Re: systemd-networkd: IPv6 prefix delegation lost when changing prefixes

[Tobias Brink <tobias.brink@gmail.com>]

Hello Vincent,

Vincent Truchseß <debian-user@v-tr.org> writes:

> I've had the same Issue here delegating prefixes to my VPN-Gateway in my
> home-hetwork.

Good to know that it's not just me. For posterity, I did some further
tests:

* It does not matter if the router sets the "Other Configuration" or the
  "Managed" flags, as long as I use "ForceDHCPv6PDOtherInformation=yes"
  in systemd-networkd. The behavior is the same.

* Playing around with tcpdump, I found that while a new prefix on my
  external interface enp3s0 is announced (and received) correctly via
  router advertisement, neither the ISP's router nor systemd-networkd
  trigger any DHCPv6 activity. I am not sure about the relevant RFCs and
  if a change in prefix should trigger a DHCPv6 Solicit or if the router
  has to send some other message, but as far as I understand, the router
  advertisement should be enough. The server *could* send Reconfigure
  messages, but my ISP's router does not seem to do it.

* Nevertheless, when systemd-networkd sends out a solicit or renew and
  the DHCPv6 server answers with NoBinding, the client is supposed to
  send a request message to try to get another prefix in reply (at least
  that's how I read RFC 8415, Sec. 18.2.10.1). It does not do this.

I also tried with the systemd-networkd from yesterday's git master of
the systemd git repo, but it behaves the same in my limited testing. I
strongly assume systemd-networkd is still buggy in this area.

An additional problem is that the routes for old prefixes do not seem to
be removed. That might also lead to problems.

> My solution back then was to ditch systemd-networkd for this setup and
> rely on configuring dhcpcd and radvd accordingly. Systemd's
> DHCP-implementation seems to a little bit out of whack, depending on the
> version.
>
> Unfortunately that VPN-Gateway got decommissioned and I don't have a
> backup of those two config-files. If I remember right, I kept the config
> close to what the ArchLinux-Wiki suggests.

Thanks for the info, but I will rather contact systemd upstream about
this, so that this software can be fixed. I found it to be a very
pleasant network managing daemon with very clear configuration and
otherwise quite a bit more reliable than NetworkManager or ifupdown, so
I think improving it is the better long-term solution.

Tobias