DNS is an internet infrastructure primarily used to convert domain names into IP addresses so that users can access websites through their browsers. However, many website administrators and internet users often encounter the problem that, despite successful DNS resolution, the IP address of the website being accessed does not immediately switch to the old IP address and remains at the old one. This phenomenon is often related to DNS caching and TTL.

　　The purpose of DNS caching:

　　The DNS caching mechanism is designed to improve efficiency and reduce frequent DNS queries. After a successful DNS resolution, the result is cached for a period of time. This allows future accesses to the same domain name to retrieve the IP address directly from the cache without having to query the DNS server again. This cache exists in multiple locations, including:

　　Local device cache: DNS resolution results are cached on devices such as users' computers, mobile phones, and browsers.

　　DNS server cache: Local DNS servers cache resolution results for a certain period of time.

　　ISP (Internet Service Provider) DNS cache: Higher-level DNS servers also cache resolution results.

　　The concept of TTL:

　　TTL is a very important parameter in DNS records, indicating how long the DNS resolution result remains valid in the cache. The TTL is measured in seconds. After the TTL expires, the record in the DNS cache is cleared, and the system will re-query the DNS server.

　　For example, if the TTL for a domain name is set to 3600 seconds (or 1 hour), after a successful DNS resolution, the system will cache the result for 1 hour. During this hour, no further DNS queries will be performed unless the TTL expires.

　　Reasons for IP Switching Delays:

　　1. Excessively Long TTL Value

　　The longer the TTL value of a DNS record, the longer the cache life, making it more difficult for DNS resolution results to be updated. If you modify a domain's DNS record (for example, by changing the server's IP address) and the TTL value is set too long, users' devices and DNS servers may continue to use the old IP address until the TTL expires.

　　For example, suppose you change the website's IP address and set the new IP address in DNS, but set the TTL to 24 hours. Even though the DNS record has been updated, most users will still access the old IP address for the next 24 hours until the TTL expires.

　　2. Local Cache Not Refreshed Timely

　　User devices (such as browsers or operating systems) also cache DNS records. This means that even after DNS resolution is complete, some user devices may still cache the old IP address. Even if the domain name's TTL expires, the cache on the user's device may not be refreshed immediately, resulting in a delay in the IP address switch.

　　3. DNS Server Cache Not Updated

　　ISPs or local DNS servers cache DNS resolution results. If these servers' caches are not updated promptly, user devices may continue to access the old IP address. ISPs typically refresh their caches regularly, but if anomalies occur or the DNS record TTL is set too long, the cache may not be updated.

　　4. DNS Record Propagation Delay

　　After a DNS record is updated, the new resolution result needs to be propagated to DNS servers worldwide. This process can take hours or even days, depending on the DNS server configuration and TTL value. Some DNS servers are slow to update their caches, causing users to still be directed to the old IP address.

　　How to resolve DNS switch delays:

　　1. Shorten the TTL value

　　If you plan to change the IP address of your domain, it's best to set the TTL value of your DNS records to a shorter value (for example, 3600 seconds or less) before the change. This allows the modified DNS records to propagate quickly to all DNS servers and devices. Once the switch is complete, you can restore the TTL value to its normal, longer value.

　　2. Clear the local DNS cache

　　Users can manually clear the local DNS cache to ensure their devices are using the latest DNS resolution results. The method for clearing the cache varies slightly depending on the operating system:

　　Windows: Open a command prompt and enter ipconfig /flushdns.

　　Mac OS: Open a terminal and enter sudo killall -HUP mDNSResponder.

　　Linux: Enter sudo systemd-resolve --flush-caches in a terminal.

　　3. Wait for the DNS records to propagate

　　After changing your DNS records and setting the TTL value to a shorter value, wait patiently for the DNS records to propagate globally. Generally, this process can take anywhere from a few hours to a day, depending on several factors.

　　4. Using a DNS Flush Tool

　　You can use third-party tools (such as nslookup or dig) to check whether the DNS records have been successfully updated. These tools can help verify whether any DNS caches have not been updated or whether the old IP address is still being accessed.

　　5. Changing DNS Servers

　　If you find that your local DNS server's cache is not updating promptly, you can try changing DNS servers. For example, you can use a public DNS server for faster and more reliable DNS resolution.

　　FAQs:

　　1. Why do I still see the old IP address after changing my DNS records?

　　This is usually because the DNS cache (including the local device cache and the DNS server cache) has not been updated. Long TTL values, delayed cache refreshes, and DNS propagation delays are the main causes of this problem.

　　2. How can I check whether the DNS records have been updated?

　　You can use tools such as nslookup or dig to query the DNS records for a domain name to see if the new IP address is returned. You can also check DNS server resolution results in different locations to determine whether global propagation has completed.

　　3. How can I speed up DNS record updates?

　　Shortening the TTL value and reducing cache time can speed up DNS record updates. Clearing the local cache and ensuring that the DNS server is refreshed promptly can also help improve update speed.

　　IP address transition delays after DNS resolution are primarily caused by DNS caching and TTL settings. Understanding the role of TTL, DNS caching mechanisms, and the record propagation process can help you better manage DNS record updates. By configuring TTL values ​​appropriately, clearing the cache, and patiently waiting for DNS record propagation, you can reduce IP address transition delays and ensure that accessed IP addresses are updated as quickly as possible.