🌐 06-02: DNS Query Process¶

📌 Introduction¶

The DNS query process is the sequence of steps used to translate a hostname into its corresponding IP address.

The process starts from the user’s local machine. If the answer is not available locally, the request moves through the global DNS hierarchy step by step until the correct IP address is found.

🧭 DNS Query Process (Conceptual Steps)¶

DNS Query

Step 1: Check Local DNS Files¶

Before contacting any DNS server, the user machine checks its local DNS configuration files:

/etc/hosts
Stores manually defined hostname-to-IP mappings
/etc/resolv.conf
Specifies the IP address of the local DNS server

Step 2: Contact the Local DNS Server¶

If the hostname is not resolved locally, the user machine sends the query to the Local DNS Server.

💡 The Local DNS Server is also called: - DNS Resolver - Recursive Resolver

Step 3: Check DNS Cache¶

The local DNS server first checks its DNS cache:

If found → return immediately
If not → continue lookup

Step 4: Query the Root Server¶

If the answer is not cached, the resolver queries a Root Server.

Root server provides the location of the appropriate TLD server

Step 5: Query the TLD Server¶

The resolver queries the Top-Level Domain (TLD) server.

TLD server returns the authoritative nameserver for the domain

Step 6: Query the Authoritative Nameserver¶

The resolver queries the Authoritative Nameserver.

Stores actual DNS records
Returns the final IP address

Step 7: Return the Result¶

Resolver caches the result
IP address is returned to the user machine

🔄 Summary of Conceptual Flow¶

User → Local Files → Resolver → Cache → Root → TLD → Authoritative → IP Address

🔁 Recursive vs Iterative Query¶

🔹 Recursive Query¶

The client asks the resolver for the final answer
The resolver performs all steps (Root → TLD → Authoritative)
The client receives the final IP directly

💡 Used between: - User → Local DNS Server

🔹 Iterative Query¶

Each DNS server returns the best possible answer
It may not be final, but points to the next server

Example flow: - Root → "Ask TLD server" - TLD → "Ask authoritative server"

💡 Used between: - DNS servers (Resolver ↔ Root ↔ TLD ↔ Authoritative)

🧪 DNS Query Process (Example: www.example.net)¶

DNS Query

Step 1: Check `/etc/hosts`¶

The system checks:

/etc/hosts

If found → use IP
Otherwise → continue

Step 2: Check `/etc/resolv.conf`¶

The system checks:

/etc/resolv.conf

Example:

nameserver 10.0.2.3

This gives the local DNS server

Step 3: Query Local DNS Server¶

The system sends:

www.example.net

Resolver checks cache
If not found → continues

Step 4: Query Root Server¶

dig @a.root-servers.net www.example.net

Response:

net.    IN   NS   a.gtld-servers.net.
net.    IN   NS   e.gtld-servers.net.

👉 Root server points to .net TLD servers

Step 5: Query TLD Server¶

dig @a.gtld-servers.net www.example.net