How to bind IPv6 server sockets behind home router using dynamic prefix delegation?

Question

I'm trying to run a personal web service from my lan, on IPv6. I want it to be visible on the public internet, and behind an AAAA record. I need to balance ease of setup, and graceful reconfiguration when the IPv6 prefix from my ISP changes.

Which address should I bind to? A local IPv6 address + NAT, or should I embrace globally routable options pushed down to the host via prefix delegation?

My hosts are behind an OpenWRT router, IPv4 + IPv6. OpenWRT gets a /56 prefix delegated by my ISP (Telus in Canada). However, this prefix is not static. It changes once in a while, in the order of days. I can expedite change sometimes by doing resets.

I’m willing to give this no-NAT thing on IPv6 a go. I’m okay with punching ports/IPs open in the stateful firewall, but I would like to avoid address translation.

On my stateless+stateful IPv6 setup, my hosts on the lan do autoconfiguration, and also do DHCPv6 to get leased addresses. On my ubuntu box:

192.168.1.4
fd00:cafe::4/128                    # ULA local (by dhclient -6)
2001:2:3:4:a8a8:efcf:d96d:1315/64   # slaac+privacy global
2001:2:3:4:22f:bcff:fe12:1234/64    # slaac+EUI64 (macaddr)
fd00:cafe::a8a8:efcf:d96d:1315/64   # slaac+privacy local
fd00:cafe::22f:bcff:fe12:1234/64    # slaac+EUI64 (macaddr)
fe80::21f:bcff:fe08:c07a/64         # link local

The 2001:2:3:4: addresses are routable on the public internet. The fd00:cafe:: are routable only locally on my subnet. fd00:cafe:: is a prefix I've configured in OpenWRT (ULA) for just my internal network. The fe80: of course isn't routable. OpenWRT does not lease out a 2001:2:3:4::4 in this configuration, by design (I would very much like that however).

The v6 IP I pick for my service will have to end up in an AAAA DNS record somewhere, so it would be preferable if it didn't change every hour or so.

• The slaac+EUI addresses say a bit about my mac address, and I don't like that.
• The slaac+privacy addresses are a tad more privacy-preserving, but rotate every few hours, and that is less desirable. Addresses stick around if they are still in use, but they would be released every time I started the service (and dns would need to be updated -- which takes minimum 5min).
• Another option is to just statically pick a suffix I like for my service (e.g. ::d00d), with the same globally routable prefix and statically assign that to my nic, like so:

$ sudo ip -6 addr add 2001:2:3:4::d00d/64 dev eth0

Then in my applications, I could bind to that address only (or to "::" on the device holding that ip).

# ./myserver -l 2001:2:3:4::d00d/64 -p 8080

Now, once I’ve got a listening socket bound to a publically routable IP on the host, what's the best way to inform the router to let the SYN packets flow in for it? I need a hole punched here. UPnP, NAT-pmp?

Am I thinking about this the wrong way around? What’s the typical setup on IPv6?

Update

I tried using the hostid dhcp setting in my OpenWRT leases /etc/config/dhcp. It supposedly allows specifing the last 32bits of a dhcpv6 reservation. I was hoping I to receive an extra IPv6 with the wan prefix and my chosen suffix, but no luck. The dhcp6 client on my host still doesn't receive that extra IP. Probably related to this odhcpd issue 61.

Answer 1

If you are intending to configure an AAAA record, then configure that address as a static IP address on the host that requires a static IP address. DHCP can be used to serve static IP addresses, but it is a poor second choice solution.

OpenWRT has used dnsmasq for DHCP (v4 and v6) and DNS. dnsmasq can be configured to supply a static IPv6 address if the client using DHCPv6. It might be appropriate use these as a destination for a proxy or VIP (Virtual IP. The latest version of OpenWRT appears to use odhcpd for some if not all DHCP services.

Use the firewall to open ports from the Internet to the server. (The address you assigned will be its address from the Internet as well as your LAN.) You should be able to connect locally to the server without opening any ports on the firewall.

odhcpd does support static IP address by adding a host section to its configuration. I haven't my router to the lastest OpenWRT version, so I am using dnsmasq which can also provide RA and DHCPv6 services. You should be able to specify the static IP address at the bottom of the DHCP and DNS configuration page.

IPv6 should not require NAT, although they may be the destination behind a Proxy or VIP (Virtual IP). Build a /64 from the /56 delegated to you, and assign an address from that /64 to your web server. This is the address you should use. Your web server will need a Dynamic IP client that can update your DNS whenever it's address change. Until you get DHCP static IP addresses working, the MAC based address will be your stable address.

Address in the network block with fc00:/8 should not be used for communications with the internet. These are intended for use as unique local addresses, similar to the IPv4 private addresses.

You may need to develop your own dynamic IP address scripts. On the router you will need to update the firewall when the /56 changes, and may need to make some other changes. On the web server, you will need to update DNS, and possibly assign a new static IP address. (I think assignment on IP address prefix change may be simpler than DHCP static IPv6 address assignment.)