Domain Name System (DNS)

DNS Hierarchy

graph TD
    A(Root Level Domain '.') --> B('.com');
    A --> C('.org');
    A --> D('.net');

    B --> E(Second-Level Domain
e.g., 'google.com');
    E --> F(Subdomain
e.g., 'www.google.com');

    C --> G(Second-Level Domain
e.g., 'wikipedia.org');
    G --> H(Subdomain
e.g., 'en.wikipedia.org');

DNS Query Path

sequenceDiagram
    participant User
    participant RecursiveResolver as Recursive Resolver

    box "Internet DNS Infrastructure"
        participant RootServer as Root Server
        participant TLDServer as TLD Server
        participant AuthoritativeServer as Authoritative Server
    end

    User->>RecursiveResolver: What is the IP for www.example.com?

    note over RecursiveResolver: First, I'll check my cache.
Assuming it's not there...

    RecursiveResolver->>RootServer: Query for www.example.com
    RootServer-->>RecursiveResolver: Here is the .com TLD Server address
    RecursiveResolver->>TLDServer: Query for www.example.com
    TLDServer-->>RecursiveResolver: Here is the example.com Authoritative Server address
    RecursiveResolver->>AuthoritativeServer: Query for www.example.com
    AuthoritativeServer-->>RecursiveResolver: The IP is 192.0.2.1

    note over RecursiveResolver: Great! I'll cache this answer
and send it to the user.

    RecursiveResolver-->>User: The IP for www.example.com is 192.0.2.1

Common DNS Record Types

graph TD
    subgraph "DNS Records"
        direction LR
        subgraph "Address Records"
            direction TB
            A[A Record] -->|Maps to| B(IPv4 Address);
            C[AAAA Record] -->|Maps to| D(IPv6 Address);
        end

        subgraph "Routing & Alias Records"
            direction TB
            E[CNAME Record] -->|Alias for| F(Another Domain);
            G[MX Record] -->|Directs mail to| H(Mail Server);
            I[NS Record] -->|Delegates to| J(Authoritative Server);
        end

        subgraph "Data Records"
            direction TB
            K[TXT Record] -->|Contains| L(Text Information);
        end
    end

DNS Caching

flowchart TD
    A(Start: User queries a domain) --> B[Recursive Resolver checks its cache];
    B --> C{Is the record in the cache?};
    C -- No --> D[Perform Full DNS Lookup
to find the IP address];
    D --> E[Store the result in the cache];
    E --> F[Return IP to User];
    C -- Yes --> F;
    F --> G(End: User receives IP);

DNSSEC Chain of Trust

graph LR
    subgraph sg [Signed Chain of Trust]
        direction TB
        A(Root Zone) -- Signs Key of --> B(TLD Zone);
        B -- Signs Key of --> C(Domain Zone);
        C -- Signs --> D(DNS Records);
    end

    E(Recursive Resolver);

    sg -- is validated by --> E;

• TLD: Top-Level Domain
• SLD: Second-Level Domain