Subnet option to disable dns server

[Zang MingJie]

Multiple NIC/subnet may attach to a VM instance, if both subnets provide dns servers, only the last one will be used (overrides the previous one), so we need a method to disable dns server on some subnets.

[Salvatore Orlando (salvatore-orlando)]

Wouldn't you have the same problem with the router option provided by the DHCP server?

I think DNS server options are not sent for a subnet if there are no DNS servers there.
In cases like the following:
hosts A,B,C with A on S1, C on S2 and B on S1 and S2, then globally disabling DNS for S2 (for instance) is not a solution. However, I don't see any criterion for specifying which of the two DNS servers B should use.
Do you have already in mind a strategy?

[Zang MingJie (zealot0630)]

For security reason some subnet can be totally local and isolate, there is no l3-router, even the dnsmasq won't be able to talk to any other dns servers, if any VM choose it as its dns server, the VM lost its dns function. So I want to disable dns on such subnets.

The routes can be fully controlled by gateway option and host routes, there is no routing problem to setup multiple subnets.

[Nachi Ueno (nati-ueno)]

IMO, This is not a bug. blueprint is needed for this.

[Launchpad Janitor (janitor)]

[Expired for neutron because there has been no activity for 60 days.]

[OpenStack Infra (hudson-openstack)]

Change abandoned by Kyle Mestery (<email address hidden>) on branch: master
Review: https://review.openstack.org/89745
Reason: This review is > 4 weeks without comment and currently blocked by a core reviewer with a -2. We are abandoning this for now. Feel free to reactivate the review by pressing the restore button and contacting the reviewer with the -2 on this review to ensure you address their concerns.

[Dr. Jens Harbott (j-harbott)]

We have the same use case as in comment #2, an isolated subnet shared between tenants in order to share some internal services. Greatest issue is multiple tenants choosing identical instance names, resulting in DNS doing round-robin across all of them.

My proposed solution is to redefine the behaviour when setting `dns_servers = [ "0.0.0.0" ]`, currently this has this effect that each dhcp agent will announce only itself as DNS server, which seems less useful than the behaviour when dns_servers is unset, where it will announce all agents. I'll upload a patch for that, please let me know what you think about it.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/495781

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/495781
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=584b7561c162ec461e5c6a8dff5c012402cac5e0
Submitter: Jenkins
Branch: master

commit 584b7561c162ec461e5c6a8dff5c012402cac5e0
Author: Jens Harbott <email address hidden>
Date: Mon Aug 21 09:13:33 2017 +0000

    Allow to disable DNS server announcement per subnet

    Currently there is no way to have DHCP agents not announce DNS servers
    for a subnet. The current behaviour when the dns_nameservers option is
    set to '0.0.0.0' is that each agent will only announce itself instead of
    announcing the list of all dhcp agents for that subnet, which seems not
    too useful. So we redefine the meaning of this option to instruct the
    DHCP agent to not announce any DNS server in that case.

    Actually, going back to square one, it would be more natural to swap the
    meaning of "option unset" and "option 0.0.0.0", but that would change
    the default behaviour for all existing installation and thus does not
    seem feasible.

    Change-Id: I32d943360162c483ac1364100a21ab56b13517fb
    Closes-Bug: 1311040

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/neutron 12.0.0.0b1 development milestone.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: stable/pike
Review: https://review.openstack.org/544985

[OpenStack Infra (hudson-openstack)]

Change abandoned by Jens Harbott (frickler) (<email address hidden>) on branch: stable/pike
Review: https://review.openstack.org/544985

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: master
Review: https://review.openstack.org/614555

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/614555
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=698e6d475cd12b7a8484c4e2eb1a44426780fef4
Submitter: Zuul
Branch: master

commit 698e6d475cd12b7a8484c4e2eb1a44426780fef4
Author: Jens Harbott <email address hidden>
Date: Wed Oct 31 15:36:04 2018 +0000

    Update docs for disabling DNS server announcement

    The functionality allowing to disable DNS server announcement from the
    DHCP agent for certain subnets was introduced in [0]. We should mention
    this feature in the corresponding documentation.

    [0] https://review.openstack.org/495781

    Change-Id: I4adfa7ba789a59f967297ebb6c571deb0652c7ca
    Related-Bug: 1311040

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: stable/rocky
Review: https://review.openstack.org/619715

[OpenStack Infra (hudson-openstack)]

Related fix proposed to branch: stable/queens
Review: https://review.openstack.org/619716

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/619715
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=3206492a3b053d847c21ba93ceb9d709856d3d08
Submitter: Zuul
Branch: stable/rocky

commit 3206492a3b053d847c21ba93ceb9d709856d3d08
Author: Jens Harbott <email address hidden>
Date: Wed Oct 31 15:36:04 2018 +0000

    Update docs for disabling DNS server announcement

    The functionality allowing to disable DNS server announcement from the
    DHCP agent for certain subnets was introduced in [0]. We should mention
    this feature in the corresponding documentation.

    [0] https://review.openstack.org/495781

    Change-Id: I4adfa7ba789a59f967297ebb6c571deb0652c7ca
    Related-Bug: 1311040
    (cherry picked from commit 698e6d475cd12b7a8484c4e2eb1a44426780fef4)

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/619716
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=ac490d7d99c264330a0e3f56d1b1066c41567b6f
Submitter: Zuul
Branch: stable/queens

commit ac490d7d99c264330a0e3f56d1b1066c41567b6f
Author: Jens Harbott <email address hidden>
Date: Wed Oct 31 15:36:04 2018 +0000

    Update docs for disabling DNS server announcement

    The functionality allowing to disable DNS server announcement from the
    DHCP agent for certain subnets was introduced in [0]. We should mention
    this feature in the corresponding documentation.

    [0] https://review.openstack.org/495781

    Change-Id: I4adfa7ba789a59f967297ebb6c571deb0652c7ca
    Related-Bug: 1311040
    (cherry picked from commit 698e6d475cd12b7a8484c4e2eb1a44426780fef4)