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Chapter 26 — DNS

A complete networking lesson based on the course chapter menu.

20 topicsPractical examplesTroubleshooting checksReview Q&A
Estimated reading time0% read

Chapter 26: DNS

This chapter follows the topics shown in the Networking chapter menu. Work through each section in order, then use the review questions to check recall and troubleshooting reasoning.

26.1 DNS Purpose

DNS translates names into resource records such as IP addresses, aliases, mail-routing information, and service data. Client caching and TTL values affect how quickly changes become visible.

Example: a user can reach 203.0.113.20 but cannot reach server.example by name. That difference points toward name resolution, DNS reachability, record content, cache state, or search-suffix behavior rather than basic IP routing.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain DNS Purpose without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.2 DNS Hierarchy

DNS translates names into resource records such as IP addresses, aliases, mail-routing information, and service data. Client caching and TTL values affect how quickly changes become visible.

Example: a user can reach 203.0.113.20 but cannot reach server.example by name. That difference points toward name resolution, DNS reachability, record content, cache state, or search-suffix behavior rather than basic IP routing.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain DNS Hierarchy without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.3 Root Servers

Root Servers is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Root Servers in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain Root Servers without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.4 TLD Servers

TLD Servers is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place TLD Servers in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain TLD Servers without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.5 Authoritative DNS

DNS translates names into resource records such as IP addresses, aliases, mail-routing information, and service data. Client caching and TTL values affect how quickly changes become visible.

Example: a user can reach 203.0.113.20 but cannot reach server.example by name. That difference points toward name resolution, DNS reachability, record content, cache state, or search-suffix behavior rather than basic IP routing.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain Authoritative DNS without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.6 Recursive Resolver

Recursive Resolver is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Recursive Resolver in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain Recursive Resolver without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.7 Forward Lookup

Forward Lookup is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Forward Lookup in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain Forward Lookup without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.8 Reverse Lookup

Reverse Lookup is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place Reverse Lookup in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain Reverse Lookup without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.9 A Record

A Record is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place A Record in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain A Record without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.10 AAAA Record

AAAA Record is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place AAAA Record in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain AAAA Record without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.11 CNAME

CNAME is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place CNAME in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain CNAME without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.12 MX

MX is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place MX in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain MX without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.13 PTR

PTR is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place PTR in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain PTR without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.14 NS

NS is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place NS in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain NS without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.15 TXT

TXT is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place TXT in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain TXT without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.16 SRV

SRV is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

Example: place SRV in a small office network containing clients, switches, a router, wireless access, DNS/DHCP services, and an internet connection. Identify which device or layer owns the function and what evidence you would inspect to verify it.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain SRV without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.17 DNS TTL

DNS translates names into resource records such as IP addresses, aliases, mail-routing information, and service data. Client caching and TTL values affect how quickly changes become visible.

Example: a user can reach 203.0.113.20 but cannot reach server.example by name. That difference points toward name resolution, DNS reachability, record content, cache state, or search-suffix behavior rather than basic IP routing.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain DNS TTL without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.18 DNS Caching

DNS translates names into resource records such as IP addresses, aliases, mail-routing information, and service data. Client caching and TTL values affect how quickly changes become visible.

Example: a user can reach 203.0.113.20 but cannot reach server.example by name. That difference points toward name resolution, DNS reachability, record content, cache state, or search-suffix behavior rather than basic IP routing.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain DNS Caching without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.19 DNSSEC

DNS translates names into resource records such as IP addresses, aliases, mail-routing information, and service data. Client caching and TTL values affect how quickly changes become visible.

Example: a user can reach 203.0.113.20 but cannot reach server.example by name. That difference points toward name resolution, DNS reachability, record content, cache state, or search-suffix behavior rather than basic IP routing.

What to check

  • Identify which OSI/TCP-IP layer and device type are primarily responsible.
  • Check configuration, interface or service state, counters, logs, and a simple end-to-end test.
  • Verify both normal operation and one realistic failure case so you understand what changes when the feature breaks.
Practice: Practice: explain DNSSEC without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

26.20 DNS Troubleshooting

DNS translates names into resource records such as IP addresses, aliases, mail-routing information, and service data. Client caching and TTL values affect how quickly changes become visible.

Example: a user can reach 203.0.113.20 but cannot reach server.example by name. That difference points toward name resolution, DNS reachability, record content, cache state, or search-suffix behavior rather than basic IP routing.

What to check

  • Confirm the symptom and determine whether the problem affects one host, one segment, one site, or many sites.
  • Compare actual configuration and measurements with the intended design, baseline, or documentation.
  • Change one variable at a time, verify the result, and document both the cause and the final fix.
Practice: Practice: explain DNS Troubleshooting without reading the definition. Then draw or describe one network where it is used, name one failure symptom, and list the first two checks you would perform.

Chapter 26 Review Questions

1. What should you remember about DNS Purpose?

Answer: DNS translates names into resource records such as IP addresses, aliases, mail-routing information, and service data. Client caching and TTL values affect how quickly changes become visible.

2. What should you remember about Root Servers?

Answer: Root Servers is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

3. What should you remember about Authoritative DNS?

Answer: DNS translates names into resource records such as IP addresses, aliases, mail-routing information, and service data. Client caching and TTL values affect how quickly changes become visible.

4. What should you remember about Forward Lookup?

Answer: Forward Lookup is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

5. What should you remember about A Record?

Answer: A Record is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

6. What should you remember about MX?

Answer: MX is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

7. What should you remember about NS?

Answer: NS is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

8. What should you remember about SRV?

Answer: SRV is one of the core topics in DNS. Understand what the term represents, where it operates in the network, what information it uses, and what observable behavior confirms that it is working correctly.

9. What should you remember about DNS Caching?

Answer: DNS translates names into resource records such as IP addresses, aliases, mail-routing information, and service data. Client caching and TTL values affect how quickly changes become visible.

10. What should you remember about DNS Troubleshooting?

Answer: DNS translates names into resource records such as IP addresses, aliases, mail-routing information, and service data. Client caching and TTL values affect how quickly changes become visible.