IPv6 for lazy dummies

flips · March 2021

I just realized it's been too long since I read about and learned IPv6 (and so much stuff has also changed since back then).

Got a new VPS, and correct config is something like:

IP/subnet: 2b01:dead:beef:42::2/64
Gateway: 2b01:dead:beef::1

And this works fine. To me this looks like the gateway is on a different subnet (/48) than the interface.
Anyone wants to shed any light on this?

Brueggus · March 2021

@flips said:
And this works fine. To me this looks like the gateway is on a different subnet (/48) than the interface.

It is. They most likely have 2b01:dead:beef::/48 and assigned 2b01:dead:beef:42::/64 to you. You should have a static route towards their /48 or their gateway IP somewhere.

rm_ · March 2021

Anyone wants to shed any light on this?

ip -6 route might.

alwyzon · March 2021

What you’ve got there is a setup in which your VPS believes it’s in its own little subnet that consists of just itself. That works on IPv4 too, for example, the sample below works on Debian and in a fictional local 10.0/8 network.

auto eth0
iface eth0 inet static
        address 10.1.2.3/32
        gateway 10.0.0.1
        pointopoint 10.0.0.1

This will disable all local traffic and just allow packets towards the gateway. This can be preferential in a shared environment (such as VPS or dedis) where the neighboring IPs aren‘t yours and you usually don‘t want any noise from them anyhow.

flips · March 2021

@Brueggus said: It is. They most likely have 2b01:dead:beef::/48 and assigned 2b01:dead:beef:42::/64 to you. You should have a static route towards their /48 or their gateway IP somewhere.

Ah, makes sense.

@rm_ said: ip -6 route might.

But of course ... I didn't pay attention. Did run ip -6 r s, but didn't noticed the added static route to the gw:

2b01:dead:beef::1 dev eth0 proto static metric 100 pref medium
2b01:dead:beef:42::/64 dev eth0 proto kernel metric 100 pref medium
fe80::/64 dev eth0 proto kernel metric 100 pref medium
default via 2b01:dead:beef::1 dev eth0 proto static metric 100 pref medium

This is on CentOS 8.3, I thought that static route would be in a separate route6-eth0 file or similar in /etc/sysconfig/network-scripts/. But there is no such file.

Brueggus · March 2021

I think DEFAULTGW=2b01:dead:beef::1 is sufficient on Centos 7/8 to create

2b01:dead:beef::1 dev eth0 proto static metric 100 pref medium

on the fly. CentOS 6 would complain since the GW is not on the net and therefore unreachable.

IPv6 for lazy dummies

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