In domain name resolution management, TTL (Time To Live) is a crucial parameter that determines how long DNS records live on the caching server. While a high TTL can reduce the number of DNS queries and thus lower server load, an excessively high TTL can also lead to delayed resolution updates, especially when domain name records need to be frequently changed or migrated. If your business involves frequent IP changes, CDN node switching, or server migration, a high TTL can cause significant problems, such as users still accessing the old IP, resulting in access failures or pages failing to update. Therefore, properly adjusting the TTL setting is essential to ensure timely DNS resolution.

　　How TTL works:

　　When a DNS server receives a record, it caches it locally, and the cache expires according to the TTL setting. For example, an A record with a TTL of 86400 seconds (24 hours) will have its cached result returned directly by the local DNS server within 24 hours when a user requests a record, without querying the source DNS server again. This reduces the pressure on the source DNS and improves resolution speed. However, when the IP changes, all cached records will still retain the old records until the TTL expires. This is the root cause of "resolution latency." Therefore, businesses that frequently modify DNS records are not suitable for setting excessively high TTL values.

　　How to Adjust the TTL Value?

　　Adjusting TTL requires comprehensive consideration of business characteristics and resolution frequency. If your website or application is deployed on a CDN backend and requires frequent node switching or load balancing, an excessively long TTL will lead to DNS switching latency. Generally, a short TTL value between 300 and 600 seconds is reasonable, allowing record updates to be completed within 5 to 10 minutes, enabling visitors to obtain the latest IP almost in real time without causing long access times to old nodes. For stable and unchanging resources, TTL can be set longer, such as one day (86,400 seconds) or even one week, but these records typically do not involve frequent modifications.

　　In practice, different DNS service providers have slightly different methods for adjusting TTL, but most support direct modification in the management panel or API. For example, in BIND DNS, TTL can be set in the zone file:

$TTL 600
example.com.  IN  A   123.45.67.89
www         IN  CNAME   example.com.

　　In the example above, the default TTL for all records is set to 600 seconds. If any special record requires a shorter or longer TTL, it can be specified individually.

ftp         IN  A   123.45.67.90    ; TTL 300 Second

　　After modifying the TTL, you need to reload the DNS service to ensure the new TTL takes effect.

rndc reload

　　For cloud DNS services, the TTL value can be adjusted directly in the console. Generally, a default TTL option, manual TTL input, and automatic TTL optimization are provided. It's important to note that once the TTL is modified, cached DNS records will not immediately become invalid on recursive DNS servers around the world; they will continue to follow the original TTL until it expires. This means that after a TTL adjustment, it usually takes up to the longest possible time of the original TTL for it to take effect. This is why it's necessary to adjust the TTL before switching IPs or CDN nodes: reducing the TTL to a shorter time first, waiting for the cache to refresh, and then making the change can minimize resolution latency.

　　Besides the TTL setting itself, resolution latency is also affected by the user's local DNS cache. Operating systems and browsers cache DNS resolution results. Even if the recursive DNS server returns a new IP, the old IP will continue to be used if the local cache has not expired. For example, in Linux systems, the local DNS cache can be cleared using the following command:

sudo systemd-resolve --flush-caches

　　Or for macOS:

sudo killall -HUP mDNSResponder

　　Windows systems can use:

ipconfig /flushdns

　　When adjusting TTL, it is recommended to simultaneously clear the local cache to test whether the new DNS record is effective and ensure a smooth service switchover.

　　In actual network operations and maintenance, to avoid DNS resolution latency affecting services, a "TTL degradation strategy" is typically adopted. The steps are as follows: First, adjust the TTL of the target DNS record to a lower value (e.g., 300 seconds) in advance, wait for the existing cache to expire, and allow the global DNS cache to refresh to the new TTL. Then, when it is necessary to modify the IP or CDN node, simply wait for the new TTL to expire for a quick switch. After the switchover is complete, restore the TTL to the original higher value to reduce long-term DNS query pressure. This method is widely used in large websites, cross-border CDNs, load balancing switchovers, and disaster recovery drills, and can minimize access interruptions and user experience degradation.

　　Common misconceptions about TTL adjustment:

　　1. A shorter TTL value is not necessarily better. An excessively short TTL increases the number of DNS queries, leading to increased load on recursive servers and potentially increasing the probability of resolution failures. It is generally recommended that the TTL not be lower than 60 seconds, unless it is a temporary record for high-frequency switching.

　　2. TTL changes do not take effect immediately. As mentioned above, cached DNS records will continue to use the old TTL. Therefore, the TTL must be reduced beforehand before a planned switch.

　　3. TTL only affects the DNS cache's effective period and does not directly improve network speed or reduce latency. Solving slow access issues still requires a combination of network optimization and CDN acceleration.

　　Frequently Asked Questions about TTL Adjustments:

　　Q: Will a shorter TTL increase DNS resolution costs?

　　A: Most cloud DNS services are billed per record. Increasing the number of resolutions usually does not significantly increase costs, but it may incur additional overhead under extremely high traffic.

　　Q: Will adjusting the TTL affect SEO?

　　A: No, search engines focus more on website content and user experience. TTL only affects DNS resolution time.

　　Q: Can the TTL be modified in real time?

　　A: TTL changes take effect immediately in the source DNS, but global cache refresh still requires the original TTL time.

　　Q: Can different record types have different TTLs?

　　A: Yes, for example, 300 seconds for A records and 86400 seconds for MX records, to balance update speed and query pressure.

　　Q: Is it necessary to adjust the TTL every time I change my IP address?

　　A: It is recommended to lower the TTL in advance and restore the original TTL after the switch is complete to ensure smooth access.

　　Summary: Delayed DNS updates caused by excessively high TTL settings are a common problem in DNS management, especially during domain migration, CDN switching, or high-availability deployments. By properly adjusting the TTL, planning switches in advance, clearing local caches, using dig/nslookup to check for updates, and combining CDN and load balancing strategies, DNS latency can be effectively reduced, ensuring stable business operation. TTL adjustment is a basic but crucial DNS optimization measure. Mastering the correct methods can make website access more stable, user experience smoother, and avoid traffic waste and access anomalies caused by DNS latency.