SYMC: DNS does not work when DHCP in a neutron subnet is disabled

[Karthik Krishnan Ramasubramanian]

According to https://github.com/Juniper/contrail-controller/wiki/Subnet-Options, DHCP and DNS are separate knobs, but when I disable DHCP on a neutron subnet, DNS also does not work.

contrail version: R2.21 Build 15

STEPS TO REPRODUCE:

1. Create network

neutron net-create dhcp-test-net

2. Create subnet for dhcp-test-net, disable dhcp.

neutron subnet-create dhcp-test-net 10.10.10.0/24 --name test-dhcp-subnet --disable-dhcp

3. Create a port with fixed IP 10.10.10.3

neutron port-create --fixed-ip subnet_id=$SUBNET_ID_CREATED_ABOVE,ip_address=10.10.10.3 dhcp-test-net

4. Create VM with port created above.

nova boot --flavor $FLAVOR_ID --image $IMAGE_ID --nic port-id=$PORT_ID_CREATED_ABOVE --security-groups default --key-name mykey dhcp-test-vm-
```

5. Try to resolve DNS @10.10.10.2.

Now, I have no DHCP server running, so I login to the VM via console, configure things appropriately, interface IP and so on and try to do a DNS query @10.10.10.2 and I get no response. Queried contrail config and network_ipam attribute has `dns_server_address: "10.10.10.2"`, `enable_dhcp: false,` attributes.

I look at the vrouter interface config for the tap interface and I see `dhcp_service: "Disable",` and `dns_service: "Disable",`.

[Karthik Krishnan Ramasubramanian (doublek)]

The workaround for this is to re-enable DHCP on the affected subnet. However, this triggers another bug, wherein the route for default gateway x.y.z.1/32 disappears on all hypervisors that have VMs in the affected subnet. Restarting contrail-vrouter-agent on affected hypervisors re-downloads the route and things get back to normal.