Comment 2 for bug 1979528

It seems to me I can get neutron to not push a default route.

The subnet gateway actually can be unset. The API allows is:

https://docs.openstack.org/api-ref/network/v2/?expanded=create-subnet-detail#create-subnet
"To specify a subnet without a gateway, set the gateway_ip attribute to null in the request body."

With neutron client you could (and still can though neutron client is deprecated) leave the subnet gateway unset:

neutron subnet-create NET CIDR --no-gateway

I believe it is a bug of openstack client that leaving the gateway unset is not possible. I will open another bug report for that.

But if I do that and set the port's dhcp option 'router' to empty then I believe no default route is pushed:

openstack network create net0
neutron subnet-create net0 10.0.4.0/24 --name subnet0 --no-gateway
openstack port create port0 --network net0
openstack port set port0 --extra-dhcp-option name=router,value=,ip-version=4
openstack server create --flavor ds1G --image u1804 --nic port-id=port0 --wait vm0

Then I log in to this vm and inside:

# dhclient ens2

and in parallel I see this dhcp reply:

# tcpdump -n -vvv -i ens2 port 67 or port 68
...
    10.0.4.1.67 > 10.0.4.55.68: [udp sum ok] BOOTP/DHCP, Reply, length 333, xid 0xbc39bb14, Flags [none] (0x0000)
          Your-IP 10.0.4.55
          Server-IP 10.0.4.1
          Client-Ethernet-Address fa:16:3e:ac:bd:06
          Vendor-rfc1048 Extensions
            Magic Cookie 0x63825363
            DHCP-Message Option 53, length 1: ACK
            Server-ID Option 54, length 4: 10.0.4.1
            Lease-Time Option 51, length 4: 86400
            RN Option 58, length 4: 43200
            RB Option 59, length 4: 75600
            Subnet-Mask Option 1, length 4: 255.255.255.0
            BR Option 28, length 4: 10.0.4.255
            Domain-Name-Server Option 6, length 4: 10.0.4.1
            Domain-Name Option 15, length 14: "openstacklocal"
            Hostname Option 12, length 14: "host-10-0-4-55"
            Classless-Static-Route Option 121, length 9: (169.254.169.254/32:10.0.4.1)
            MTU Option 26, length 2: 1450
            END Option 255, length 0

I believe here we are only pushing a route for metadata but no default route.

I did not try yet, which part was necessary: unsetting the subnet gateway and/or using the router dhcp option (as empty). Please give it a try. I hope it works for you as a well.

I will link to the openstack client bug from here when I opened it. But until then neutron client should work.