Can't specify gateway when creating subnet with subnetpool and without cidr

[Hong Hui Xiao]

When creating a subnet by using the subnetpool(without specifying cidr), I can't specify the gateway. Either using --no-gateway or --gateway, the newly created subnet will have the first ip in the pool as the the gateway ip.
Looks like the code don't consider gateway ip at [1], and always set the first ip in pool as gateway ip in [2].

I fount this when creating an external network, which don't have a gateway indeed. The unnecessary gateway ip will create meaningless default route in router.

[1] https://github.com/openstack/neutron/blob/master/neutron/ipam/requests.py#L290
[2] https://github.com/openstack/neutron/blob/master/neutron/ipam/subnet_alloc.py#L126

[Carl Baldwin (carl-baldwin)]

I don't think I have any links to share about this but there was a lot of discussion on this in the blueprint spec and possibly the ML. The discussion was centered around how does one specify such a gateway? You don't know the network address of the subnet so you can't specify the gateway address.

In some versions of the spec, I had proposed that the gateway address and allocation pools could be specified by using 0 as the network address. For example, you'd specify 0.0.0.0.3 if you wanted to use the third address in the resulting. So, if the subnet ended up being 192.168.0.128/25 then your gateway would end up being 192.168.0.131. There were objections to using the gateway field for this because it would be somewhat non-RESTful to return a different gateway than the one in the request. I then proposed using a new input-only field called gateway_template or something.

In the end, a few people argued that if you don't care what the subnet is then why would you care what gateway address or allocation pool you get in the subnet? We settled on not allowing you to specify the gateway address in this use case.

[Hong Hui Xiao (xiaohhui)]

Hi Carl.
Thanks for the explanation. I understand the point in your comment, and now I see why the spec says that user might "using 0 as a wildcard prefix " at [1]. However, in the real neutron, I have to specify the exact cidr and allocation pool, even if I have used a subnetpool.

My concern in this bug is that when user create a subnet from subnetpool. And the user don't want the gateway ip. Now there are 2 ways: a) specify the exact cidr and then the --no-gateway will take effect. b) create the subnet from subnetpool, and then update the subnet with --no-gateway.

I think neutron can at least open the gate for --no-gateway + auto allocation subnetpool. Yes, I don't care what the subnet is, and I don't care what the gateway ip is. I just don't want the gateway ip. What is your opinion about it?

[1] https://specs.openstack.org/openstack/neutron-specs/specs/kilo/subnet-allocation.html

[Carl Baldwin (carl-baldwin)]

Thanks for the reply. I think enabling the --no-gateway use case could be worth pursuing. I'm happy to leave this bug as triaged. I don't have any plans to work on this at the moment but someone (you, maybe) could pick it up.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/251758

[OpenStack Infra (hudson-openstack)]

Change abandoned by Hong Hui Xiao (<email address hidden>) on branch: master
Review: https://review.openstack.org/251758
Reason: This patch doesn't draw much attention, and fails the jenkins.

[Armando Migliaccio (armando-migliaccio)]

This bug is > 180 days without activity. We are unsetting assignee and milestone and setting status to Incomplete in order to allow its expiry in 60 days.

If the bug is still valid, then update the bug status.

[Launchpad Janitor (janitor)]

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