Improper DNS resolution TTL settings: a hidden risk that website owners must pay attention to.

　　During website maintenance, many website owners focus on server configuration, bandwidth, and CDN acceleration, often overlooking a crucial DNS resolution parameter—TTL (Time To Live). In fact, many website issues such as "unexplained slowdowns," "access problems after switching servers," and "inaccessibility in some regions" can ultimately be traced back to improper TTL settings.

　　I. What is DNS TTL? Why is it so important?

　　TTL, commonly known as "Time to Live," indicates how long DNS resolution results are retained in the local or ISP's cache.

　　For example, when a user accesses your domain for the first time, the local DNS queries the authoritative server for the IP address and caches the result. If the TTL is set to 600 seconds, then for the next 10 minutes, all accesses will directly use the cached result without querying the authoritative DNS again.

　　The essence of TTL is controlling the balance between the speed of DNS updates and the cache hit rate.

　　A TTL that is too short or too long will cause a series of problems.

　　II. Common Misconceptions for Beginners

　　Many website owners believe that a smaller TTL is always better, and a larger TTL saves resources. In reality, both extremes carry risks.

　　TTL is not about "shorter for more flexibility" or "longer for more stability," but rather about dynamically adjusting it based on the business scenario.

　　III. Common Problems Caused by Insufficiently Short TTL Settings

　　1. Increased DNS Query Frequency, Leading to Resolution Failures

　　When TTL is set to 30 seconds or even 10 seconds, it means that almost every user has to re-request DNS. This increases the pressure on authoritative DNS servers, renders ISP caching ineffective, and easily leads to resolution timeouts during peak periods. This is extremely dangerous for websites with moderate traffic.

　　2. Slower First-Time Website Access

　　DNS queries themselves take time. An excessively short TTL leads to a lower cache hit rate, requiring many users to re-resolve domain names, significantly increasing the first-time download time. This manifests as longer loading times for web pages and delayed initial page load. This effect is particularly pronounced when accessing overseas servers or from other regions.

　　3. Frequent Jitter in Multi-Line DNS Scheduling

　　If you are using multi-line DNS, when the TTL is extremely short, users may be rescheduled to different nodes multiple times in a short period. The result is session interruptions, abnormal API requests, and game or real-time service outages. The stability is actually worse than with a high TTL.

　　IV. Excessively Long TTL Settings Also Pose Significant Risks

　　Many beginners, for the sake of "save time," directly set the TTL to 3600, 7200, or even 86400 seconds. While this seems stable, it actually hides greater risks.

　　1. Users Still Accessing Old IPs After Server Switching

　　This is the most typical problem. When you change servers or relocate data centers, everything seems normal in your own tests, but some users still access the old server. The reason is that the ISP's cache has not yet expired. The longer the TTL, the more uncontrollable the switching cycle becomes. In severe cases, it can last for hours or even a day.

　　2. Extremely Slow Fault Recovery

　　If a server goes down, and the TTL is 1 hour, it means that a large number of users will still access the failed node for the next 60 minutes. This is a fatal blow to business continuity.

　　3. Multi-line strategies fail to take effect immediately

　　When you adjust the IP addresses of China Telecom, China Unicom, and China Mobile lines, the old cache prevents the new configuration from taking effect immediately. The behavior varies significantly across different regions, making troubleshooting extremely difficult. Novices often mistakenly believe it's a DNS misconfiguration, when in fact it's simply a TTL issue.

　　V. Chain reactions caused by improper TTL settings

　　An unreasonable TTL is often not a single point of failure, but rather triggers a series of chain reactions. For example, CDN origin pull anomalies, SSL certificate verification failures, API domain name drift, unstable search engine crawling, and occasional 502/504 errors in some regions. These problems may seem chaotic on the surface, but they all stem from uncontrolled DNS caching.

　　VI. Special considerations for TTL in multi-line DNS scenarios

　　If you are using multi-line DNS, you also need to pay attention to the following:

　　1. Keep the TTL consistent across all lines to avoid situations where: China Telecom uses 60 seconds, China Unicom uses 600 seconds, as this will lead to completely different behaviors from different operators.

　　2. The default line must be synchronized with the TTL. The default line is a fallback strategy; if the TTL is inconsistent, some users may experience resolution problems.

　　3. Lower the TTL before switching lines. The standard practice is to lower the TTL 24 hours in advance before migrating or switching. This minimizes cache residue.

　　TTL may seem like a small number in DNS, but it directly affects website access speed, fault recovery capabilities, multi-line performance, and user experience. Many website owners spend money upgrading servers and increasing bandwidth, only to find that incorrect TTL settings prevent them from achieving the desired effect. Truly professional operations and maintenance are often reflected in these details. Use a short TTL during debugging, and a medium TTL after stabilization; never go to extremes. Following this principle will put you ahead of most novice website owners.