Im interested in thoughts for a scenario where you want to do small-scale multi-site activities, with site-to-site connectivity.

Here’s a couple of constraints:

  • you’re not going to pay the money to get an assignment, you’ll just have ISP global.

  • your two or more sites will have different ISPs.

  • You’re doing VPN between sites instead of provider managed. The sites might be running some normal enterprise services like active directory, or other internal corporate norms.

  • you might have the need for a backup Internet connection. Load balancing would not be required.

With the fact that the globals could change at a site, would you consider using ULA? Or just stick with global and update DNS in the event of change. I know there’s a preference problem with ipv4 being chosen over ULA, so the ULA thing wouldn’t be very easy unless you went straight v6.

If ULA, would you pattern/convention match the global in each site or create one organization wide ULA and assign it something like /48 per site?

What precautions do you take on gateways to ensure globals aren’t used outside of the tunnel? ULA prevents this, but so does proper configuration I assume.

How would you do this?

I keep asking about ULA because I heard/read enough articles where the author says don’t do it, but they seem to be geared at large enterprise or hosting where they would definitely get dedicated blocks, peering, etc. I’m interested in the little guy.

  • @ScooptaEnglish
    2•5 months ago

    The main disadvantage of ULAs is in dual stack networks windows prefers IPv4 over them. In principle Linux should too but glibc follows an older RFC and as a result in practice picks ULAs over IPv4. If your GUA space is subject to change I would definitely recommend ULAs. Dynamic DNS is more headache than it’s worth. As others have mentioned I would keep IPv4 out of your internal DNS so that ULAs are preferred, if you want to dual stack your internal DNS then there are ways to configure clients to prefer ULAs over v4. Personally I run both ULAs and GUAs internally even with my own direct allocation but that’s because of dn42. What I do on my gateways to prevent leaks is I have a routing policy that returns an ICMP host unreachable if source is fd00::/8 and destination is 2000::/3 that way the gateway blocks any address mismatch. I also have a policy for the opposite GUA to ULA scenario. One other note, technically ULAs are supposed to be random /48s, others have mentioned generating a /40 but that’s not technically in spec. Ideally you would generate one /48 per site or use a single /48 and then do a /56 per site. Obviously do what you want and what makes the most sense for you but I’m going to put that info out there.