The operation of the internet relies on an invisible “address book” that converts human-readable website names into machine-readable digital addresses—a process known as domain name resolution. Whenever you enter a website address in a browser, a precise, efficient, and globally coordinated series of queries takes place behind the scenes.
The basic concepts and hierarchical structure of domain names
To understand parsing, it is first necessary to understand what a domain name is. A domain name is the name of a computer or a group of computers on the Internet, used to identify their electronic location during data transmission. It is not just a random string of characters; instead, it has a strict hierarchical structure.
The tree-like structure of the Domain Name System (DNS)
The Domain Name System (DNS) uses an inverted tree-like structure, known as the “domain name space.” The root of this tree is the “root domain,” which is represented by a period (.). Below the root domain are the top-level domains, followed by second-level domains, third-level domains, and so on, with each level further subdividing the domain space.
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For example, in the domain name “www.example.com”, when read from right to left: “.com” is the top-level domain, “example” is the second-level domain, and “www” is the hostname (or subdomain). The complete domain name is actually “www.example.com.”; the period at the end (the root domain) is often omitted.
Classification of Top-Level Domains
Top-level domains are mainly divided into two categories: generic top-level domains (gTLDs) and country-code top-level domains (ccTLDs). Generic top-level domains, such as .com, .org, and .net, were initially used to distinguish the type of organization. Country-code top-level domains, such as .cn (China), .uk (United Kingdom), and .jp (Japan), represent specific countries or regions.
In recent years, a large number of new generic top-level domains (TLDs) have emerged, such as .app, .blog, .cloud, etc., greatly expanding the range of domain name options available. This hierarchical structure not only facilitates management but also lays the foundation for subsequent distributed resolution of domain names.
The core components of domain name resolution
Domain name resolution is not performed by a single server; it relies on a distributed system composed of multiple key components working together.
DNS Record Types
DNS records are instructional entries stored on DNS servers, which are used to respond to domain name queries. The most common types of records include:
Record A: Points the domain name to an IPv4 address.
- AAAA record: It points the domain name to an IPv6 address.
CNAME Record: An alias record that points one domain name to another domain name, allowing both domain names to reference the same IP address.
MX Record: A mail exchange record that specifies the server responsible for receiving emails for that domain name.
NS Record: Specifies which DNS server is responsible for resolving the domain name.
TXT record: Typically used to store verification information or SPF (Sender Policy Framework) anti-spam policies.
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Recursive parsers and authoritative servers
The entire parsing process involves two types of core servers: recursive resolvers and authoritative domain name servers.
A recursive resolver is typically provided by your internet service provider or a public DNS service provider. Its role is similar to that of a “helpful librarian”: it receives the user’s query and searches the global DNS system on their behalf to find the desired answer (i.e., the corresponding IP address), before returning it to the user.
Authoritative domain name servers are the “true repositories of knowledge.” They store the official DNS records for specific domains. For example, the authoritative servers for the .com top-level domain know which subordinate authoritative servers are responsible for managing all domain names that end with .com; the authoritative server for example.com, on the other hand, knows exactly what the IP address corresponding to its www subdomain is.
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The complete process of domain name resolution
When you enter “www.example.com” in your browser and press Enter, a journey of information retrieval begins, spanning the entire globe in an instant.
Local queries and recursive processes
First, the computer will check the local cache, including the browser cache and the operating system cache, to see if the domain name has been accessed recently and if any IP address records have been saved. If so, the computer will use the existing information directly, and the resolution process will be completed.
If the required resource is not available locally, the query request is sent to the recursive resolver specified in the local network configuration. The recursive resolver also checks its own cache first. If the cache does not contain the required resource, it initiates a series of iterative queries to retrieve the information.
Iterative Queries and Root Domain Guidance
The recursive resolver first sends queries to 13 global root domain name servers to ask for the IP address of “www.example.com”. The root servers do not provide the answer directly; instead, they provide the addresses of the authoritative servers responsible for the .com top-level domain.
Next, the recursive parser queries the authoritative server for the .com domain. Similarly, the .com server will provide the address of the authoritative server responsible for the “example.com” domain.
Finally, the recursive resolver sends a query to the authoritative server for “example.com”. This time, the authoritative server will search its zone data file to find the A record or CNAME record corresponding to the “www” hostname, and then return the final IP address to the recursive resolver.
The result is returned from the cache.
After receiving the final IP address, the recursive resolver returns this result to the user’s computer. At the same time, it caches this record for a certain period of time. The user’s computer also stores this record in its local cache and then passes the IP address to the browser. The browser then sends an HTTP/HTTPS request to port 80 or 443 of the IP address, and the loading of the web page begins.
