When your computer needs to resolve a domain name (like “google.com”) into an IP address and doesn’t find it in its local cache, it relies on a Recursive DNS Resolver to perform the query. Here’s how that process works:
1. Initiating the Query
• Local Cache Miss: When your computer or device needs to access “google.com” and the IP address is not found in its local cache, it sends a DNS query to the Recursive DNS Resolver.
• Recursive DNS Resolver: This is typically provided by your ISP (Internet Service Provider) or a public DNS service like Google DNS (8.8.8.8) or Cloudflare DNS (1.1.1.1). The resolver’s job is to find the IP address associated with the domain name.
2. Recursive Resolver Queries the Root DNS Server
• What is a Root DNS Server?
• The Internet has 13 logical Root DNS Servers distributed worldwide, each identified by letters (e.g., A, B, C, … M). These servers are operated by different organizations and are highly redundant to ensure reliability.
• The Root DNS Servers are the starting point in translating human-readable domain names into IP addresses. They contain information about where to find the Top-Level Domain (TLD) servers, which handle domains like .com, .org, .net, etc.
• Query to the Root DNS Server:
• The Recursive Resolver sends a query to one of these Root DNS Servers. For “google.com”, the query essentially asks: “Where can I find the DNS servers responsible for .com domains?”
• The Root DNS Server doesn’t know the IP address for “google.com”, but it knows where to find the .com TLD DNS servers, so it responds with the IP address of the TLD DNS server for .com.
3. Query to the TLD DNS Server
• Top-Level Domain (TLD) Servers:
• TLD servers manage the top-level domains, such as .com, .org, .net, and country-specific domains like .uk, .de, etc.
• These servers know where to find the Authoritative DNS Servers for domains within their TLD. For example, the .com TLD server knows how to find the DNS servers responsible for “google.com”.
• Query to the TLD DNS Server:
• The Recursive Resolver now sends a query to the .com TLD DNS server, asking, “Where can I find the DNS servers that are authoritative for google.com?”
• The TLD DNS server doesn’t know the IP address for “google.com” itself but knows which DNS servers are authoritative for the “google.com” domain. It responds with the IP address of these Authoritative DNS Servers.
4. Query to the Authoritative DNS Server
• Authoritative DNS Servers:
• These servers hold the definitive records for specific domain names. They contain the actual DNS records, such as A records (which map a domain to an IP address), MX records (which specify mail servers), and others.
• For “google.com”, the Authoritative DNS Servers are operated by Google itself. They know the exact IP addresses associated with various subdomains and services under “google.com”.
• Final Query to Authoritative DNS Server:
• The Recursive Resolver now contacts one of the Authoritative DNS Servers for “google.com”. It asks, “What is the IP address for google.com?”
• The Authoritative DNS Server responds with the IP address of “google.com” (e.g., 142.250.190.46).
5. Response and Caching
• Receiving the IP Address:
• The Recursive Resolver receives the IP address from the Authoritative DNS Server and passes it back to your computer.
• Caching the Response:
• Recursive Resolver Cache: The Recursive DNS Resolver caches the IP address along with the domain name for a certain amount of time, defined by the TTL (Time to Live) value in the DNS record. This means that if another query for “google.com” comes in within the TTL period, the resolver can respond immediately from its cache, without going through the entire process again.
• Local Cache: Your computer also caches the IP address for the domain, so if you need to visit “google.com” again soon, it doesn’t need to query the Recursive Resolver.
Additional Considerations
• TTL (Time to Live):
• The TTL value in a DNS record dictates how long a DNS resolver or client can cache the record. Once the TTL expires, the resolver must query the authoritative source again to ensure the information is still accurate.
• Load Balancing:
• Often, large services like Google have multiple IP addresses for load balancing and redundancy. The Authoritative DNS Server might return different IP addresses in different queries to distribute traffic across various servers.
• Redundancy and Fault Tolerance:
• The DNS infrastructure is highly redundant. Multiple Authoritative DNS Servers exist for each domain to ensure that if one server is unreachable, others can provide the required information. Similarly, the 13 Root DNS Servers are distributed globally with additional mirrors to handle requests efficiently.
• Recursive vs. Iterative Queries:
• The process described above involves recursive queries where the Recursive Resolver does all the work on behalf of the client. However, DNS also supports iterative queries, where each step would be handled by the client, but this is less common in typical scenarios.
• DNSSEC (DNS Security Extensions):
• DNSSEC adds a layer of security to DNS by enabling DNS responses to be cryptographically signed. This helps protect against certain types of attacks, such as cache poisoning, by ensuring that the response received is authentic and has not been tampered with.