DNS resolution is often overlooked in website optimization and maintenance, yet it's a crucial factor in determining the website's user experience. When a user opens a webpage, their browser first requests domain name resolution from a DNS server to obtain the corresponding IP address. This process may seem simple, but a small parameter—the TTL (time to live)—can directly impact website access speed, stability, and the duration of resolution. Properly setting the TTL can improve DNS resolution efficiency and reduce access latency; however, improperly setting it can lead to slow website updates, access anomalies, and even increase the risk of DNS hijacking.

　　In the DNS system, TTL is a parameter that specifies the duration for which DNS records are cached. Simply put, it defines how long DNS resolution results remain in the cache, typically in seconds. After a DNS server or browser obtains the IP address corresponding to a domain name, it stores it in the cache for a period of time. If a user accesses the same domain name again before the TTL expires, the resolution result will be retrieved directly from the cache, without re-querying the authoritative DNS server.

　　For example, if the TTL is set to 3600 seconds (or one hour), then for one hour after the resolution record is updated, all users using that cache will continue to access the old IP address. Only after the TTL expires will the cache be cleared, and the next access request will require a new resolution result. This mechanism was originally intended to reduce pressure on DNS servers and improve access speed, but it also presents a problem: a TTL that is too long can delay updates after a record modification, while a TTL that is too short can increase the resolution load.

　　Therefore, the key to setting DNS TTL is to strike a balance between speed and stability.

　　A short TTL value means a shorter DNS cache lifespan, allowing users to retrieve the latest resolution records more quickly. This is particularly useful in situations like website migrations, CDN switching, and frequent load balancing policy adjustments. For example, if you're planning to change your server IP address or deploy new smart DNS traffic diversion rules, a short TTL can ensure that resolution changes take effect quickly and reduce access errors. Common short TTL values ​​include 300 seconds (5 minutes) and 600 seconds (10 minutes).

　　However, too short a TTL can also have side effects. Because the cache time is shortened, each user access may trigger a new DNS request, increasing the load on the recursive server and slightly decreasing overall resolution speed. If the website has high traffic, this may also significantly increase DNS query latency and even trigger the DNS service provider's QPS (query per second) limits.

　　Conversely, a longer TTL value can improve access stability and efficiency. Because the resolution results are cached for a longer time, DNS server queries are less frequent, resulting in faster user resolution. This makes it suitable for domain name resolution that remains unchanged for a long time. For example, corporate websites, static content websites, and API services often use a higher TTL, such as 86400 seconds (1 day) or 43200 seconds (12 hours). This setting significantly reduces DNS resolution requests and improves global access consistency.

　　However, when you need to modify records, a higher TTL can delay updates. For example, after a website changes servers or CDN nodes, some users may still access the old address, resulting in "cannot be opened" or "access errors." Therefore, a longer TTL is not necessarily better; it should be appropriately configured based on the actual business scenario.

　　In practice, TTL settings can follow these guidelines:

　　1. Use short TTLs for frequently changing records: For records that may change at any time (such as load balancing portals, test domains, and temporary resolution during migration), a TTL of 300-600 seconds is recommended. This ensures that resolution changes take effect within a few minutes, facilitating testing and migration.

　　2. Use medium- to long-term TTLs for stable services: For fixed servers or long-term primary domain names, such as corporate websites or CDN resolution for main sites, a TTL of 14,400-86,400 seconds (4 hours to 1 day) can be set. This reduces DNS load while ensuring access speeds.

　　3. Dynamic TTLs when combining CDN and Smart DNS: If your website uses CDN or Smart DNS services, you can use the dynamic TTL mechanism provided by the service provider. Some platforms automatically adjust the TTL based on node health, lowering it to speed up migration when nodes are experiencing issues and increasing it to improve performance when they are stable.

　　4. Temporarily reduce the TTL before migration or maintenance: 24 hours before planning to switch IP addresses or modify resolution, you can change the TTL from 1 day to 300 seconds. This way, when the switch is officially completed, the new resolution will take effect globally within minutes, reducing the risk of downtime. After the migration is complete, simply restore the original TTL value.

　　In large website architectures, different types of DNS records should be configured differently. A records (host addresses) and CNAME records (alias records) are typically set to a medium TTL; TXT and MX records (email records) that rarely change can be set to a long TTL; and load balancing entry points or dynamic switching nodes can be set to a short TTL. Furthermore, it's important to note the relationship between TTL and DNS caching mechanisms. DNS caching operates at multiple levels:

• Browser cache: The browser caches the resolution results itself;
• Operating system cache: For example, Windows' DNS cache service;
• Recursive DNS cache: The ISP or public DNS cache;
• Authoritative DNS: Provides the final resolution results.

　　This means that even if you modify the authoritative DNS records, the browser or ISP may still have the old cache, resulting in the resolution not being updated for a short period of time. This phenomenon is particularly noticeable when the TTL is long. Therefore, planning your TTL strategy ahead of time is essential.

　　From a performance and security perspective, TTL can also be used in conjunction with DNSSEC and DoH protocols. DNSSEC ensures that DNS records have not been tampered with, while DoH prevents DNS hijacking. In these encrypted scenarios, appropriately extending the TTL not only reduces query times but also lowers the system overhead of encrypted requests.

　　In summary, the DNS TTL is a simple yet crucial parameter, striking a crucial balance between website access speed, resolution stability, and update efficiency. A well-designed TTL setting ensures that a website remains responsive while also providing sufficient flexibility to adapt to changes. For stable websites, a longer TTL improves performance; for systems undergoing deployment or migration, a shorter TTL allows for quick adjustments and recovery. Mastering TTL setting techniques will not only make DNS resolution smarter and more efficient, but also ensure a stable and reliable website experience globally.