Managing public IPs

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Find out how to assign, move and release public IP addresses on SNC Cloud Platform using independent Neutron ports that survive instance recreation

Objective

Practical guide to assigning, moving, and releasing public IP addresses on this platform via independent Neutron ports. The additional-IP feature is landing with major release #2; in the meantime, keeping the same public address across instances calls for dedicated management, detailed here. Target audience: anyone provisioning instances on this platform.

This guide describes how to manage public IP addresses on this platform so they can be detached from one instance and reattached to another without being lost — a common need (instance recreation, migration, switching a test environment) that, without care, results in the IP disappearing.

Overview

The additional-IP feature will be available starting with the platform's major release #2. In the meantime, a public IP is obtained by attaching a second network interface directly to the shared external network Ext-Net — which calls for dedicated management to keep the same address across instances.

The approach to favour is creating the Neutron port independently of any instance, then attaching it to instance A. You can then detach that independent port from instance A and attach it to instance B without losing the address.

This guide details how to implement this approach, including one limitation to be aware of (see the note in step 1).

Requirements

The openstack CLI configured (see the Terraform guide for clouds.yaml-based authentication).
The ID of the Ext-Net external network and its subnet:

openstack network show Ext-Net -f value -c id
openstack subnet list --network Ext-Net -f value -c ID

Instructions

Independent port vs. implicit port

Whether a Neutron port survives an instance's lifecycle depends entirely on how it was created, not on its attachment state at any given moment:

	Implicit port	Independent port
Creation	`openstack server create --nic net-id=<network>` — Nova creates the port itself	`openstack port create --network <network>` — created separately, before any instance
Attachment	`--nic net-id=` at instance creation	`--nic port-id=<port>` at creation, or `openstack server add port` afterwards
Instance deletion	The port is automatically deleted along with the instance — the IP goes back to the pool	The port survives instance deletion
Live detach (`server remove port`)	The port is deleted immediately, not just detached (empirically verified behaviour, see known pitfalls)	The port stays detached, reattachable at will

Practical consequence: always create the port separately, before the instance, whenever you want to keep the option of reusing the IP. There is no way to "convert" an already-attached implicit port into an independent one afterwards. Attempting it (detach then reattach) destroys the port and releases the IP back to the pool, just like any deletion.

Step 1 — Creating an independent port on `Ext-Net`

EXTNET_ID=$(openstack network show Ext-Net -f value -c id)

openstack port create --network $EXTNET_ID --security-group <secgroup> my-public-port

Neutron automatically assigns a free IP from the Ext-Net pool. Retrieve the address:

openstack port show my-public-port -f value -c fixed_ips

Info

Good to know: on Ext-Net, the address is always auto-assigned from the pool — picking a specific IP ahead of time is not possible:

openstack port create --network $EXTNET_ID \
  --fixed-ip subnet=<subnet-id>,ip-address=192.0.2.10 my-public-port
# ForbiddenException: 403 ... rule:create_port:fixed_ips:ip_address ... disallowed by policy

The benefit of an independent port remains fully intact (portability across instances); only the ability to pick the address ahead of time is missing. On a private network (e.g. an internal network you created), choosing a specific IP works normally.

Step 2 — Attaching the port to a new instance

openstack server create \
  --flavor <flavor> \
  --nic net-id=<private-network>       `# private interface, implicit: fine, no public IP to lose` \
  --nic port-id=my-public-port         `# public interface, ALWAYS via port-id` \
  --image <image> \
  my-instance

Step 3 — Attaching the port to an already-existing instance

If the instance was created before the port, or already exists without a public interface:

openstack server add port my-instance my-public-port

Verify:

openstack server show my-instance -f value -c addresses

Step 4 — Moving the IP to another instance

This is the target scenario of this guide: reusing the same public IP after recreating or replacing an instance.

# detach from instance A
openstack server remove port instance-a my-public-port

# reattach to instance B
openstack server add port instance-b my-public-port

The port — and therefore the IP — is never returned to the pool: it moves directly from one instance to the other. Expect a few seconds of network interruption during the detach/attach.

Warning

This operation is only safe on a port created independently (step 1). On an implicit port, server remove port destroys it immediately instead of detaching it — see known pitfalls.

Step 5 — Finding which IP is attached to which instance

# all IPs across all instances
openstack server list -f table -c Name -c Networks

# port-level detail (ID, fixed IP, owning instance)
openstack port list -f table -c ID -c "Fixed IP Addresses" -c "Device ID"

# find the port for a specific IP
openstack port list -f value -c ID -c "Fixed IP Addresses" | grep 192.0.2.10

A port with an empty Device ID column is detached (independent and currently free, ready to be attached to an instance).

Step 6 — Releasing a port

If the IP is no longer needed, deleting the port returns it to the Ext-Net pool:

# detach first if still attached
openstack server remove port <server> <port>

openstack port delete <port>

Known pitfalls (troubleshooting)

Symptom	Cause	Fix
The public IP changes after deleting/recreating an instance	The port carrying the IP was implicit (`--nic net-id=` at creation), so it was automatically deleted along with the instance	Always create the port separately (step 1) before creating any instance you may later recreate
`openstack server remove port` makes the port disappear instead of detaching it (`openstack port show` returns `No Port found`)	Attempting to detach an implicit port — Nova deletes it immediately rather than freeing it, whether via instance deletion or manual detach	Create a new independent port (step 1) and attach it — the old IP is lost, a new one is assigned from the pool
`403 Forbidden` on `openstack port create --fixed-ip ip-address=...` on `Ext-Net`	Picking a specific public IP is not available on this platform	Accept the auto-assigned IP from the pool; the portability benefit of the independent port remains fully intact
The instance gets no public IP after `server add port`	Port's security group too restrictive, or port on the wrong network	Check the security group attached to the port (`openstack port show <port> -c security_group_ids`) and the target network (`openstack port show <port> -c network_id`)
IP needs reconfiguring inside the OS after a public IP change	The VM has no cloud-init (Windows VM), so nothing re-reads the IP from metadata at boot — on a Linux VM with cloud-init, this is generally not needed	Reconfigure the IP manually inside the OS after any port change — see the Creating a Windows Server VM guide

Go further

Terraform guide — network attachment pattern (second interface on Ext-Net), section "No floating IP / Neutron router".

Creating a Windows Server VM — manual IP configuration after a change, on a Windows VM.

Official OpenStack CLI documentation on ports: docs.openstack.org — Network v2.

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