Introduction

A nameserver migration can move the main domain successfully while a delegated subdomain still uses the old DNS provider. The apex zone may resolve correctly from the new platform, but app.example.com, shop.example.com, or another child zone keeps following separate NS records that were never updated during the cutover.

Treat this as a delegated-zone problem instead of a generic propagation delay. Start by checking whether the failing hostname belongs to a separately delegated subdomain, because child-zone authority can remain on the old provider even when the parent domain already points somewhere new.

Symptoms

  • A delegated subdomain still uses the old DNS provider after nameserver migration
  • The main domain works on the new provider, but one subdomain still resolves from legacy DNS
  • Changes made in the new parent zone do not affect the failing subdomain
  • DNS responses for the subdomain show different authoritative nameservers than the apex domain
  • Only services under one delegated child zone still point to the previous environment
  • The issue started after registrar nameserver change or DNS provider migration

Common Causes

  • The subdomain has its own NS delegation that still points to the old provider
  • Teams moved the parent zone but forgot the separately delegated child zone
  • The delegated subdomain is managed in a different account or by a different team
  • Validation focused on apex DNS instead of delegated application or service subdomains
  • Internal documentation assumed the parent zone controlled all records
  • The old child zone remains authoritative even though the parent domain has migrated

Step-by-Step Fix

  1. Identify the exact subdomain that still behaves incorrectly and confirm it is separate from the apex issue, because delegated child zones must be checked individually.
  2. Query the authoritative nameservers for the failing subdomain and compare them with the parent domain, because different NS answers usually reveal a delegation path that survived the migration.
  3. Check whether the subdomain exists as its own zone at the old provider, because child-zone authority often remains in a separate account or service.
  4. Compare the delegated subdomain design with the intended post-migration DNS ownership, because some migrations move the parent zone but intentionally or accidentally leave the child zone behind.
  5. Update the child NS delegation at the actual parent-zone control point only after confirming the destination provider is ready to serve the subdomain, because changing delegation without a live zone creates downtime.
  6. Verify the destination zone contains the required records before or immediately after the delegation change, because the new provider must answer authoritatively once the old child zone is retired.
  7. Retest the subdomain through authoritative lookups and confirm it now resolves from the intended provider, because editing the zone file alone does not prove delegation changed.
  8. Review other delegated subdomains created in the same environment, because DNS migrations often miss more than one child zone.
  9. Document every delegated subdomain and its authority model after recovery, because child NS records are easy to overlook during future nameserver migrations.