named fails to listen dynamic ipv6 address on startup

[Ko-Zu]

On rebooting up ubuntu 20.04, named detects available ipv6 address assigned via router advertisement but fails to listen on it.

`journalctl -b0` shows something like this.
```
Jan 01 15:27:12 cdns named[200]: IPv6 socket API is incomplete; explicitly binding to each IPv6 address separately
Jan 01 15:27:12 cdns named[200]: listening on IPv6 interface lo, ::1#53
Jan 01 15:27:12 cdns named[200]: listening on IPv6 interface eth0, fd42:xxxxxxx#53
Jan 01 15:27:12 cdns named[200]: creating TCP socket: address not available
```

It seems named.service is starting up too early. I have tried adding `After=network-online.target` for named.service but nothing changed.

I could reproduce this on LXD container ubuntu-daily:20.04 and current version
bind9/focal-updates,now 1:9.16.1-0ubuntu2.4 amd64 [installed]

```
lxc launch ubuntu-daily:20.04 cdns
lxc exec cdns -- apt install -y bind9
lxc restart cdns
lxc exec cdns -- ss -tupln | grep :53
```

The final command should print that named listening on a unique local address generated by LXD.
```
tcp LISTEN 0 10 [fd42:xxxxxxxx]:53 [::]:*
```

A few users noticed this issue and posted workaround on askubuntu.
https://askubuntu.com/questions/1261187/dns-server-does-not-listen-on-ipv6-after-a-reboot

Their workaround, delaying service startup resolved this issue for me.

```
# systemctl edit named
[Service]
ExecStartPre=-/bin/sleep 10
```

[Nick Tait (nick.t)]

FYI I've reported a similar (but different) bug: https://bugs.launchpad.net/ubuntu/+source/bind9/+bug/1965521

I believe the workaround that I've offered is slightly more optimal than the 10-second sleep offered above. (It is however reassuring to know that you solved this problem in almost exactly the same way.)

Thanks,
Nick.

[Paride Legovini (paride)]

Thanks Ko-Zu for reporting this bug and sorry if we didn't reply earlier, it somehow fell under the radar of our bug triage process

I agree with Nick in that this bug and LP: #1965521 are likely to have the same underlying issue: named is started before networking is ready.

As LP: #1965521 already elaborates more on the issue I'm tempted to mark this bug as a duplicate of it, but before doing I'd like to hear Ko-Zu's opinion, so I'm just marking this as Incomplete for now.

[Simon Déziel (sdeziel)]

FYI, this problematic behavior is fixed in Jammy/bind9 (I tested with 1:9.18.0-2ubuntu2) where the automatic-interface-scan feature works properly and detects IPs appearing/disappearing.

[Paride Legovini (paride)]

Thanks Simon for checking and clarifying! I'm marking the devel task as Fix Released then, and adding a Confirmed Focal task to reflect your IRC comment:

<sdeziel> paride: I can also confirm that Focal's broken and there, neither automatic-interface-scan nor interface-interval paper over the problem... bind9 simply never notices late showing IPs

We'll tackle LP: #1965521 on its own.