Rebuilding an OpenStack instance and keeping the same fixed IP

OpenStack, and in particular the compute service, Nova, has a useful rebuild function that allows you to rebuild an instance from a fresh image while maintaining the same fixed and floating IP addresses, amongst other metadata.
However if you have a shared storage back end, such as Ceph, you’re out of luck as this function is not for you.
Fortunately, there is another way.

Prepare for the Rebuild:

Note the fixed IP address of the instance that you wish to rebuild and the network ID:

$ nova show demoinstance0 | grep network
| DemoTutorial network                       | 192.168.24.14, 216.58.220.133                     |
$ export FIXED_IP=192.168.24.14
$ neutron floatingip-list | grep 216.58.220.133
| ee7ecd21-bd93-4f89-a220-b00b04ef6753 |                  | 216.58.220.133      |
$ export FLOATIP_ID=ee7ecd21-bd93-4f89-a220-b00b04ef6753
$ neutron net-show DemoTutorial | grep " id "
| id              | 9068dff2-9f7e-4a72-9607-0e1421a78d0d |
$ export OS_NET=9068dff2-9f7e-4a72-9607-0e1421a78d0d

You now need to delete the instance that you wish to rebuild:

$ nova delete demoinstance0
Request to delete server demoinstance0 has been accepted.

Manually Prepare the Networking:

Now you need to re-create the port and re-assign the floating IP, if it had one:

$ neutron port-create --name demoinstance0 --fixed-ip ip_address=$FIXED_IP $OS_NET
Created a new port:
+-----------------------+---------------------------------------------------------------------------------------+
| Field                 | Value                                                                                 |
+-----------------------+---------------------------------------------------------------------------------------+
| admin_state_up        | True                                                                                  |
| allowed_address_pairs |                                                                                       |
| binding:vnic_type     | normal                                                                                |
| device_id             |                                                                                       |
| device_owner          |                                                                                       |
| fixed_ips             | {"subnet_id": "eb5db27f-edad-480e-92cb-1f8fec8848a8", "ip_address": "192.168.24.14"}  |
| id                    | c1927578-451b-4682-8888-55c7163898a4                                                  |
| mac_address           | fa:16:3e:5a:39:67                                                                     |
| name                  | demoinstance0                                                                         |
| network_id            | 9068dff2-9f7e-4a72-9607-0e1421a78d0d                                                  |
| security_groups       | 5898c15a-4670-429b-a414-9f59671c4d8b                                                  |
| status                | DOWN                                                                                  |
| tenant_id             | gsu7j52c50804cf3aad71b92e6ced65e                                                      |
+-----------------------+---------------------------------------------------------------------------------------+
$ export OS_PORT=c1927578-451b-4682-8888-55c7163898a4
$ neutron floatingip-associate $FLOATIP_ID $OS_PORT
Associated floating IP ee7ecd21-bd93-4f89-a220-b00b04ef6753
$ neutron floatingip-list | grep $FIXED_IP
| ee7ecd21-bd93-4f89-a220-b00b04ef6753 | 192.168.24.14   | 216.58.220.133     | c1927578-451b-4682-8888-55c7163898a4 |

Re-build!

Now you need to boot the instance again and specify port you created:

$ nova boot --flavor=m1.tiny --image=MyImage --nic port-id=$OS_PORT demoinstance0
$ nova show demoinstance0 | grep network
| DemoTutorial network                       | 192.168.24.14, 216.58.220.133                     |

Now your rebuild has been completed, you’ve got your old IPs back and you’re done. Enjoy 🙂

_This post was originally published on McWhirter’s blog. Superuser is always looking for interesting content, email us at [email protected] to get involved._

Cover photo by Yann // CC BY NC-ND 2.0