[systemd-devel] IPv6 dhcp-acquired prefix delegation?

[Kevin P. Fleming]

You have to accept RAs from upstream, unless you have a static route
to the ISP's gateway. DHCPv6 does not provide routing information,
only end-node addressing information, so with RAs ignored you won't
have a default route out of your network.

This is probably the largest, and most surprising, difference between
DHCPv4 and DHCPv6.

On Sat, May 16, 2020 at 8:40 PM John Ioannidis <systemd-devel at tla.org> wrote:
>
> I am running systemd v241, the one that comes with debian-10.
>
> Is the following scenario possible natively (that is, without using a standalone dhcpv6 client)?
>
> My residential ISP will normally hand me a /64, but will give me a /56 if I ask for it. While technically not a statically-allocated prefix, it changes very rarely: I had had the same prefix for over three years until I started experimenting with systemd-networkd and killed my old lease files.
> I have several local vlans, and I want to give a different /64 to each one of them, but if the prefix I get from upstream changes, I want them to automatically renumber.
> I'm perfectly happy relying on SLAAC for the local vlans, but I'm not against having to also run a dhcpv6 server to hand out addresses and things.
>
> My previous solution, using ifupdown instead of systemd was this:
>
> dhcpv6 client with -P --prefix-len-hint 56, which gets the /56 and the default route.
> A script in /etc/dhcp/dhclient-exit-hooks.d/ that builds an /etc/radvd.conf file with bits 56-63 appropriately numbered, and then restarts radvd.
>
> I have been totally unable to reproduce this behavior with pure networkd; in fact, I cannot even get a dhcpv6 session going. I do not want to rely on RAs from upstream, just DHCPv6. Here is a minimal .network file for the external interface ("ethwan")
>
> [Match]
> Name=ethwan
>
> [Network]
> DHCP=yes
> IPForward=yes
> IPMasquerade=no
> IPv6PrivacyExtensions=no
> IPv6AcceptRA=no
>
> [DHCP]
> UseDNS=no
> UseNTP=yes
> UseRoutes=yes
>
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