Many website owners using .com domains encounter a seemingly simple yet extremely tricky problem: DNS configurations have been correctly added, but they fail to take effect. Whether it's the A record, CNAME record, or the DNS server connection, everything seems fine, yet access still redirects to the old address or displays an "inaccessible" message. This situation is extremely common both domestically and internationally, but the underlying reasons are not limited to "cache not refreshed." More often, it's related to DNS mechanisms, global synchronization, differences in registrar settings, TTL, authoritative DNS status, CDN caching, and even the local network environment.

　　When a .com domain's DNS configuration fails to take effect, the first thing to check is whether the domain is correctly bound to an authoritative DNS service. Many website owners, after changing their DNS registrar, only add records on the registrar platform but fail to synchronize the NS server changes in the domain registrar's backend. For .com domains, the global DNS root servers only trust the NS address recorded by the registrar. If the NS still points to the old platform, the new DNS records added on the new platform will naturally not propagate. Checking this is very simple; you can use the following command:

nslookup -type=ns yourdomain.com

　　If the returned NS address is not equal to the NS address provided by the current DNS provider, the configuration will never take effect, and you need to log in to the domain control panel immediately to manually change it.

　　After the NS configuration is correct, the second common problem is that an excessively high TTL setting causes propagation delays. TTL represents the lifespan of a DNS record in the cache. Some users set the TTL to 3600 seconds or even 86400 seconds for "stability," causing it to take several hours or even a day for a DNS record to refresh globally after a change. For sites that need to frequently switch IPs, such as during server migration, CDN switching, or DDoS protection scheduling, the TTL should not be set too long. Generally recommended:

Record A: 300 seconds
CNAME: 300-600 seconds
MX: 1800 seconds

　　Especially during the testing phase, setting the TTL to 60-120 seconds can significantly improve DNS update speed. Many website owners mistakenly believe that "if the resolution doesn't work, it's a DNS problem," but in reality, it's often because they initially set the TTL too high, causing slow propagation.

　　After users confirm the TTL is normal, if it still doesn't work, it's necessary to check if the DNS resolver's records conform to the standard format. Common errors include mistakenly mixing CNAME and A records, not deleting the existing A record when adding a CNAME, using a CNAME for the root domain that the platform doesn't support, or the target CNAME pointing to another domain that needs further resolution, causing content redirection and delays in effectiveness. Especially when using a CDN, CNAME is the most easily misconfigured part. For example:

　　Correct:

www.example.com CNAME xxx.cdnprovider.net

　　mistake:

example.com CNAME xxx.cdnprovider.net ← 不允许根域名 CNAME

　　If the root domain wants to use a CDN, it needs to use A + CNAME Flattening or the ANAME mechanism provided by its DNS provider; otherwise, the DNS resolution will never take effect.

　　After troubleshooting, if the new IP is still inaccessible, consider the possibility of a local DNS cache. Windows, macOS, and Linux all cache DNS resolution results, and even browsers have independent DNS caches. Simply restarting the browser won't completely refresh the page; you need to execute the following command:

　　Windows：

ipconfig /flushdns

　　macOS：

sudo killall -HUP mDNSResponder

　　Linux：

sudo systemd-resolve --flush-caches

　　Browsers can refresh the page by clearing their internal DNS cache. For example, Chrome's internal operation address is:

chrome://net-internals/#dns

　　After clearing these caches, most websites should return to normal.

　　However, if clearing the local cache still doesn't work, you need to check your ISP, router, and local network environment. Many ISPs have default DNS caching issues, cache hijacking, or even advertising hijacking, causing very slow DNS updates or even returning incorrect results. In this case, the most direct solution is to change the DNS.

　　In some public Wi-Fi or community broadband environments, the router may be hijacked, with its internal DNS pointing to an unknown address, causing all devices to fail to resolve the issue. This type of problem can only be solved by manually changing the router's DNS or using encrypted DNS (DoH, DoT).

　　If the above methods still don't work, the problem may lie with the authoritative DNS itself. Some DNS providers experience delays during global Anycast node synchronization, which may cause some regions to access the site normally while others still point to the old IP. In this case, you generally need to wait for the DNS provider to complete the synchronization or manually switch to a more stable DNS platform. Additionally, when .com domains involve domain transfers, DNSSEC activation failures, or DNS provider system maintenance, temporary ineffectiveness may also occur; you need to check the announcements or notices on the relevant platforms.

　　A common misconception is confusing CDN caching with DNS resolution. When the CDN returns old content, website owners often mistakenly believe that the DNS hasn't been updated, but in reality, the resources cached by the CDN nodes haven't been cleared. Even if the DNS points to a new CDN CNAME, if the CDN edge nodes still retain the old cache, accessing the old data will continue without a refresh. The correct troubleshooting path is to first confirm that the DNS is pointing to the correct CDN CNAME, and then check if the CDN cache has been refreshed.

　　Another misconception is ignoring IPv6 resolution. Some platforms add AAAA records to domains by default. If the server itself isn't configured with IPv6 or is misconfigured, some users will experience access failures due to IPv6 connection issues, mistakenly believing that the DNS is not working. Many website owners are unaware that AAAA records are automatically generated in the background, so they should proactively check the IPv6 configuration. If the server doesn't have IPv6 enabled, clear the AAAA records, keeping only the IPv4 A records to ensure correct access.

　　In summary, DNS configuration failures for .com domains are not always due to problems with the DNS resolution service itself. Multiple factors can cause abnormal results, including: NS not being switched over, excessively high TTL, configuration conflicts, local cache retention, ISP hijacking, IPv6 misconfiguration, and CDN cache misjudgment. The correct troubleshooting steps should proceed from the outside in: authoritative DNS → DNS records → TTL → local environment → ISP → CDN cache, checking sequentially. This approach can quickly reduce troubleshooting costs.