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Difficulty: HardConfigure Azure DNS Private and Public Zones

An administrator needs to migrate the DNS resolution of an active public domain named `contoso.com` from an external DNS hosting provider to Azure DNS. The migration must occur with zero downtime for users accessing the domain's web services.

Which sequence of steps should the administrator perform to complete the migration successfully?

  1. 1Create the public DNS zone `contoso.com` in Azure DNS.
  2. 2Recreate all existing active DNS resource records (such as A, CNAME, and MX records) within the Azure DNS zone.
  3. 3Update the name server (NS) delegation at the domain registrar to point to the name servers assigned by Azure DNS.
  4. 4Wait for the Time to Live (TTL) duration of the original name server (NS) records to expire.
  5. 5Delete the DNS zone configuration from the external DNS hosting provider.

Answer

Create the public DNS zone in Azure DNS, recreate all resource records, update the name server delegation at the registrar to Azure DNS name servers, wait for the TTL of the original NS records to expire, and finally delete the DNS zone on the external provider.
The correct order follows a zero-downtime migration strategy: first, instantiate the public DNS zone in Azure DNS; second, populate it with all necessary resource records to ensure that the zone is ready to handle queries; third, point the domain registrar's NS records to the new Azure DNS name servers to redirect traffic; fourth, wait for the duration of the original NS TTL to expire so that no cached queries continue to hit the old servers; and fifth, safely decommission the old DNS provider zone once all traffic has moved.

Step-by-Step Solution

1
Create the `contoso.com` zone in Azure DNS.
The zone is initialized in Azure, and four unique name servers are allocated.
You cannot populate resource records or point the registrar delegation to Azure DNS without first having the zone resource created.
2
Add the duplicate resource records (A, MX, etc.) to the new Azure DNS zone.
Azure DNS name servers are fully prepared to resolve queries for the domain.
To ensure zero downtime, the zone must be ready to resolve requests before any traffic is routed to it. Shifting delegation to empty name servers will cause immediate lookup failures.
3
Update the domain registrar delegation with the new name servers.
Traffic begins transitioning from the old DNS servers to the new Azure DNS servers.
This initiates the migration of active queries to the new host. Due to DNS caching, some resolvers will immediately query Azure, while others will continue querying the old host until caches expire.
4
Wait for the TTL of the old NS records to expire.
All recursive DNS resolvers globally clear their cache of the old delegation records and begin querying Azure DNS exclusively.
If the old DNS servers are shut down before cache expiration, queries directed to the cached servers will fail, causing localized downtime.
5
Delete the zone from the external provider.
The migration is successfully completed with no active dependencies remaining on the previous host.
Once the cached TTL has expired, no queries will reach the old provider, allowing it to be safely decommissioned.

Key Concept

Azure DNS Public Zone migration and delegation flow to prevent downtime.
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Topics

Configure Azure DNS Private and Public Zones

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