Important Technologies and Optimizations in the Parsing Process
To improve parsing speed and ensure security and reliability, the DNS system has introduced a series of important technologies and optimization measures.
DNS Cache Mechanism
Caching is key to the efficient operation of the DNS system. As mentioned earlier, caching is implemented at nearly every stage of the process, from browsers and operating systems to recursive resolvers and authoritative servers at various levels. Each DNS record has a Time To Live (TTL) value, which determines how long the record can be cached on non-authoritative servers. A properly set TTL value helps to strike a balance between data consistency and query efficiency.
DNS Security Extensions
Traditional DNS queries are transmitted in plaintext, making them vulnerable to hijacking and spoofing attacks. DNS Security Extensions (DNSSEC) address this issue by adding digital signatures based on public-key cryptography to DNS data. These signatures enable recursive resolvers to verify whether the received DNS responses come from legitimate authoritative servers and whether the data has been altered during transmission, thereby ensuring the security of the resolution process.
Load balancing and high availability
Simple load balancing can be achieved through DNS. For example, a website can configure multiple A records for its domain name, each pointing to a different server IP address. When responding to queries, the DNS server can return one of these IP addresses in a round-robin manner or using other allocation strategies, distributing user traffic across multiple servers and thereby improving the system’s processing capacity and availability. Additionally, by setting up primary and secondary authoritative DNS servers, it is ensured that even if the primary server fails, the secondary servers can continue to provide resolution services.
summarize
Domain name resolution is a fundamental internet service that converts easy-to-remember domain names into machine IP addresses. It operates efficiently through a distributed, hierarchical global database system, involving the coordination of local caches, recursive resolvers, root servers, and multiple authoritative servers. The resolution process not only aims for millisecond-level speeds but also continuously optimizes reliability, security, and performance through technologies such as DNS caching, DNSSEC (Domain Name System Security Extensions), and load balancing. Understanding the entire chain of events that occurs from entering a website address to actually opening the corresponding web page is an essential part of gaining a deep understanding of the internet’s infrastructure.
FAQ Frequently Asked Questions
修改 DNS 记录后多久生效？
The effective time primarily depends on the TTL (Time To Live) value of the record and the DNS caching at various levels. Theoretically, after the TTL expires, recursive DNS resolvers around the world will refresh their caches and obtain the new record. However, due to the uncontrollable caching on users’ local devices and by their local ISPs, it may take several minutes to up to 48 hours for the changes to take effect globally. It is recommended to make modifications during off-peak business hours and to reduce the TTL value in advance to speed up the refresh process.
What is the difference between public DNS and ISP (Internet Service Provider) DNS?
The ISP’s DNS (Domain Name System) is the resolver automatically assigned by your internet service provider, which typically results in the lowest latency when accessing websites. However, it may come with issues such as advertising insertion or DNS hijacking. Public DNS services, provided by third-party providers like Cloudflare’s 1.1.1.1 or Google’s 8.8.8.8, place a greater emphasis on privacy protection, security filtering, and the stability of resolution speeds, making them a viable alternative to the ISP’s DNS.
Why does the message “DNS resolution failed” sometimes appear when accessing a website?
This usually indicates that the DNS lookup process was interrupted at some point. Possible reasons include: an abnormal local network connection; a failure or unavailability of the specified DNS server; incorrect configuration of the domain name records; a firewall or security software blocking the DNS query request; or the domain name having been suspended for resolution by the registration authority due to expiration or other reasons.
Can CNAME records and A records exist simultaneously?
For the same hostname, it is generally not possible to set both a CNAME record and other types of records at the same time. A CNAME record specifies that the hostname is an alias for another domain name, and all resolutions for that hostname should follow the settings of the target domain name. If other records are also present, it can lead to conflicts and uncertainties in the DNS resolution process.
What's next, what's next?
Extended reading and practical knowledge
The following are related to the topic of this article and are suitable for further in-depth reading. Prioritize starting with the article that is closest to your current problem, and gradually expanding to surrounding topics usually works better.
- Starting from scratch: A step-by-step guide on how to efficiently apply for and configure a personal website domain name
- What is a domain name? A comprehensive guide for beginners to experts, from registration to resolution.
- A detailed explanation of the entire domain name resolution process: from entering a website address to the behind-the-scenes journey of loading the web page
- What is a domain name? A comprehensive explanation of its definition, types, and common questions.
- Domain Name Resolution and DNS Configuration: A Comprehensive Guide from Beginner to Expert
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