[Bug 1796164] Re: After interface/IP changes, bind9 can fail to respond to queries on the new interface

[Andreas Hasenack]

I can confirm this on cosmic with a libvirt network (virbrN). The logs
say named started listening on the new nic, but queries don't work. In
fact, even with query logging enabled, nothing is logged when a query
comes in via that interface.

tcpdump shows (virbr1 is 10.0.3.1):
listening on any, link-type LINUX_SLL (Linux cooked), capture size 262144 bytes
12:48:26.477271 IP 10.0.3.1.36985 > 10.0.3.1.53: 38421+ [1au] A? fakehost.maas. (54)
12:48:26.477440 IP 10.0.3.1.53 > 10.0.3.1.36985: 38421 Refused$ 0/0/0 (31)

It worked with a real nic hot plugged into the VM, though. I then just
used plain old "ifconfig ens3 10.0.5.5 netmask 255.255.255.0 up" (i.e.,
no netplan generate/apply, for "fear" it would restart things), bind
started listening on it, and dig @10.0.5.5 worked.

Let me investigate the libvirt (virbrN) case a bit more.

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https://bugs.launchpad.net/bugs/1796164

Title:
  After interface/IP changes, bind9 can fail to respond to queries on
  the new interface

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