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Difficulty: MediumDNS Infrastructure and Record Types

A workstation user attempts to access a web portal at `portal.corp.contoso.com`. The local recursive DNS resolver receives the request with an empty cache. During the lookup, the resolver discovers that `portal.corp.contoso.com` is configured as a CNAME alias pointing to `hosting-node-01.external-provider.net`. What is the correct sequence of steps the recursive DNS resolver takes to resolve the final IP address for the workstation?

  1. 1The recursive resolver queries the authoritative server for `corp.contoso.com` and receives a CNAME record pointing `portal.corp.contoso.com` to `hosting-node-01.external-provider.net`.
  2. 2The recursive resolver extracts the target alias and initiates a new DNS resolution sequence for `hosting-node-01.external-provider.net`.
  3. 3The recursive resolver queries the authoritative server for `external-provider.net` requesting the host A record for `hosting-node-01.external-provider.net`.
  4. 4The authoritative server for `external-provider.net` responds with the IPv4 address (A record) for `hosting-node-01.external-provider.net`.
  5. 5The recursive resolver caches both the intermediate CNAME and final A records, then returns the resolved IPv4 address to the workstation stub resolver.

Answer

The resolution sequence begins by querying the original domain's authoritative DNS server to receive the CNAME alias mapping, extracting the canonical hostname to start a new lookup, querying the canonical domain's authoritative server for its A record, receiving the IPv4 address response, and finally delivering the IP address to the requesting workstation while caching both records.
When resolving a domain configured with a CNAME record, a recursive DNS resolver must follow a multi-stage process. First, it queries the authoritative server for the original domain (`corp.contoso.com`), which returns a CNAME record pointing to an alias target (`hosting-node-01.external-provider.net`). Recognizing that a CNAME provides a canonical hostname rather than an IP address, the resolver starts a new lookup chain for the target hostname. It queries the authoritative server for `external-provider.net` to obtain the A record. Once the target server responds with the IPv4 address, the resolver caches both the intermediate CNAME mapping and the final A record, and finally delivers the IP address back to the workstation stub resolver.

Step-by-Step Solution

1
Query initial domain authoritative server
Received CNAME record mapping portal.corp.contoso.com to hosting-node-01.external-provider.net
The resolver must query the domain specified in the original request to identify its record type and mapping.
2
Process alias and initiate canonical lookup
New name lookup target established for hosting-node-01.external-provider.net
CNAME records do not contain IP addresses; they require an additional resolution path for the target canonical name.
3
Query canonical domain authoritative server
A record request sent to authoritative DNS server for external-provider.net
Authoritative servers for external-provider.net store the actual host IP mapping for hosting-node-01.
4
Receive IPv4 address response
Received A record containing the host IP address
Provides the actionable network layer address required to establish network connections.
5
Cache records and respond to client
Workstation receives the IP address, and resolver stores CNAME and A records in cache
Fulfills the recursive client request and optimizes future name resolution requests via caching.

Key Concept

DNS Recursive Resolution and CNAME Alias Chaining
Estimated Time:1m 30s
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Topics

DNS Infrastructure and Record Types

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