When website traffic increases and a single server can't handle the load, you might consider adding more servers to distribute the pressure. So, how do you distribute user requests across these different servers? The most basic and commonly used method is load balancing via DNS. This method is simple but very practical, and many companies initially used this strategy to cope with traffic growth.

The core idea of ​​DNS load balancing is simple. Typically, one domain name corresponds to one server IP address. DNS load balancing assigns one domain name to multiple IP addresses. When a user accesses your domain, the DNS server selects one of these IP addresses and returns it to the user. The user then accesses the server corresponding to that IP address. By controlling the "selection" rules, you can roughly distribute traffic to different servers. The most common selection rule is round-robin. Let's assume you have three servers with IPs of 1.1.1.1, 2.2.2.2, and 3.3.3.3. When the first user queries the domain name, the DNS returns 1.1.1.1; the second user queries, it returns 2.2.2.2; the third user queries, it returns 3.3.3.3; and the fourth user starts from the beginning again, returning 1.1.1.1, and so on. In this way, probabilistically, traffic is evenly distributed across the three servers.

This method is primarily implemented by manipulating your domain's DNS resolution records. You don't need to install any special software on the servers; you only need to manage your resolution records with your DNS service provider. For example, you originally had an A record pointing `www.yourcompany.com` to `203.0.113.10`. Now you've bought two new servers with IPs `203.0.113.11` and `203.0.113.12`. Then, in your DNS management backend, you add three A records for the same host record `www.yourcompany.com`, pointing to each of the three IP addresses respectively. After saving and applying the changes, the DNS load balancing is essentially in effect. Of course, round-robin is just the most basic method. In a real production environment, you might need more granular control. For example, you might have three servers with different configurations: two new, high-performance machines and one older, backup machine. You want the newer machines to handle more traffic. In this case, weighted round-robin is useful. You can assign higher weights (e.g., weight 2) to the IPs of the two newer machines and lower weights (e.g., weight 1) to the older machine. When the DNS system returns IPs, it will select the high-performance machine more frequently in a 2:2:1 ratio. Most cloud service providers or professional DNS service providers (such as Alibaba Cloud DNS, DNSPod, AWS Route 53, etc.) allow you to intuitively set these weights in their consoles.

Simply distributing traffic isn't enough. What if a server goes down? A smart DNS load balancing solution incorporates a health check mechanism. The DNS service will periodically (e.g., every minute) proactively probe a specific port (e.g., port 80) or URL path of the backend server IPs you've configured to check if the service is functioning correctly. If a server is detected as inaccessible, the DNS will automatically and temporarily remove its IP address from the response list. This ensures subsequent user queries only return healthy server IPs, preventing users from being redirected to inaccessible pages. Once the server recovers and the health check is successful, its IP address will be automatically added back to the list. This feature is crucial for ensuring service availability and is offered by most mainstream commercial DNS services.

Beyond simple round-robin and weighted sorting, more advanced DNS services can perform geolocation-based intelligent resolution. This strategy is particularly suitable for companies with widely distributed operations. For example, if your company has data centers in Hangzhou and Guangzhou, you can configure a rule like this: when a user is from East China (e.g., Shanghai, Zhejiang), the DNS returns the IP address from your Hangzhou data center; when the user is from South China (e.g., Guangdong, Fujian), the DNS returns the IP address from your Guangzhou data center. This allows users to access servers closer to their location, resulting in lower network latency and faster access speeds. This strategy is typically not implemented through the standard DNS protocol itself, but rather as a value-added service provided by DNS providers using their globally distributed resolution nodes and IP geolocation databases.

While DNS load balancing is simple and inexpensive to implement, it has some significant limitations that you should be aware of. The biggest problem is caching. To alleviate the pressure on DNS servers at all levels and speed up resolution, DNS resolution results are cached from the user's local network operator's DNS servers to the user's own computer or mobile phone. The caching time is controlled by the TTL value. If you set the TTL to 10 minutes, then within those 10 minutes, even if the user's server IP has failed and been removed by the DNS service provider, the user with the cached faulty IP will continue to try to access that broken machine until the cache expires. Therefore, when using DNS load balancing, it is usually necessary to set a shorter TTL value, such as 1 to 5 minutes, to speed up failover. However, setting the TTL too short will increase the frequency of DNS queries, putting pressure on the DNS server.

Another problem is that DNS load balancing itself is stateless. It only intervenes in the initial domain name resolution stage. Once the user obtains an IP address and establishes a connection with the server, subsequent sessions occur entirely between the user and that specific server. The DNS system is unaware of whether the server's current connection count or load is too high. Suppose you use a simple round-robin approach, assigning user A to server 1 and user B to server 2. If user A's session is very resource-intensive (e.g., downloading a large file), while user B is simply browsing a webpage, server 1 will be heavily loaded while server 2 will be idle. DNS cannot dynamically adjust for this. This is called load imbalance. Therefore, DNS load balancing is more suitable for stateless, short-connection services, such as ordinary HTTP web browsing. For long-connection, stateful, or server-load-sensitive services, it's usually necessary to combine a reverse proxy load balancer (such as Nginx or HAProxy) or Layer 4/Layer 7 load balancers behind the DNS load balancer for more granular traffic management and session persistence.

In summary, DNS load balancing is an effective and fundamental method for distributing traffic at the network entry point. It's easy to deploy, can be combined with various advanced strategies (such as health checks and region resolution), and is a common first line of defense for building highly available, high-performance website architectures. Understanding its working principles, configuration methods, and applicable boundaries will help you make more appropriate technology choices in actual operations and maintenance.