In daily internet use, when a website becomes inaccessible, many beginners' first reaction is to clear the DNS cache, assuming the problem stems from "expired or incorrect local DNS records." However, even after clearing the DNS cache, sometimes the website remains inaccessible, which is perplexing. Why does this happen? Is the problem with the computer, router, or server?

　　In reality, DNS resolution is only one link in the website access chain. If inaccessibility persists after clearing the cache, the problem likely lies at multiple other levels, not just the local cache. Understanding the nature of these issues is crucial for troubleshooting and resolving access problems.

　　First, it's important to understand that DNS caching exists at multiple levels. Besides the local operating system cache, there are browser caches, router caches, ISP-provided recursive DNS caches, cloud service or CDN caches, etc. Clearing the local DNS cache only affects the resolution results of the local operating system records; it doesn't immediately refresh the caches of the ISP's recursive servers or remote authoritative DNS servers. If these caches still hold expired or incorrect resolution records, access will still fail or resolve to the wrong IP address. Therefore, clearing the local cache is only one part of the troubleshooting process and cannot guarantee a solution to all access problems.

　　Secondly, website inaccessibility may be related to the authoritative DNS configuration itself. The final DNS resolution result is determined by the authoritative DNS. If the authoritative DNS is misconfigured—for example, A or CNAME records pointing to the wrong IP, inconsistent NS records, or an unreasonable TTL setting—users may still receive incorrect results even after clearing their cache. In this case, it's necessary to check the records on the domain registrar and DNS service provider's end, ensuring all NS records are consistent, A records point to the correct IP, and the TTL is neither too long nor too short to guarantee correct DNS resolution globally.

　　Furthermore, network factors are also a common cause. Even with correct DNS configuration, problems with the user's network path can lead to inaccessibility. For example, cross-border access may fail to connect to the target server due to international link congestion, line fluctuations, or ISP restrictions. Sometimes, the ISP's DNS servers may be poisoned or hijacked, returning incorrect IPs. Even after clearing the local cache, incorrect resolution results will be retrieved again from the poisoned upstream server. In this case, try changing to a different public DNS or using encrypted DNS (DoH/DoT) to bypass ISP poisoning.

　　Finally, server-side issues also need to be considered. Even if DNS resolution is correct, server issues such as downtime, port congestion, firewall restrictions, or excessive load can prevent users from accessing the site. This typically manifests as successful ping or traceroute, but browser access times out or fails. Clearing the DNS cache in this case won't improve access, as the problem lies not in the resolution process, but in the server's responsiveness or network security policies.

　　Another possibility is access problems caused by browser or application-level caching. Some browsers cache DNS or HTTP response results, or applications may use hard-coded IP addresses to access resources. Even clearing the operating system's DNS cache may continue to access incorrect addresses. In this case, try clearing the browser cache, restarting the application, or testing access on different devices to confirm if the problem is related to application caching.

　　Furthermore, for websites using CDNs or cloud services, caching policies can also affect access. If CDN nodes cache old IPs or resources, even if the local DNS is refreshed, expired content may still be accessed, causing loading failures or connection failures. In this case, check the CDN configuration to ensure the caching policy is synchronized with DNS, and manually refresh the CDN node cache if necessary.

　　Finally, incorrect operating system or firewall configurations can also cause access problems. For example, incorrect entries in the hosts file can overwrite DNS resolution results; firewall rules blocking the target IP or port can cause access failure; local security software or router policies can restrict network access. Novices often overlook these local factors during troubleshooting, but they can still cause access problems even after clearing the DNS cache.

　　In summary, if access is still impossible after clearing the DNS cache, possible causes include: multi-level DNS cache not being refreshed, authoritative DNS configuration errors, network line or ISP restrictions, DNS poisoning or hijacking, server-side failure, browser or application caching, CDN node caching, and local operating system or firewall settings. Solving the problem requires a multi-pronged approach: checking DNS configuration and synchronization status, using reliable DNS, investigating network paths, confirming server availability, clearing application cache, and checking CDN configuration and local system settings.

　　FAQs:

　　Q: Why does clearing the DNS cache sometimes not work?

　　A: Because DNS caching exists at multiple levels, including the local system, browser, router, ISP, and CDN. Clearing the local cache only affects local records and cannot immediately refresh other levels of cache.

　　Q: Why are some regions still inaccessible after DNS configuration changes?

　　A: This may be because the TTL cache has not expired, or the ISP DNS has not yet synchronized the new records. You can try lowering the TTL and waiting for synchronization, or test access using public DNS.

　　Q: What are the consequences of DNS pollution?

　　A: It may return incorrect IPs, resolve to inaccessible servers, or even redirect to malicious websites. Clearing the local cache is ineffective; a reliable or encrypted DNS bypass is required.

　　Q: Will server downtime affect DNS?

　　A: Server downtime itself does not affect DNS resolution, but it will cause access failures. Even if the DNS resolves correctly, users will not be able to connect to the server.

　　Q: Is it necessary to clear browser and application caches?

　　A: Yes, it is necessary. Browsers or applications may cache DNS or resources, overwriting local resolution results, leading to access abnormalities.

　　In summary, DNS caching is only one link in the access chain. Clearing the local cache is only one step in troubleshooting and does not guarantee that access problems will be resolved. When beginners encounter access problems, they should systematically check DNS configuration, network, server status, caching policies, and local settings to find the root cause from a holistic perspective in order to truly restore access stability.