Comment 1 for bug 2037420

we use domain name if
```
def _subnet_uses_dns(subnet):
    return (
        subnet is not None
        and not subnet.dns_servers
        and (subnet.vlan.dhcp_on or subnet.vlan.relay_vlan_id is not None)
    )
```

and this is why in your case we don't return the rack IP directly. We should add a check on the `allow_dns`. I'll fix this