External DNS Resolution¶
|Author:||Carl Baldwin <firstname.lastname@example.org>|
|Copyright:||2015 Hewlett-Packard Development Company, L.P.|
This blueprint builds from where the Internal DNS Resolution blueprint leaves off . That one reconciles the Nova VM name with the hostname assigned by Neutron DHCP and with a DNS name that can be used to look up the host using Neutron’s internal DNS. The next step is to integrate DNSaaS so that instances can be found using their instance names plus a DNS domain name from outside the cloud.
As instances come and go and floating IPs are associated and disassociated, changes may be required in an external DNS service to reflect them. Currently, these changes need to be made independently. This is often a manual process which is prone to error. It can be scripted using an API to the external DNS service but that adds complication for API users and their scripts.
This change will link public IP addresses on Neutron external networks with an external DNS service such as OpenStack Designate. Tenants will expect that any create, update, or delete of an IP address on a network will result in a set of corresponding changes in DNS, covering both A/AAAA and PTR records.
A very important question to consider at this point is whether the A/AAAA record(s) pointed to by the DNS name are associated with an instance or with the public IP itself. This question will be discussed in detail in the Data model impact section.
The change will introduce an interface and a reference implementation of that interface which will integrate with the OpenStack Designate service. This reference implementation will be as easy to configure and use as possible.
This driver will be administratively configured with credentials for the external service in a way similar to how Nova is configured to access the Neutron service for port creation when an instance gets created.
Split Horizon DNS¶
Consider an IPv4 situation where NAT is used and a DNS domain has been associated with a network. One might expect DNS to respond to queries on that domain with the private IP address for VMs that happen to reside on the same network. Similarly, a reverse DNS request for the private IP address would return the VMs DNS name.
The advantage of split horizon DNS is that any machine can use the same names for other machines regardless of whether the machine is on the same network or a different network.
This is the kind of thing that has been accomplished using BIND 9 dns server views. It is typical to disable recursive DNS on the external zone but not on the internal zone.
This blueprint will accomplish this by leveraging the existing dnsmasq implementation for internal resolution and integrate with an external dns service for the external part.
Currently, the domain name is set by Neutron to openstacklocal. To enable split horizon DNS, dnsmasq for a network will be configured with the domain name associated with that network instead of openstacklocal. If no domain is associated then it will default to openstacklocal.
There are some limitations to split horizon DNS.
- If two networks are connected privately by a neutron router then split horizon DNS will not work between them. It will only work for names of instances on the same network.
- It will probably not work for names that come from the the floating IP, as explained in the Data model impact
Data Model Impact¶
+–––––––––––––+ +–––––––––––––+ +–––––––––––––+ +–––––––––––––+ | FloatingIP | | Port | | Network | | DNSDomain | +–––––––––––––+ +–––––––––––––+ +–––––––––––––+ +–––––––––––––+ | ... |––> | ... |––> |... | -->| | | | | | | | | | | dns_domain | | | | | | dns_domain | | dns_name | | dns_name | | | | | +–––––––––––––+ +–––––––––––––+ +–––––––––––––+ +–––––––––––––+ \ | \ V \ +––––––––––––––––––––––+ \ | External DNS service | \ +––––––––––––––––––––––+ +––––––––––––––––––––––––––––––––––––––> | dns_domain | | dns_name | | ... | | | +––––––––––––––––––––––+
The diagram above summarizes the changes that will be made to the data model and how these changes will support the interactions with the external DNS service. Such interactions are triggered when a public IP is created, updated or deleted. Three cases will be supported:
Floating IP with null dns_domain and dns_name:
In this case, DNS names are linked to instances. More precisely, they are linked through the port attached to the instance. When a floating IP association changes, the name and domain used for DNS will come from the backing port's dns_name and the DNSDomain associated to the port's network. The dns_name will match either the Nova display name or the Neutron generated name consistent with the first part of this blueprint.
