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📘 1. Basics of DNS

What Is DNS?

The Domain Name System (DNS) is the Internet’s phonebook. It translates human-friendly domain names (like www.example.com) into IP addresses (like 192.0.2.1) that computers use to identify each other on the network.

How DNS Resolution Works

When you enter a URL into your browser, the DNS resolution process unfolds as follows:

1. Recursive Resolver: Your device queries a recursive DNS resolver, typically provided by your ISP or a public DNS service.
2. Root Name Server: The resolver contacts a root name server, which directs it to the appropriate Top-Level Domain (TLD) server (e.g., .com, .org).
3. TLD Name Server: The resolver then queries the TLD server, which provides the authoritative name server for the specific domain.
4. Authoritative Name Server: Finally, the resolver contacts the authoritative name server, which returns the IP address associated with the domain name.
5. Caching: The resolver caches this information for a duration specified by the Time to Live (TTL) value to expedite future requests.

Image Source: ResearchGate

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📘 2. DNS Record Types

DNS records are instructions stored in DNS servers that provide information about a domain. Here are some common types:

Record Type	Purpose	Example
A	Maps a domain to an IPv4 address	example.com → 192.0.2.1
AAAA	Maps a domain to an IPv6 address	example.com → 2001:db8::1
CNAME	Alias of one domain to another	www.example.com → example.com
MX	Mail exchange; directs email to a mail server	example.com → mail.example.com
TXT	Holds text information, often for verification	example.com → "v=spf1 include:_spf.example.com ~all"
SRV	Specifies a port for services like SIP or XMPP	_sip._tcp.example.com → sipserver.example.com:5060
NS	Indicates authoritative name servers for the domain	example.com → ns1.example.com
SOA	Start of authority; provides administrative information about the zone	Contains details like the primary name server, email of the domain admin, domain serial number, etc.

Image Source: ASMED

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📘 3. TTL and DNS Propagation

What Is TTL?

Time to Live (TTL) is a value in a DNS record that tells DNS resolvers how long to cache a query before requesting a new one. It’s measured in seconds.

• High TTL (e.g., 86400 seconds / 24 hours): Reduces load on DNS servers but delays propagation of changes.
• Low TTL (e.g., 300 seconds / 5 minutes): Allows quicker propagation of changes but increases DNS query traffic.

DNS Propagation

When a DNS record is updated, the change doesn’t take effect immediately across the Internet. Instead, it propagates as cached records expire based on their TTL values.

Image Source: ClouDNS

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📘 4. Public vs. Private DNS

Public DNS

• Accessible to anyone on the Internet.
• Used to resolve domain names of public websites.
• Examples: Google Public DNS (8.8.8.8), Cloudflare (1.1.1.1).

Private DNS

• Restricted to a private network.
• Used within organizations to resolve internal hostnames.
• Enhances security by preventing exposure of internal DNS records.

Image Source: NordLayer

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📘 5. Reverse DNS (PTR Records)

What Is Reverse DNS?

Reverse DNS (rDNS) translates an IP address back into a domain name, the opposite of the usual DNS lookup.

PTR Records

• Pointer (PTR) records are used for reverse DNS lookups.
• They map an IP address to a domain name.
• Commonly used in email servers to verify the domain name associated with an IP address, helping to prevent spam.

Certainly! Here’s a comprehensive explanation of how the Domain Name System (DNS) works, accompanied by a detailed diagram to illustrate the process.

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🌐 How DNS Works: Step-by-Step Explanation

The Domain Name System (DNS) functions as the Internet’s phonebook, translating human-readable domain names (like www.example.com) into machine-readable IP addresses (like 192.0.2.1). This translation is essential for locating and accessing websites.

🔄 DNS Resolution Process

When you enter a URL into your browser, the DNS resolution process unfolds as follows:

1. Browser Cache Check: The browser checks its cache to see if it has recently resolved the domain. If found, it uses the cached IP address. (DNS Resolution Process | Cycle.io)
2. Operating System Cache Check: If not in the browser cache, the operating system checks its own cache. (DNS Resolution Process | Cycle.io)
3. Recursive DNS Resolver Query: If the IP address isn’t cached locally, the query is sent to a recursive DNS resolver, typically provided by your Internet Service Provider (ISP). (DNS Resolution Process | Cycle.io)
4. Root Name Server Query: The resolver queries a root name server, which responds with the address of the appropriate Top-Level Domain (TLD) name server (e.g., .com, .org).
5. TLD Name Server Query: The resolver then queries the TLD name server, which responds with the address of the domain’s authoritative name server.
6. Authoritative Name Server Query: The resolver queries the authoritative name server, which responds with the IP address of the requested domain. (How does the Domain Name System (DNS) lookup work?)
7. Response to Client: The resolver returns the IP address to the browser. (DNS Resolution Process | Cycle.io)
8. Website Access: The browser uses the IP address to access the website. (Como funciona o DNS? – ManageEngine Blog)

Throughout this process, each server caches the response for a duration specified by the Time to Live (TTL) value, improving efficiency for future queries.

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🖼️ DNS Resolution Diagram

To visualize this process, here’s a diagram illustrating each step of the DNS resolution:

Image Source: ResearchGate

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Understanding this process is crucial for anyone working with networked systems, as DNS plays a vital role in the functionality of the Internet.

If you have further questions or need more detailed explanations on any part of this process, feel free to ask!