Public bug reported:

The gateways/firewalls in our DC are highly available and when there is
a failover their IPv6 VIP (fe80::1) moves from the master to the backup
one.

We found that only our Bionic VMs behind those gateways had issues after
a failover. Those Bionic VMs were all running systemd-networkd (from
netplan) and before the failover they had:

$ ip -6 route
...
default via fe80::1 dev eth0 proto ra metric 1024 pref medium

But after a failover:

$ ip -6 route
...
default proto ra metric 1024
        nexthop via fe80::1 dev eth0 weight 1
        nexthop via fe80::210:18ff:febe:6750 dev eth0 weight 1

And after another failover:

$ ip -6 route
...
default proto ra metric 1024
        nexthop via fe80::1 dev eth0 weight 1
        nexthop via fe80::210:18ff:febe:6750 dev eth0 weight 1
        nexthop via fe80::210:18ff:fe77:b558 dev eth0 weight 1


This is problematic as those then use fe80::210:18ff:fe77:b558%$IFACE as their 
default gateway even when this gateway is unavailable:

$ ip -6 route get ::
:: from :: via fe80::210:18ff:fe77:b558 dev eth0 proto ra src 
fe80::a800:ff:fe51:8c37 metric 1024 pref medium


We concluded it was a systemd-networkd bug after checking that the following 
combinations were NOT affected:

1) Xenial+4.4 kernel
2) Xenial+4.15 kernel
3) Bionic+ifupdown


Additional information:

$ apt-cache policy systemd
systemd:
  Installed: 237-3ubuntu10.3
  Candidate: 237-3ubuntu10.3
  Version table:
 *** 237-3ubuntu10.3 500
        500 http://archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
        100 /var/lib/dpkg/status
     237-3ubuntu10 500
        500 http://archive.ubuntu.com/ubuntu bionic/main amd64 Packages

$ lsb_release -rd
Description:    Ubuntu 18.04.1 LTS
Release:        18.04

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: systemd 237-3ubuntu10.3
ProcVersionSignature: Ubuntu 4.15.0-38.41-generic 4.15.18
Uname: Linux 4.15.0-38-generic x86_64
ApportVersion: 2.20.9-0ubuntu7.4
Architecture: amd64
Date: Wed Oct 31 08:47:28 2018
Lspci: Error: [Errno 2] No such file or directory: 'lspci': 'lspci'
Lsusb: Error: [Errno 2] No such file or directory: 'lsusb': 'lsusb'
MachineType: QEMU Standard PC (i440FX + PIIX, 1996)
ProcEnviron:
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_US.UTF-8
 SHELL=/bin/bash
ProcKernelCmdLine: BOOT_IMAGE=/boot/vmlinuz-4.15.0-38-generic 
root=UUID=43b7ee2e-2ab1-4505-8e0b-d9fe0563a034 ro console=ttyS0 net.ifnames=0 
vsyscall=none kaslr nmi_watchdog=0 possible_cpus=1
SourcePackage: systemd
UpgradeStatus: No upgrade log present (probably fresh install)
dmi.bios.date: 04/01/2014
dmi.bios.vendor: SeaBIOS
dmi.bios.version: Ubuntu-1.8.2-1ubuntu1
dmi.chassis.type: 1
dmi.chassis.vendor: QEMU
dmi.chassis.version: pc-i440fx-xenial
dmi.modalias: 
dmi:bvnSeaBIOS:bvrUbuntu-1.8.2-1ubuntu1:bd04/01/2014:svnQEMU:pnStandardPC(i440FX+PIIX,1996):pvrpc-i440fx-xenial:cvnQEMU:ct1:cvrpc-i440fx-xenial:
dmi.product.name: Standard PC (i440FX + PIIX, 1996)
dmi.product.version: pc-i440fx-xenial
dmi.sys.vendor: QEMU

** Affects: systemd (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: amd64 apport-bug bionic

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https://bugs.launchpad.net/bugs/1800836

Title:
  systemd-networkd doesn't IPv6 RA properly

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