Alibaba Cloud DNS:Basic DNS concepts

DNS

DNS (Domain Name System) translates domain names into IP addresses for both the Internet and corporate intranets. This process is called domain name resolution. As a distributed database, DNS lets users access resources by name instead of memorizing IP addresses. The DNS protocol runs over UDP on port 53. By network environment, DNS is categorized into public DNS and internal DNS. By functional role, it is categorized into authoritative DNS and recursive DNS.

Domain hierarchy

Domain names follow a hierarchical tree structure, with dot-separated labels forming a unique name for each Internet-connected host or router.

Examples

DNS server hierarchy

The DNS resolution process involves 4 levels of DNS servers, as follows:

DNS resolution process

When you access a website using example.com, DNS resolution proceeds as follows.

1. The user enters example.com in the browser, which queries the local DNS resolver. If the resolver has cached data for example.com, it returns the IP address directly (skip to step 9). Otherwise, it continues to step 2.

2. The local DNS resolver queries the root name server.

3. The root name server returns the address of the .com TLD name server to the local DNS resolver.

4. The local DNS resolver sends a query request for example.com to the .com TLD name server.

5. The .com TLD name server returns the address of the authoritative name server that provides authoritative resolution for example.com to the local DNS resolver.

6. The local DNS resolver sends a query request to the authoritative name server.

7. The authoritative name server returns the IP address corresponding to the domain name example.com to the local DNS resolver.

8. The DNS resolver feeds the IP address into the browser.

9. The Web browser accesses the website server using the IP address.

10. The website server returns the webpage information.

Recursive resolution (Recursive Query)

A single DNS server handles the entire query process and returns the final result directly. For example:

• The client submits "Please find the IP of www.example.com" to the local DNS (recursive server).

• If the recursive server does not know the answer, it will continuously query the root DNS, TLD, and authoritative DNS, following each step until it obtains the final answer.

• The user only needs to wait for the recursive server to provide the final resolution result.

Characteristics

• The user only needs to ask once and wait once.

• The recursive server bears higher load because it completes the entire query.

• This is the most common method between clients and local DNS.

Common applications

• Regular users/PCs/browsers: Only use recursive queries and do not process iterative queries.

• Local DNS servers (such as your broadband carrier, public 8.8.8.8, or company intranet DNS): Usually take on the role of recursive resolution and initiate iterative queries externally (to root, TLD, authoritative DNS).

• Public DNS servers: Recursive for users, iterative for higher-level DNS.

Iterative resolution (Iterative Query)

The DNS server only provides the next referral it knows. If it lacks the answer, it tells you where to ask next, and you query each step yourself. For example:

• The client asks the local DNS: "What is the IP of www.example.com?"

• The local DNS does not know and queries the root DNS. The root DNS does not directly answer with the IP but responds: "You should ask the TLD server." Here, the root DNS is performing iterative resolution.

• The local DNS asks the TLD server, and the TLD tells you "go find this authoritative DNS." Here, the TLD server is performing iterative resolution.

• This continues until the authoritative server is found and the final result is obtained.

Characteristics

• The client needs to ask at each step.

• The server load is relatively low, but the client burden is high (not typically used this way).

Common applications

• Root DNS servers, TLD servers, authoritative DNS servers.

• Self-built DNS servers for enterprises, schools, etc.

• Public DNS servers: Recursive for users, iterative for higher-level DNS.