====== DNS Explained ======

The **Domain Name System (DNS)** is often described as the "phonebook of the Internet." It translates human-readable domain names (such as `example.com`) into IP addresses (such as `93.184.216.34`) that computers use to communicate.

Without DNS, users would have to remember the IP address of every website they visit.

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===== How DNS Works =====

When you enter a domain name into your browser, the following steps occur:

  - **Resolver** (usually your ISP's DNS server or a public resolver such as Google DNS `8.8.8.8` or Cloudflare DNS `1.1.1.1`) receives the query.
  - The resolver checks whether it already has the answer cached.
  - If not cached, the resolver queries a **Root DNS Server**.
  - The root server directs the resolver to the appropriate **Top-Level Domain (TLD) Server** (such as `.com`, `.org`, or `.ir`).
  - The TLD server tells the resolver which **Authoritative DNS Server** is responsible for the domain.
  - The authoritative server returns the requested DNS record (A, AAAA, MX, TXT, etc.).
  - The resolver caches the answer according to its **TTL (Time To Live)** and returns it to your browser.

<code>
Browser
   │
   ▼
Recursive Resolver
   │
   ▼
Root Server
   │
   ▼
TLD Server
   │
   ▼
Authoritative DNS Server
   │
   ▼
DNS Record
</code>

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===== Common DNS Record Types =====

^ Record ^ Purpose ^ Example ^
| **A** | Maps a hostname to an IPv4 address | `example.com → 93.184.216.34` |
| **AAAA** | Maps a hostname to an IPv6 address | `example.com → 2606:2800:220:1:248:1893:25c8:1946` |
| **CNAME** | Alias for another hostname | `www.example.com → example.com` |
| **MX** | Mail server for a domain | `10 mail.example.com` |
| **TXT** | Text information (SPF, DKIM, DMARC, ownership verification, etc.) | `v=spf1 include:_spf.example.com ~all` |
| **NS** | Authoritative name servers for a zone | `ns1.example.com` |
| **SOA** | Zone configuration and serial number | `ns1.example.com admin.example.com 2026072001` |
| **PTR** | Reverse DNS (IP → hostname) | `93.184.216.34 → example.com` |
| **CAA** | Specifies which Certificate Authorities may issue TLS certificates | `0 issue "letsencrypt.org"` |

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===== Recursive Resolver vs Authoritative Server =====

It is important to distinguish between these two server types.

^ Recursive Resolver ^ Authoritative Server ^
| Receives queries from users | Stores the official DNS records |
| Performs recursive lookups | Answers only for its own domains |
| Caches results | Does not perform recursion |
| Usually operated by ISPs or public DNS providers | Usually operated by the domain owner or DNS provider |

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===== What is an Authoritative DNS Server? =====

An **authoritative DNS server** stores the official DNS records for a domain.

Example:

<code>
example.com
    │
    └── ns1.example.com
            ├── A      → 93.184.216.34
            ├── AAAA   → 2606:...
            ├── MX     → mail.example.com
            ├── TXT    → v=spf1 include:_spf.example.com ~all
            └── CAA    → letsencrypt.org
</code>

Whenever DNS information changes, these servers are the source of truth.

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===== What is TTL? =====

**TTL (Time To Live)** specifies how long a DNS record may be cached before it must be queried again.

Example:

| TTL | Meaning |
| 300 | Cache for 5 minutes |
| 3600 | Cache for 1 hour |
| 86400 | Cache for 24 hours |

Short TTL values allow DNS changes to propagate more quickly but increase the number of DNS queries.

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===== Why do different DNS servers sometimes return different answers? =====

DNS responses may vary because:

  * Different resolvers may still have cached records.
  * CDN providers return different IP addresses based on the user's geographic location.
  * DNS propagation may still be in progress after a recent change.
  * Some providers implement GeoDNS or Anycast routing.

This is why querying multiple DNS servers is useful when troubleshooting DNS issues.

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===== DNS Propagation =====

After changing a DNS record, not all users will see the change immediately.

Propagation time depends on:

  * The TTL of the previous record.
  * Resolver cache expiration.
  * ISP caching policies.

Although changes are published immediately on the authoritative server, recursive resolvers around the world may continue serving cached data until the TTL expires.

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===== Telenegar DNS Check =====

**Telenegar DNS Check** queries DNS records from multiple DNS providers and geographic locations, allowing you to:

  * Compare responses from different DNS servers.
  * Detect DNS propagation issues.
  * Verify GeoDNS and CDN behavior.
  * Query A, AAAA, MX, TXT, NS, SOA, PTR, CNAME, and other DNS record types.

Use our [[https://telenegar.ir/dnscheck/|DNS Check]] tool to investigate DNS resolution from multiple locations.

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{{tag>dns networking dnssec authoritative resolver propagation ttl}}