Thank you very much for your bug report. Let me ask some clarifying
questions / notes:

1/ ipv6.ignore-auto-dns: "true" => This is about not picking up a
nameserver via IPv6 autoconfiguration / DHCP, so unrelated to the
default route.

2/ ipv6.never-default: "true" => This asks NetworkManager to never set
the default on this interface / connection.

Both settings are correctly set in Netplan's "passthrough" section. The
"ip route" output that you showed does not contain any default route (as
requested), i.e. nothing specifying the "default" or "::/0" target
network. I can only see "2a00:1398:300:14::/64" and "2000::/3", which
are specific target networks, not the "default" fallback.


Interestingly, the route-metric is relatively low (50), which means if there's 
any traffic towards those specific networks, it might choose those routes, as 
the default route usually has a higher metric of 100. But for a VPN this is 
usually what we want. If you do not want traffic to be routed through "tun0", 
you could try increasing the "ipv6.route-metric", to e.g. 150 (to make it 
higher than your "default" route).


So I do not fully understand the question.. The configuration seems to be 
matching the routing table. Can you please elaborate on your expected outcome a 
bit more?


Reference: https://networkmanager.dev/docs/api/latest/settings-ipv6.html

** Changed in: netplan.io (Ubuntu)
       Status: New => Incomplete

** Summary changed:

- Network manager still adds default ipv6 route despite setting to ignore it
+ NetworkManager still adds default ipv6 route despite setting to ignore it

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https://bugs.launchpad.net/bugs/2106017

Title:
  NetworkManager still adds default ipv6 route despite setting to ignore
  it

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