IPv6 added with random prefix

[NiceRath]

We have seen this issue happen on multiple debian11/12 root-servers (running at Hetzner):

Version: 3.0.0-1.1 (latest in default apt-repo)

Note: xxxx is redacted

1. We configure an IP-address in /etc/network/interfaces.d/:

   auto eno1
   iface eno1 inet6 static
     address 2a01:4f8:2xxx:xxxx::1
     netmask 64
   

2. Perform a ifreload -a or reboot the server

3. We see a 'random' IP-address on the interface (output of ip a)

   inet6 74ca:5341:1400:200:2a01:4f8:2xxx:xxxx/32 scope global 
   

---

When using the 'legacy' ifupdown - every works as expected.

As a workaround - this worked:

  auto eno1
  iface eno1 inet6 static
    up ip address add 2a01:4f8:2xxx:xxxx::1/64 dev eno1

Interesting enough - adding another interface like this (for Hetzner vswitch) works without any issues:

auto eno1.4000
iface eno1.4000 inet6 static
  address 2a01:4f8:fxxx:xxxx::1
  netmask 64
  vlan-raw-device eno1
  mtu 1400

[julienfortin]

Try removing static:

auto eno1
iface eno1
  address 2a01:4f8:2xxx:xxxx::1
  netmask 64