What is TTL? What is the most appropriate TTL setting for DNS?

　　TTL stands for Time to Live, which is essentially the lifespan of a record. While it might seem abstract at first glance, it's like a "freshness" label on a package. This label, applied to DNS resolution results, specifies how long DNS servers and your computer can store this "addressing route" in a cache.

　　If you think of it as a treasure map, and you ask a friend to save the map to a secret location on their phone, that's the TTL. TTL is the lifespan of a record in the cache, such as 3600 seconds, 86400 seconds, etc.

　　When a friend asks for directions, they'll first check their phone's cache. If it's not there, or the map has expired, they'll ask you for a new one. Therefore, the core of TTL is finding a balance between "query efficiency" and "information freshness." When facing specific business needs, you have to consider whether to increase or decrease your TTL (Time To Live).

　　First, increasing the TTL value provides stability and efficiency, but slows down the process.

　　This is like giving a map a very long shelf life.

　　What are its advantages? Firstly, it reduces the pressure on DNS lookups. Let's say you set your blog's TTL to 86,400 seconds (24 hours). This means that once someone visits your site, all users within the next 24 hours can directly obtain the IP address from their local or ISP's cache, without having to query the authoritative server multiple times. For corporate websites and static resource sites with high traffic but infrequent server changes, a high TTL effectively reduces bandwidth and the load on the authoritative server.

　　Secondly, it offers faster resolution speeds. Users read directly from the cache with almost zero latency, effectively improving page loading speed.

　　What are its limitations? The biggest problem is the slow address switching. When you must change your server IP, a 24-hour TTL means that for the entire next day, users will be cached and directed to the old server. If the old server is no longer in use, users will be unable to access it. Similarly, if a server goes down, users relying on old caches will still be redirected to the downed node, severely impacting business recovery time.

　　Second, lowering the TTL value results in a rapid response, but at a high cost.

　　Now, let's shorten the "shelf life," for example, to 300 seconds (5 minutes).

　　What is its most attractive advantage? The core advantage is rapid change effectiveness. When performing server migrations, A/B testing, or responding to sudden attacks, global user traffic can be redirected to new servers or backup nodes in a short time. For scenarios like e-commerce flash sales, game server launches, or disaster recovery switchovers, a low TTL is a lifesaver.

　　However, we must also acknowledge its disadvantages.

　　Increased DNS resolution overhead: With a very low TTL, users have to re-query each time it expires, significantly increasing the pressure on authoritative DNS servers, and potentially causing resolution timeouts during peak periods.

　　Potentially slows down performance: DNS queries themselves take time. Excessively short TTLs lead to frequent cache invalidation, potentially slowing down initial user access. Even a few milliseconds of latency can accumulate and significantly impact the user experience.

　　Side effects of multi-line strategies: In multi-line DNS, extremely short TTLs can cause frequent changes in intelligent scheduling, session interruptions, or API request anomalies.

　　Since both increasing and decreasing TTLs present different trade-offs, finding the optimal balance is a perennial question. Let's examine this in practical scenarios:

　　Stable main business: For infrastructure such as corporate websites, SaaS service main sites, and API gateways, a range of 3600 seconds (1 hour) to 86400 seconds (24 hours) is recommended.

　　Dynamic or high-availability businesses: For applications like e-commerce promotional pages, social media, game servers, and dynamic scheduling systems, a range of 300 seconds (5 minutes) to 1800 seconds (30 minutes) is recommended.

　　Scenarios using CDN acceleration: Many CDNs require origin servers to have short TTLs for flexible node switching. The recommended range is typically between 300 and 600 seconds (5-10 minutes).

　　Email Service (MX, TXT Records): Email systems have retry mechanisms and are less sensitive to TTL. A range of 3600 seconds (1 hour) to 14400 seconds (4 hours) is recommended, allowing time to resolve email delivery issues while maintaining caching efficiency.

　　Infrastructure and Verification Records: TXT records used for verifying domain ownership or configuring SSL certificates can be set to 600 seconds (10 minutes) or shorter, and adjusted as needed after verification.

　　Additionally, it's worth mentioning the preparatory work before planned changes. If you plan to switch servers next month, reduce the TTL to your lowest acceptable value (e.g., 300 seconds) at least 24-48 hours in advance. Wait for the old global cache to expire, switch entirely to the new IP, and once the new records are confirmed to be working correctly, restore the TTL to its normal value.

　　Finally, regarding TTL adjustments, you might encounter some common pitfalls. I've compiled a list of common misconceptions to help you avoid them.

　　Myth 1: Lower TTL means faster resolution.

　　This is a widely held misconception. In reality, every user accesses the internet and first performs a DNS lookup. If the TTL is too low, the cache expires frequently, forcing the user to repeatedly perform this lookup. Compared to reading directly from the local cache, this process itself adds extra time, easily causing a brief loading screen error on the first page. TTL should strike a balance between "flexibility for change" and "regular resolution efficiency."

　　Myth 2: Using the same TTL for all DNS records.

　　This was a common practice in early operations, but now it seems rather crude. The update frequency and business importance of different record types often vary greatly. A worthwhile practice is: A/AAAA records (core business IPs) can be set to 5-30 minutes for flexible switching; MX records (mail exchange) can be set to 1-4 hours with a retry mechanism as a fallback; and TXT records (verification/configuration) can be set to 10-30 minutes for quick modification. Divide and conquer often leads to better overall management results.

　　Myth 3: Changing TTL takes effect instantly.

　　This is often a misconception. In practice, after you change the TTL, caching servers around the world do not immediately apply the new value. The new TTL value itself needs to wait for the old cache to expire before it can be retrieved. Therefore, the time for the entire network to fully take effect is approximately "old TTL value + new TTL value". Understanding this principle will give you more confidence when developing change plans.

　　Myth 4: TTL can be reduced indefinitely, for example, to 1 second.

　　Technically, some paid DNS services do allow settings down to 1 second. However, in a larger real-world network environment, many public DNS servers have their own caching policies that ignore configurations below 30 seconds. Therefore, to achieve "instant" updates, the difference between 1 second and 30 seconds is often not significant at a macro level.

　　Myth 5: I only need to set it once and don't need to worry about it anymore.

　　This can be considered the root cause of many subsequent business change incidents. TTL (Time To Live) management should be a continuous and dynamic process, aligned with the business lifecycle. Use a higher TTL during stable operation to ensure efficiency; lower the TTL in advance for planned changes to accelerate the switchover; and restore the TTL to normal values ​​after the switchover is complete and stability is verified. This phased strategy of "high TTL during stable periods – low TTL during change periods – high TTL during recovery periods" is an engineering practice worth adhering to long-term.

　　TTL is hidden in a corner of your domain management backend, rarely noticed, but it is actually a crucial parameter in DNS, bridging the gap between different stages. It's not complex, but understanding it can significantly improve your control over website access speed and business changes. Hopefully, these thoughts and breakdowns will help you use this small tool more effectively.