DNS option in dhcp always provided

Issue

• Although we have no DNS option configured in Horizon GUI the neutron dhcp server always offers the DNS as the 2 IPs of the dhcp pools:

[root@controller ~]#  neutron subnet-show 2b5d9bb8-eb8b-4132-9f90-0f2383393
+------------------+------------------------------------------------------------+
| Field            | Value                                                      |
+------------------+------------------------------------------------------------+
| allocation_pools | {"start": "192.168.22.100", "end": "192.168.22.200"}     |
| cidr             | 192.168.22.0/24                                           |
| dns_nameservers  | 0.0.0.0                                                    |
| enable_dhcp      | True                                                       |
| gateway_ip       | 192.168.22.254                                            |
| host_routes      | {"destination": "0.0.0.0/0", "nexthop": "192.168.22.254"} |
| id               | 2b5d9bb8-eb8b-4132-9f90-0f2f5579975b                       |
| ip_version       | 4                                                          |
| name             | test_net                                             |
| network_id       | e902fab2-9724-4ead-b8af-bd89f4586888
| tenant_id        | 6fdb261f3ec74f51a926a3003c8b7a87                           |
+------------------+------------------------------------------------------------+

[root@controller ~]# cat /var/lib/neutron/dhcp/e902fab2-9724-4ead-b8af-bd89f4586888/opts
tag:tag0,option:classless-static-route,169.254.169.254/32,192.168.22.101
tag:tag0,249,169.254.169.254/32,192.168.22.101
tag:tag0,option:router,192.168.22.254
tag:tag0,option:dns-server,192.168.22.100,192.168.22.101  <===

• How to disable this DNS option being offered ?