DNS Query Process

The DNS query process involves several steps that work together to resolve a human-readable domain name into an IP address. Here's a detailed look at each step:

1. User Request

• Initiation: When you type a domain name (e.g., www.example.com) into your browser and press Enter, the browser needs to determine the IP address associated with that domain to load the webpage.

2. DNS Resolver

• Local Resolver: Your computer or network router first checks its local DNS cache to see if it already has the IP address for the domain. If the address is found, it is returned immediately.
• Query to Resolver: If the address is not cached, the query is sent to a DNS resolver. This resolver is typically provided by your Internet Service Provider (ISP) or a third-party DNS service.

3. Root DNS Servers

• Initial Query: The resolver queries a root DNS server. The root servers are responsible for directing the resolver to the appropriate top-level domain (TLD) servers.
• Response: The root server responds with a referral to the TLD DNS servers for the domain’s TLD (e.g., .com).

4. TLD DNS Servers

• Query to TLD Servers: The resolver then queries the TLD DNS servers for the domain's TLD.
• Response: The TLD servers respond with a referral to the authoritative DNS servers for the specific domain (e.g., example.com).

5. Authoritative DNS Servers

• Query to Authoritative Servers: The resolver now queries the authoritative DNS servers for the domain. These servers hold the actual DNS records for the domain.
• Response: The authoritative DNS server provides the final answer, which includes the IP address associated with the domain name.

6. Return IP Address

• Final Response: The resolver returns the IP address to your browser. The browser can then use this IP address to request the website’s content from the web server.

7. Caching

• Local Caching: The resolver and your browser cache the DNS result to speed up future requests for the same domain. This caching helps reduce the time needed for subsequent lookups and minimizes the load on DNS servers.
• Time-to-Live (TTL): Each DNS record has a TTL value that specifies how long the record should be cached. After this period, the cached record is considered stale, and the resolver must perform a new DNS query.

Summary of the DNS Query Process

1. User enters domain name.
2. Local DNS resolver checks cache.
3. If not cached, query is sent to root DNS servers.
4. Root DNS servers direct the resolver to TLD DNS servers.
5. TLD DNS servers direct the resolver to authoritative DNS servers.
6. Authoritative DNS servers provide the IP address.
7. The resolver returns the IP address to the browser.
8. The browser uses the IP address to connect to the web server.
9. Results are cached for future use.

Key Points

• Recursive Query: The process described is a recursive query where the DNS resolver performs all the steps to find the IP address and then returns it to the user.
• Iterative Query: In an iterative query, the resolver may return a referral to another DNS server instead of performing the full resolution itself.