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Comment by simoncion | Hacker News Reader

To be clear, what you have is a router that's asking your ISP for a DHCPv6-PD prefix, assigning slices of that to one or more interfaces on that router, and what you want is for your dhcpd on that router to assign prefix-oblivious addresses to specific hosts on your LAN?

In other words, you want things to work like this?

  ISP-provided-PD-prefix 2001::/64 + Host address ::22 = Assigned address 2001::22
  ISP-provided-PD-prefix 2001:1:/64 + Host address ::22 = Assigned address 2001:1::22

If so, I'll poke around the docs to see if this is possible. I'm running both dhcpcd and ISC dhcpd on my LAN and have a hobbyist's experience with them.

But -honestly- what I've done is just relied on SLAAC to handle the globally-routable addresses, and advertised a ULA prefix for stable addresses. These go into my local DNS, but you could just as easily use that for DHCPd.

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Not sure if this is what you were describing, but my dhcpd server is a separate machine to the router.

I’m just using an off the shelf ASUS router because it’s actually surprisingly good at the basics. But I wanted PXE booting so set up ISC dhcpd on a home server.

To be fair, it might actually be possible to do this on my ASUS router. I’ve not actually checked. I’ve had the same setup up for years. Easily more than a decade. Only updating hardware when necessary. So I might be missing a trick with these latest ASUS routers.

> Not sure if this is what you were describing, but my dhcpd server is a separate machine to the router.

That was not what I was describing. I was figuring that your DHCPv6 client (that talks to your ISP) and your DHCPd would be on the same machine, but maybe that's okay. How does your dhcpd server get its address? A DHCPv6 request to the router? If so, the following report might (might!) be useful to you:

So, while I DID find out about dhcp-eval(5), it doesn't look to me like ISC DHCPd will do what you want. I didn't see any parameters documented in the dhcpd.conf manual that looked like they were prefix-independent.

Probably your best bet is to template your dhcpd.conf and known_hosts files, then use your network manager's [0] "on address change" hooks to fill in the currently-assigned prefix, write out new files, and bounce dhcpcd.

[0] NB: NOT (neccessarily) NetworkManager (that nasty, wretched thing), but maybe like dhcpcd's run hooks.

> How does your dhcpd server get its address?

It’s hardcoded. For IPv4 it doesn’t need to be dynamic because NAT allows you to hardcode private address ranges. But that whole concept of networking doesn’t translate (no pun intended) to IPv6

This is the problem I’m running into with deploying IPv6. I don’t know what address ranges to allocate because the dhcp server doesn’t perform any handshakes with the ISP. And I’m a bit reluctant to rearchitect the network topology for IPv6 because everything already works really well without IPv6.

So ideally I’d want a way of sliding in IPv6 without having to break what’s already working.

Every solution I’ve explored thus far hasn’t achieved that. But there’s lots of good information shared here today so I’ll have another read and maybe they’ll offer up an insight I’d previously missed.

Oof.

Yeah, because you're gonna have to have a DHCPv6 client running on your router (and because your ISP is almost certainly using DHCPv6-PD the router is where you're pretty much going to have to first learn about your LAN-side DHCPv6 prefixes), it's probably going to be a bit tricky (but probably not impossible) to do what you want.

Best of luck. If you figure out how to do it within the HN comment freeze period (I think it's 14 days?), please do leave a follow-up comment. I'd be very interested in hearing what you come up with.

I have had success running a hybrid IPv4/6 network by reading this guide for inspiration:

https://blog.infected.systems/posts/2024-12-07-building-an-i...

This allows me to have a mixture of both protocols and even some boxes that have both IPv4 and IPv6 addresses. I still have some issues writing routing rules that does not fail for link-local addresses, but the network has now been fully operational for well over a month.

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