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Zorluk: ZorTroubleshooting DNS and Name Resolution Services

A network administrator is investigating reports that internal workstations are unable to connect to a web application at `payroll.internal.net`. To diagnose the issue, the administrator executes a hostname lookup using `nslookup` on a client machine, which returns the following output:

text
$ nslookup payroll.internal.net
Server: 10.1.1.5
Address: 10.1.1.5#53

Name: payroll.internal.net
Alias: app-server-04.internal.net

*** 10.1.1.5 can't find app-server-04.internal.net: Non-existent domain (NXDOMAIN)

Which of the following represents the primary root cause of this name resolution failure?

  1. The CNAME record for `payroll.internal.net` points to a canonical target (`app-server-04.internal.net`) that lacks a corresponding A or AAAA record in DNS.Cevap
  2. B
    The client system failed to establish a TCP port 53 handshake, which is mandatory for resolving CNAME alias records.
  3. C
    The DNS server returned an authoritative error because `payroll.internal.net` requires an MX record rather than a CNAME alias.
  4. D
    The `nslookup` utility failed because it only inspects the local HOSTS file when resolving canonical alias addresses.

Cevap

The CNAME record for `payroll.internal.net` points to a canonical target (`app-server-04.internal.net`) that lacks a corresponding A or AAAA record in DNS.
A CNAME (Canonical Name) record acts as an alias pointing to another domain name rather than directly to an IP address. When a client requests `payroll.internal.net`, the DNS server resolves `payroll.internal.net` to `app-server-04.internal.net` and then attempts to resolve `app-server-04.internal.net` to an IP address. Because `app-server-04.internal.net` does not have a valid A/AAAA host record in the zone, the DNS server returns an NXDOMAIN error for the target, causing the entire resolution attempt to fail.

Adım Adım Çözüm

1
Analyze the `nslookup` command output.
The lookup successfully resolved `payroll.internal.net` to its alias target `app-server-04.internal.net`.
This confirms that the CNAME entry for `payroll.internal.net` exists and is functional.
2
Examine the second stage of the DNS resolution chain.
The DNS server returned `Non-existent domain (NXDOMAIN)` when attempting to resolve `app-server-04.internal.net`.
Resolving a CNAME record requires a valid A (IPv4) or AAAA (IPv6) record for the destination canonical hostname. Without an A/AAAA record for `app-server-04.internal.net`, final IP address resolution cannot complete.
3
Identify the corrective action required to fix the issue.
Create an A record for `app-server-04.internal.net` mapping to the server's correct IP address.
Adding the missing host record will allow future CNAME chain resolution to complete successfully.

Anahtar Kavram

DNS CNAME Record Resolution and Chaining Dependencies
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Troubleshooting DNS and Name Resolution Services

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