📘 03-05: DNS¶

❓ What Is DNS?¶

DNS (Domain Name System) translates human-friendly names (FQDNs) into IP addresses, so users don't memorize numbers.

Port: UDP 53 (TCP 53 for zone transfers and large responses).
An FQDN (Fully Qualified Domain Name) specifies the full path in the hierarchy: server1.sales.example.com.

📌 The Name Resolution Process¶

Client → Recursive resolver → Root (.) → TLD (.com) → Authoritative (example.com) → answer

Answers are cached along the way (with a TTL) to speed up repeat lookups.

🌐 Common DNS Record Types¶

Record	Maps	Purpose
A	Name → IPv4	Host address
AAAA	Name → IPv6	Host address (quad-A)
CNAME	Name → another name	Alias
MX	Domain → mail server	Email routing (has priority)
NS	Domain → name server	Delegation
PTR	IP → name	Reverse lookup
TXT	Text	SPF/DKIM, verification
SRV	Service location	e.g., SIP, LDAP

🌐 The DNS Hierarchy¶

                . (root)
        /       |        \
      .com     .org     .edu     ← TLDs
       |
   example.com              ← second-level domain
       |
 sales.example.com          ← subdomain

🔍 Troubleshooting¶

Use nslookup or dig to query DNS directly and diagnose resolution problems.

🧭 Scenario: Connectivity Works, Names Fail¶

If a user can ping 8.8.8.8 but cannot browse to example.com, Layer 3 connectivity is probably working and DNS is the first service to test.

Recommended checks:

Confirm the client has the correct DNS server address.
Query the configured resolver: nslookup example.com.
Query a known resolver for comparison: nslookup example.com 8.8.8.8.
Check whether only one record type is failing, such as A, AAAA, or MX.
Consider cache and TTL effects after a DNS change.

Common exam distinction: recursive resolvers do the lookup work for clients, while authoritative DNS servers hold the official records for a domain.

Tip

Key idea — DNS resolves names → IPs over port 53 using a hierarchy (root → TLD → authoritative) with caching. Know the record types, especially A, AAAA, CNAME, MX, PTR.

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