Migrating servers is a common process for almost every website. However, many webmasters encounter a frustrating phenomenon after the migration: some users can access the website normally, while others receive messages like "Unable to connect to server," "Page not found," or "DNS resolution error." This "partial user access anomaly" makes it difficult to determine whether it's a server issue or a network issue. In fact, in most cases, this phenomenon is directly related to DNS resolution delays or caching.

　　To understand this issue, you first need to understand how DNS works. DNS converts human-friendly domain names (such as example.com) into computer-readable IP addresses. When a user enters a website address, the system first obtains the corresponding IP address from a DNS server before accessing the content on that server. When a website moves servers, the IP address also changes, and DNS servers around the world take time to synchronize this change. Before synchronization is complete, some users' DNS nodes may still store the old resolution record, causing their requests to be directed to the old server or an invalid address, resulting in access anomalies.

　　This is the fundamental reason why some users can access the website while others can't after migrating. Different users and regions use different DNS servers, and DNS cache update times vary. Most DNS resolution records have a "TTL" value, which specifies how long the resolution result remains in the cache. For example, a TTL of 24 hours means that after a DNS change is made, it may take up to 24 hours for the change to take effect in all regions. This propagation delay can be even more significant for sites with high traffic or global operations.

　　Typically, after a website migration is completed, the new DNS records will gradually take effect within a few hours. However, in some network environments (such as corporate intranets, mobile networks, or ISP-owned DNS nodes), cache updates are slower and may take one or two days to fully refresh. During this period, some users may still access the old server while others access the new one, resulting in "access inconsistency."

　　In addition to global DNS synchronization delays, local DNS cache is also an important factor leading to access anomalies. Operating systems, browsers, and even routers will save temporary records corresponding to domain names and IP addresses to speed up access. When the server IP is changed, if these caches are not updated, the user device will still try to access the old address. The solution is simple - just clear the DNS cache. For example, in Windows, open the command prompt (CMD) and enter:

ipconfig /flushdns

　　After executing, it will prompt "Successfully flushed the DNS Resolver Cache", indicating that the cache has been cleared. Mac users can enter the following in the terminal:

sudo dscacheutil -flushcache; sudo killall -HUP mDNSResponder

　　Restart your browser and try accessing the site again. In most cases, this will resolve the issue.

　　In some cases, access errors may appear to be DNS issues, but are actually related to the CDN cache or load balancing configuration. If your website uses CDN acceleration and the origin server IP address isn't updated after the server migration, the CDN nodes users visit will continue to attempt to connect to the old server, resulting in resource load failures or blank pages. Therefore, when migrating servers, ensure that the origin server configuration in the CDN console is also updated. Also, check that your firewall policies and HTTPS certificate bindings match. Otherwise, even though the new server is online, browsers may refuse to connect due to the certificate or security policy.

　　If you've ruled out CDN issues and users in some regions still have difficulty accessing the site, you can use tools to verify whether DNS latency is the cause. For example, visit dnschecker.org or whatsmydns.net and enter your website's domain name to view resolution results from various nodes around the world. If some regions still point to the old IP address, DNS propagation hasn't fully completed. Don't panic; wait for the TTL (time to live) to expire and the issue will resolve automatically. To avoid prolonged impact, you can adjust the domain name's TTL value to a shorter time (such as 300 or 600 seconds) before migrating. This will allow the DNS change to propagate more quickly.

　　Alternatively, you can verify DNS resolution using the command line. Enter:

nslookup example.com

　　If the returned IP address is still the old server address, the resolution update has not yet completed. Conversely, if the new server IP address is displayed, DNS has taken effect. If users still report access issues, you can ask them to clear their DNS cache or restart their router to re-acquire network parameters. Some carriers' DNS servers update more slowly, so users can temporarily switch to public DNS, which often resolves the issue immediately.

　　Besides DNS caching, another factor can cause access issues after migration: website configuration or firewall restrictions. Some sites forget to open necessary ports (such as 80 or 443) on the server after migration, or security group rules are incorrectly configured, resulting in external requests being rejected. Even though DNS resolution is now correctly pointing to the new server, users may still experience access failures. Therefore, after the migration is complete, ensure that the new server's web services (such as Nginx, Apache, etc.) are started, and that the firewall policy allows HTTP/HTTPS access. Use the following command:

telnet yourdomain.com 80

　　or

telnet yourdomain.com 443

　　Check whether the port is reachable. If the connection fails, check whether the server security group or cloud firewall rules are blocking external requests.

　　For websites with international or global users, geographic DNS distribution must also be considered. Different regions may use different DNS nodes. Even if records are synchronized, access latency may still occur due to varying CDN caching strategies. Therefore, before deployment, it is recommended to use an authoritative DNS service provider and enable smart resolution, which automatically connects users in different regions to the nearest node to improve availability.

　　In summary, in most cases, user access anomalies after a website relocation are indeed related to DNS resolution. DNS updates take time, and synchronization speeds vary across the globe, resulting in temporary differences in user experience. As long as DNS records are configured correctly and caches are cleared promptly, the problem will typically resolve naturally within 24 hours. Website administrators should plan DNS changes in advance, monitor the propagation process, verify resolution status, and ensure the stability of the new server environment. Understanding DNS propagation mechanisms and troubleshooting cache and network issues will ensure a smooth and seamless website migration, one that is imperceptible to users.