Intermittent inaccessibility of a domain name during daily use often perplexes website owners and operators, as the problem is not persistent but rather intermittent. Many people's first reaction is to suspect DNS poisoning, but the actual reasons may be far more complex than imagined. DNS, as the first line of defense for internet access, is susceptible to disruption if any link in the resolution chain falters, leading to website inaccessibility. Determining whether irregular domain name access failures are truly due to DNS poisoning requires a comprehensive analysis from multiple perspectives, including network environment, resolution lines, recursive DNS, TTL settings, and ISP connections.

　　When a user enters a domain name into their browser, the resolution process typically involves the local DNS cache, the ISP's recursive DNS server, the authoritative DNS server, and finally, the returned IP address. If any node in this chain experiences an anomaly, unstable domain name resolution can occur. Intermittent access failures are particularly common in scenarios such as cross-border access, improper CDN acceleration configuration, incorrect DNS settings, and inconsistent resolution lines across regions, and are not necessarily indicative of DNS poisoning. Typical signs of DNS poisoning include forced resolution results pointing to incorrect IPs, intercepted resolutions, replaced records, or forced returns of NXDOMAIN. However, intermittent inaccessibility often manifests as intermittent successful resolutions followed by timeouts, or access being normal in some regions but inaccessible in others. This phenomenon is more akin to unstable resolution link quality than poisoning.

　　From an ISP's perspective, DNS resolution is also affected by network load, outbound bandwidth pressure, and the status of DNS servers in different regions. For example, slower resolution speeds during peak hours, or packet loss and increased latency in the ISP's recursive DNS, can lead to some resolution requests failing. If a user is using the default ISP DNS (such as home broadband), resolution stability is often unreliable, making intermittent timeouts more likely when accessing across provinces or borders. While public DNS offers faster resolution speeds, it may be subject to restrictions in some regions due to routing policies, access links, and international gateways, resulting in lost resolution requests.

　　Another easily overlooked factor is the DNSTTL setting. When the TTL is set too low, the recursive DNS needs to frequently query authoritative servers for new records. Even brief fluctuations in the authoritative DNS can lead to intermittent resolution failures. When the TTL is set too high, DNS changes cannot be synchronized in a timely manner, potentially leading users to access expired IPs and causing access failures. Furthermore, some authoritative DNS services experience momentary packet loss under high concurrency, manifesting as resolution failures in some regions, making the problem appear as if it's been poisoned, when in reality it's simply due to unstable load on the DNS source.

　　If CDN acceleration is used, domain name resolution also involves intelligent scheduling, node health checks, and IP allocation strategies. CDN node anomalies, scheduling center fluctuations, and incorrect DNS line configurations can all cause intermittent access failures. For example, users in mainland China being redirected to overseas nodes, some regions being assigned to high-load nodes, and abnormal scheduling strategies from certain ISPs can all easily cause intermittent webpage inaccessibility. CDN DNS itself is "intelligent resolution," returning different IPs to different regions. Therefore, while some regions resolve normally, others experience timeouts, which doesn't exhibit "pollution" characteristics but is a problem caused by the scheduling mechanism.

　　Of course, DNS poisoning remains a possible cause, especially when accessing specific websites across borders. DNS pollution typically manifests as hijacked DNS responses, abnormal resolution results, or an inability to obtain correct responses from authoritative servers, but it is generally characterized by persistence rather than intermittent occurrence. If your site is consistently accessible in some regions while occasionally failing to resolve in others, the most likely cause is an unstable DNS resolution link. However, if all users experience a sudden inability to resolve, consistently receiving incorrect IP addresses or redirecting to strange sites, then the likelihood of pollution is extremely high.

　　The first step in diagnosing intermittent inaccessibility is to compare the resolution results from different DNS servers. You can use tools like `dig`, `nslookup`, or online DNS testing tools to check for inconsistencies in resolution results, such as whether different regions returning different IP addresses at the same time matches your configuration. If only some regions experience resolution failures, and the resolution doesn't return incorrect IP addresses but only times out or fails to respond, the problem is usually caused by network packet loss, not pollution. Websites accessing cross-border networks also need to rule out international link jitter, especially when accessing overseas servers, where peak-hour latency increases often cause DNS requests to fail.

　　Incorrect DNS resolution link configuration is also a common cause. For example, some DNS service providers support returning different IPs for different regions, such as China Telecom, China Unicom, China Mobile, and overseas. If the line configuration is incomplete or a certain line IP goes down, it will cause DNS resolution to work normally in some regions and fail in others. In some DNS resolution platforms, incomplete record configuration, incorrect priority, and IPv6 configuration conflicts can all cause intermittent access failures, which may seem similar to DNS poisoning but are fundamentally different.

　　Meanwhile, fluctuations in the server's own network can also indirectly lead to DNS resolution failures. If a user's DNS resolution is successful but the server access times out, the user often mistakenly believes it's a DNS problem, but in reality, the resolution is correct; the server simply cannot respond. When troubleshooting, it's recommended to check both ping and dig. If the resolution is successful but ping fails, it's a server network problem, unrelated to DNS. In some cases, network jitter caused by website attacks, server DDoS protection triggering, and bandwidth saturation can all lead to what appears to be "intermittent inaccessibility."

　　While intermittent domain name inaccessibility can indeed originate from DNS, it cannot be simply concluded that DNS is poisoned. True DNS pollution is often noticeable and persistent, while intermittent access failures are more often related to unstable DNS resolution links, recursive DNS problems, abnormal DNS configurations, CDN scheduling errors, and server network failures. Accurately pinpointing the cause requires a multi-pronged approach, including regional detection, DNS resolution link analysis, DNS record comparison, and server network troubleshooting, to avoid misdiagnosis and incorrect handling.

　　Finally, it is recommended to configure a stable authoritative DNS service for the website, properly configure TTL, enable multi-line intelligent resolution, avoid using unstable public DNS, and ensure smooth server network operation. If the website is accessed globally, a stable Anycast DNS service can also be enabled to reduce the risks associated with DNS resolution fluctuations. Only a comprehensive analysis can truly identify and effectively resolve the root cause of intermittent access problems, preventing business disruptions.

　　Frequently Asked Questions:

　　Q: Why is it accessible in some regions but not others?

　　A: This is usually caused by inconsistent DNS lines, uneven load on resolution nodes, abnormal CDN scheduling, or network packet loss, rather than DNS pollution.

　　Q: How can I determine if it's true DNS poisoning?

　　A: If all regions are returning abnormal IPs, being forcibly redirected, or consistently unable to resolve, it's closer to DNS poisoning.

　　Q: Will setting the TTL too high cause intermittent access failures?

　　A: It will cause users to cache old IPs. If the server changes or the IP becomes unstable, access may fail.

　　Q: Will CDN cause unstable DNS resolution?

　　A: Abnormal intelligent scheduling, node failures, or improper line configuration can all cause intermittent access problems.