Detailed Explanation of the DNS Resolution Process

Detailed Explanation of the DNS Resolution Process

Question Description: Please explain in detail the complete process of DNS resolution after you enter a web address (e.g., www.example.com) into your browser and press Enter. Include the components involved, the order of queries, and the different types of DNS queries.

Key Points:
DNS (Domain Name System) is the internet's phone book. It translates human-readable domain names (like www.example.com) into machine-readable IP addresses (like 192.0.2.1). This process is transparent to users but is the first step in network communication.

Detailed Resolution Process:

Step 1: Triggering DNS Resolution and Local Query

  1. Trigger: After entering www.example.com and pressing Enter, the application (browser) first needs to know the IP address corresponding to this domain name to establish a TCP connection. It initiates a DNS resolution request.
  2. Local DNS Cache Query: The operating system does not immediately send a query to an external server. It first checks its own local cache to see if there is a resolution record for that domain name. This cache contains recently resolved domain name results.
    • Browser Cache: Modern browsers have their own DNS caches.
    • Operating System Cache: The operating system (e.g., the hosts file in Windows and the cache in memory) also stores DNS records.
  3. Result: If the IP address corresponding to www.example.com is found in the local cache, the resolution process ends immediately, and the browser uses that IP address. This step is very fast (milliseconds).

Step 2: Recursive Query and the Recursive Resolver

  1. On a Local Cache Miss: If the required record is not in the local cache, the operating system sends the query request to the configured local DNS server (also known as the recursive resolver).
    • The IP address of this server is usually automatically assigned by your Internet Service Provider (ISP) via DHCP, such as 8.8.8.8 (Google Public DNS) or your router's own IP.
  2. Role of the Recursive Resolver: The recursive resolver promises to "recursively" find the final answer for you. It will perform a series of complex queries on behalf of your computer until it obtains the result and then returns it to you. Your computer only needs to ask it once.

Step 3: Iterative Query and the DNS Hierarchy

Now, the responsibility falls on the recursive resolver. It needs to find the answer from the globally distributed DNS system. This system is a hierarchical, tree-like structure.

  1. Query the Root Name Server:

    • The recursive resolver first queries a root name server. There are 13 groups (logically) of root servers globally (labeled A through M).
    • The recursive resolver itself has a pre-configured list of IP addresses for these root servers.
    • It asks a root server: "What is the IP address for www.example.com?"
    • The root server does not give the answer directly. It looks at the last part of the domain name (.com) and replies, "I don't know the address for www.example.com, but I know the address of the server responsible for the .com top-level domain. Go ask it." It returns a list of IP addresses for the Top-Level Domain (TLD) name servers responsible for .com.

  2. Query the Top-Level Domain (TLD) Name Server:

    • The recursive resolver then sends the same query to one of the .com TLD name servers.
    • The TLD name server looks at the next part of the domain name (example.com) and replies, "I don't know the address for www.example.com, but I know the address of the authoritative name server responsible for the example.com domain. Go ask it." It returns a list of IP addresses for the authoritative name servers responsible for example.com.

  3. Query the Authoritative Name Server:

    • Finally, the recursive resolver sends the query to one of the authoritative name servers for example.com.
    • The authoritative name server is the ultimate source of information for the domain, holding complete records for that domain. It searches its records and replies, "The IP address for www.example.com is 93.184.216.34." This answer is the final A Record (Address Record).

Step 4: Result Return and Caching

  1. After the recursive resolver obtains the IP address, it first caches this record for a period of time (determined by the TTL value returned by the authoritative server) so that subsequent identical queries can be responded to quickly.
  2. Then, the recursive resolver returns the final IP address (93.184.216.34) to your operating system.
  3. Your operating system also caches this result before passing it to the browser.

Summary of Query Types:

  • Recursive Query: The query sent by your computer to the recursive resolver. Its characteristic is "you must give me the final answer; you cannot tell me to ask someone else."
  • Iterative Query: The queries sent by the recursive resolver to the root, TLD, and authoritative name servers. Their characteristic is "I ask you, and if you don't know, tell me who to ask next."

Complete Process Diagram:
Your Computer --(Recursive Query)--> Local DNS Server (Recursive Resolver) --(Iterative Query)--> Root Server --(Returns .com server addresses)--> Local DNS Server --(Iterative Query)--> .com TLD Server --(Returns example.com authoritative server addresses)--> Local DNS Server --(Iterative Query)--> example.com Authoritative Server --(Returns IP Address)--> Local DNS Server --(Returns IP Address)--> Your Computer

At this point, the browser has successfully obtained the IP address for www.example.com. It can then initiate an HTTP request, establish a TCP connection, and start loading the webpage. This entire process is usually completed within a few hundred milliseconds.