dynamic) "home"?

  • You have to control this via split-brain DNS. Local clients' queries resolve to the HE address, while external clients get the ISP address. You could also get your own IPv6 delegation, so you'll always have a static address.. That might be the simpler option of the two. – Ron Trunk Aug 4 at 13:36
  • Split DNS seems very fragile since the prefix delegated by the ISP is dynamic and DNS gets cached a lot. I'd expect this combination to lead to a lot of service outages while DNS updates propagate. Can you talk more about getting one's own IPv6 delegation? I wasn't aware this was possible. – Caleb Aug 4 at 18:58
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Doing more research, it looks like the recommended method for handling this situation is to include the router preference options in the Router Advertisement. From https://tools.ietf.org/html/rfc4191#section-5.1:

The best router for the default route is the router with the best route toward the wider Internet. The router least likely to redirect traffic depends on the actual traffic usage. The two concepts can be different when the majority of communication actually needs to go through some other router.

For example, consider a situation in which you have a link with two routers, X and Y. Router X is the best for 2002::/16. (It's your 6to4 site gateway.) Router Y is the best for ::/0. (It connects to the native IPv6 Internet.) Router X forwards native IPv6 traffic to router Y; router Y forwards 6to4 traffic to router X. If most traffic from this site is sent to 2002:/16 destinations, then router X is the one least likely to redirect.

To make type A hosts work well, both routers should advertise themselves as default routers. In particular, if router Y goes down, type A hosts should send traffic to router X to maintain 6to4 connectivity, so router X and router Y need to be default routers.

To make type B hosts work well, router X should advertise itself with a High default router preference. This will cause type B hosts to prefer router X, minimizing the number of redirects.

To make type C hosts work well, router X should in addition advertise the ::/0 route with a Low preference and the 2002::/16 route with a Medium preference. A type C host will end up with three routes in its routing table: ::/0 -> router X (Low), ::/0 -> router Y (Medium), 2002::/16 -> router X (Medium). It will send 6to4 traffic to router X and other traffic to router Y. Type C hosts will not cause any redirects.

Note that when type C hosts process the Router Advertisement from router X, the Low preference for ::/0 overrides the High default router preference. If the ::/0 specific route were not present, then a type C host would apply the High default router preference to its ::/0 route to router X.

  • The problem with this method is it's based on the destination address. You need to adjust routing based on the source. – Ron Trunk Aug 5 at 12:19
  • I understand the route selection based on router preference to imply selection of a source IP address that's routed through that same link though. Sending traffic to a specific route from an address with a different return route sounds sounds like a special case, not a sensible default. – Caleb Aug 5 at 14:18
  • Following up on this, I believe what I've articulated as the theory of operation for route preference is correct, but at least on my testbench Ubuntu 19.04 system, advertising route preferences didn't affect the final choice of source IP address. I'm still digging, but so far I haven't found any evidence that route preferences are honored by the Linux kernel. – Caleb Aug 20 at 15:46

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