Changing the DNS after microstack.init has been run is more difficult than it should be

[Pen Gale]

microstack.init reads the value of config.network.dns, then writes it into $SNAP_COMMON/etc/neutron/dhcp_agent.ini

To change that value later, you need to:
1) Update the value of config.network.dns
2) Hand edit dhcp_agent.ini
3) Restart the snap

Instead, you should be able to:
1) Update the value of config.network.dns (possibly via a wrapper)
2) Invoke microstack.init in such a way that it will update the file and restart just the services that need a restart (possibly the wrapper above handles this, too)

The work involved is:
1) Refactor microstack_init to write dhcp_agent.ini via a template, rather than writing it directly in the init script.
2) Write the wrapper/add hooks to microstack.init to enable the above.

[Peter Matulis (petermatulis)]

I can't get this to work at all.

sudo snap set microstack config.network.dns-servers=10.246.112.3

The snap was restarted:

$ sudo snap restart microstack

The procedure was confirmed:

$ sudo snap get microstack config.network.dns-servers
10.246.112.3

A VM was created:

$ microstack launch cirros -n test

But the nameserver does not change from the default of 1.1.1.1 :

$ ssh -i /home/ubuntu/snap/microstack/common/.ssh/id_microstack cirros@10.20.20.85 cat /etc/resolv.conf

search bad
nameserver 1.1.1.1

I'm not sure why dhcp_agent.ini needs to also be created but I tried it, in addition to all the above:

$ cat /var/snap/microstack/common/etc/neutron/dhcp_agent.ini

[DEFAULT]

interface_driver = openvswitch
dhcp_driver = neutron.agent.linux.dhcp.Dnsmasq
enable_isolated_metadata = True
dnsmasq_dns_servers = 10.246.112.3

This was followed by a restart of the snap, as before. No change was observed upon the creation of a new VM.