IPv6 support in Metadata service


Adding IPv6 support for Metadata service.

Problem Description

The metadata service uses the well-known IP as its endpoint address. This doesn’t work in an IPv6-only environment. is not accessible from an IPv6-only Virtual Machine (VM). It is possible to add to a port’s allowed_address_pairs list and use an IPv6 link-local address for interface configuration, but the metadata proxy doesn’t even start if there are no DHCP-enabled IPv4 subnets, and IPv6 link-local addresses are unknown for Neutron and can’t be used for instance identification.

Proposed Change

The metadata proxy starts unconditionally when a network is assigned to a DHCP agent or L3 router.

The metadata proxy listens on a dual-stack socket (::).

In the case of IPv4, the metadata proxy uses IP address which belongs to the IPv4 link-local subnet ( according to [1]). In the case of IPv6, the metadata proxy joins the anycast group fe80::a9fe:a9fe on all available interfaces. The fe80::a9fe:a9fe IP address is equivalent of in the IPv6 link-local subnet which is fe80::/10 according to [2]. It is valid to do this because it is running inside a router or dhcp namespace.

The L3 agent will add a firewall rule that redirects traffic sent to the proposed anycast IP (fe80::a9fe:a9fe) to the metadata proxy port. In the case of the DHCP agent, a new IP address (fe80::a9fe:a9fe) will be configured on the tap port which belongs to the DHCP agent, the same way it is currently done for the IPv4 address (

The VM uses the address fe80::a9fe:a9fe to access Metadata service. Software like cloud-init used inside VMs must be aware of this new IPv6 address. So images used in Openstack clouds will have to be updated as well.

When the metadata proxy processes a request, it gathers the L2 addresses of a VM, and the source interface, and passes it to the metadata service.

The Metadata service, instead of using the VM IP, uses the “VM MAC” and “Gateway MAC” to identify the instance.

Documentation Impact

As this new IPv6 address proposed for metadata service isn’t currently used by any other cloud provider, we need to update our documentation to make it very clear what IPv6 address is used by the metadata service and how to configure it in at least the most popular metadata consumer, which is cloud-init. In the case of cloud-init, this new IP address can be set using the metadata_urls config option [3].

In the future we can update the cloud-init code to make this IPv6 address be one of the default IPs it uses for the OpenStack Datasource, or propose a new datasource, for example, OpenStackIPv6.


[1] https://tools.ietf.org/html/rfc3927 [2] https://tools.ietf.org/html/rfc4291 [3] https://github.com/canonical/cloud-init/blob/4f940bd1f76f50f947af533661ba6fafa3e60e59/doc/rtd/topics/datasources/openstack.rst