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Alias Records

https://blueprints.launchpad.net/designate/+spec/alias-records

ALIAS records (aka ANAME’s, root CNAME’s, root domain redirect, zone apex CNAME’s) are a relatively new “addition” to DNS. These pseudo Resource Records (RR) are used to bypass restrictions placed within the DNS protocol that forbid the presence of a CNAME RR next to any other RR. This prevents some modern usage of DNS, for example, many CDNs or other static-content hosting sites like GitHub Pages will require users to CNAME e.g. “www.example.org.” towards their frontend load balancers. A CNAME is used to ensure the provider can easily make infrastructure changes without requiring every customer to implement DNS changes in-sync with their deployment schedule.

As this is not a true feature within the DNS protocol, support for ALIAS RR’s must be implemented entirely within the end users authoritative DNS infrastructure.

Problem description

DNS RFCs do not support a CNAME record set for the root domain:

example.com IN CNAME example.github.com

As a result, this capability is not widely supported by DNS backends.

Proposed change

Designate will expose a new “ALIAS” record set type via API; the record set will be flattened on a periodic basis into an “A” or “AAAA” record set for propagation to DNS backends. This solution is backend-independent and complies with RFCs implementations.

Acceptance Criteria

  • An end user can create an “ALIAS” record set in the API and see that record set through all other API requests (e.g. get record sets, export zone, import zone)
  • All “ALIAS” records will be flattened to “A/AAAA” record sets for all backends.
  • An IP change to example.github.com will result in an update the IP address of the flattened “A” or “AAAA” record sets available on the backend.
  • Operators can configure a custom caching name sever to use for resolution and flattening of ALIAS record sets.
  • Operators can set the polling interval for the flattening process.

Note

Not recommended for use with CDN implementations.

API Changes

RRSets with the “visible” value (described below) set to “mdns” will be filtered from view within the API.

Additionally, ALIAS RRSets will not be visible in the V1 API. Instead, a dynamically generated TXT record will be included to indicate the existence of the the ALIAS record that is only editable via the V2 API.

Example ALIAS record set creation request:

POST /v2/zones/2150b1bf-dee2-4221-9d85-11f7886fb15f/recordsets HTTP/1.1
Host: 127.0.0.1:9001
Accept: application/json
Content-Type: application/json

{
  "name" : "example.org.",
  "description" : "This is an example ALIAS record set.",
  "type" : "ALIAS",
  "ttl" : 3600,
  "records" : [
    "example.github.com."
  ]
}

Flatten Task

The process of “flattening” the ALIAS record can be triggered by the user. For example, if a user starts getting error messages the resolution isn’t working, the user can make an API request to force an update.

Example Manual Flatten Request:

POST /v2/zones/<zone_id>/recordsets/<recordset_id>/tasks/flatten HTTP/1.1
Host: 127.0.0.1:9001
Accept: application/json

The request body should be empty. Also, it is recommended the user configure rate limits on this endpoint accordingly. This rate limiting is outside the scope of this spec.

Exporting

When a zone is exported as a zone file, the export should contain two lines, the ALIAS record, commented out, and the most recent A record.

For example:

; example.com. IN ALIAS example.org.
example.com.  IN  A     192.0.2.1

This allows the user to have a valid zone file to import, while providing visibility that the A record was generated from an ALIAS record.

Central Changes

Central’s create and update RecordSet methods will be updated to call a new RPC method implemented on the Zone Manager service to trigger immediate ALIAS flattening when ALIAS RRSets are created / updated. If the resolution fails, the record will be placed in the ERROR status.

Central’s delete RecordSet method will be updated to remove the associated flattened A and AAAA RecordSets - the mechanism for identifying these related RRSets is detailed in Zone Manager Changes.

Additionally, ALIAS RRSets will be treated somewhat similarly to CNAMEs. It will be invalid to place an ALIAS RRSet next to an A or AAAA RecordSet. The recordset placement validation will be updated to handle this case.

MiniDNS Changes

MiniDNS will be updated to display RRSets with the “visible” field (described below) set to ‘mdns’ or ‘all’, ensuring no attempt is made to include non-RFC compliant RRSets within AXFRs.

Zone Manager Changes

First, for each zone it manages, designate-zone-manager service will find any ALIAS recordsets associated with the zone. The zone manager will then issue a successful A/AAAA DNS query to get the IP addresses associated with the ALIAS target. Designate’s database will be updated to the “real” values, incrementing the SOA serial to trigger an AXFR towards the public facing nameservers.

If this query fails and the ALIAS target can’t be resolved, the A/AAAA records will not be updated and the ALIAS RR will be put in STALE status.

The interval at which ALIAS records are flattened will be configurable, defaulting to 1800 seconds (30 minutes).

Finally, a new RPC method will be implemented to trigger immediate ALIAS flattening for a specific Zone, allowing for newly created ALIAS RR’s to be usable before the first periodic interval is hit. This RPC method will also be utilized by the ALIAS RRset flattening task endpoint.

Storage Changes

Recordsets: update column “type”

Add “ALIAS” to “type”.

Recordsets: New Column

Field Type Null Key Default Extra
visible enum(‘all’, ‘api’, ‘mdns’) NO MUL ‘all’  

Sample data

For the example scenario described above, corresponding database values are the following (columns omitted for brevity):

Recordsets Table

id domain_id name tenant_id type visible
1230 768 example.github.com. 391 ALIAS ‘api’
0100 768 example.org. 391 A ‘mdns’

Records Table

id recordset_id data managed managed_resource_type managed_resource_id
3211 1230 example.org. 0 NULL NULL
3212 0100 192.168.1.10 1 ALIAS 3211

Alternatives

Some DNS backends support flattening processes (e.g. PowerDNS). An alternative implementation is to create a new record set type called “ALIAS” that integrates with each respective backend’s implementation.

Implementation

Assignee(s)

Primary assignee:
Eric Larson <eric.larson@rackspace.com>

Milestones

Target Milestone for completion:
Mitaka-1

Work Items

  • Implement support for hiding RRSets in the API and MiniDNS
  • Implement the periodic ALIAS flattening within the designate-zone-manager service
  • Update Central’s CUD RecordSet methods for ALIAS support

Dependencies

None