ISP's DHCPv6 in a Multi-WAN network
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Current situation: We got three contracts with two providers. One of which provides us with a DSL connection and the other one with two cable connections. We're using three (two Debian/Linux based, one pfSense/FreeBSD based) routers, because the cable provider forces us to use the same gateway for both cable connections and pfSense wouldn't properly load-balance (ie. use just either one of the cable connections) without double NATing. In it's current state our network is using IPv4 only, we would like to change that.
What has changed lately: The cable provider implemented IPv6, so now we get two /64 prefixes (one for each cable connection) from their DHCPv6 server. However those prefixes are not static, they change whenever we unplug the modems or we run out of lease time. The provider told us that this is to prevent us from running a dedicated server and there's nothing we can do about it. I applied this guide on both Debian routers. Both WAN interfaces facing the routers get IPv6 addresses assigned, but I have no clue how to tell pfSense to assign IPv6 addresses to the workstations now (with failover if possible). NPt seems to require a static external prefix, which we don't have.
I'm in desperate need of your help.