Floating IP with no null dns_domain and dns_name:
In this case, the dns_name and the dns_domain are linked to the floating IP. When the floating IP association changes there is no need to change anything in DNS. The instance name has no relevance. This covers the use case where a floating IP should always have a given name and domain regardless of which instance it is associated with. The values for dns_name and dns_domain can be overridden for each floating IP. If these values are set on the floating IP then the DNS name comes from the floating IP.
Port with no null dns_name on a public provider network with no null dns_domain:
This is the case where the user is making the ports in a provider network accesible outside the Openstack cloud. Once more, DNS names are linked to instances through their ports attached to the provider network. An instance's name and domain used for DNS will come from its backing port dns_name and the DNSDomain associated to the port's network. The dns_name will match either the Nova display name or the Neutron generated name consistent with the first part of this blueprint.
Multiple networks can be linked to the same DNS domain. This raises the potential concerns over duplicate names. Duplicate entries for a single DNS name are valid part of DNS, and are considered acceptable by this blueprint.
Because how A and AAAA records map can depend on whether the publishing DNS is running split view, it may make sense to publish the IP under both names if they exist.
For PTR records, what will be published is a pointer to the instance dns name under the private IP address and a pointer to the floating ip dns name under the floating IP address.
A DNSDomain object will be added to the data model, with a one to many relationship with the network object. This will allow to associate several networks with a DNS domain name. No relationship with a DNSDomain object means that the network object is not associated with a DNS domain name. There is no need to populate existing network objects on upgrade so the upgrade will not require any special attention.
The dns_domain in a DNSDomain object will be validated as a legal dns domain at the API level. The plugin will also validate the domain with the external service to ensure that it exists and is accessible to the tenant.
In some scenarios, a single DNS domain name may legally exist multiple times in the external service. A driver specific qualifier may be optionally appended to the dns_domain name, and will be separated from the name by a single colon (:) character. For example, assuming the domain example.org exists in the external service with a resource ID of 298c22d1-cebd-475b-b5c9-0f880ee2f23f, the qualified domain name stored by DNSDomain could be “example.org:298c22d1-cebd-475b-b5c9-0f880ee2f23f”. The use of a qualifier must be optional, and only required where a project has visibility of more than one DNS domain with the same name in the external service.
REST API Impact¶
The API will be extended to support CRUD operations on the new object DNSDomain. The create and update operations will enable users to associate dns domains with a list of networks.
The current API to manage floating ips will be extended to add optional dns_domain and dns_name fields.
The Neutron configuration files will contain driver specific configuration for accessing the external service, this will include sensitive information such as usernames/passwords, and will be similar in nature to how Nova is configured with Neutron credentials for port creation.
Other End User Impact¶
This change will need a corresponding change to python-neutronclient to add support for the API changes in REST API impact.
Other Deployer Impact¶
This change is something that has to be explicitly enabled by the deployer. It is optional, the defaults will work for existing or new deployments.
The deployer will be responsible for configuring the driver to interact with the external DNS service. This will be an optional feature and will not require any action on the part of the deployer until enabling the new feature is desired.
This functionality could be handled from Designate. In general, this has the disadvantage of excluding other DNS services.
This could be accomplished by polling Neutron for changes to public IP addresses. Doing this would require tuning the polling interval so that it is often enough that users will not complain about the time it takes for changes to take effect but not so often that it becomes a burden on the performance of the system.
This could also be done by integrating Designate with Neutron RPC so that it picks up on changes to public IP address. This has a disadvantage that it ties Designate to the implementation of Neutron. It would be difficult for designate to understand and account for the relationship between internal ports and floating IP ports. It would also be difficult to control the affinity of a DNS name.
- Extend database models.
- Add dns_name and dns_domain attributes to FloatingIP model.
- Add DNSDomain object.
- Database upgrade script.
- Extend API and validators.
- Add dns_name and dns_domain to FloatingIP.
- Implement CRUD operations for DNSDomain
- Implement interface between Neutron and external DNS service
- Implement driver for Designate.
This change requires the use of the Designate API which is not currently used by Neutron.
Designate has a full DevStack plugin, which can be used as part of integration testing within the gate. There is an open question around if this plugin needs to be included within the devstack tree, or if it can be consumed reliably from the Designate repo.