Detailed Solutions for DNS Resolution Errors Causing Subdomain Inaccessibility

　　In actual website operation, many website owners encounter this situation: the main domain is accessible normally, but a certain subdomain remains inaccessible or displays a "cannot connect to server" message. The server status is fine, and the program shows no errors. In this case, nine times out of ten, the problem lies in the DNS resolution configuration. For websites that heavily rely on subdomains, this type of problem has a very significant impact. At best, it can cause interface errors and inaccessible backends; at worst, it can paralyze the entire website.

　　What is a subdomain, and why is it prone to problems?

　　Simply put, a subdomain is a new domain name formed by adding one or more prefixes before the main domain. For example, example.com is the main domain, and www.example.com, api.example.com, and cdn.example.com are all subdomains.

　　Many websites split different functions into different subdomains, such as using independent subdomains for the frontend, API, backend, and download site.

　　Subdomains do not automatically take effect; they must be configured with separate DNS records. This is precisely why subdomains are more prone to problems than main domains. For example, forgetting to add a DNS record, incorrect record type configuration, pointing to the wrong IP address, DNS resolution not yet effective, or accidental deletion or overwriting.

　　If any one of these steps fails, the subdomain will become inaccessible.

　　Common Manifestations of Subdomain DNS Resolution Errors

　　When a subdomain resolution fails, the following situations typically occur:

　　1. The browser displays "Unable to resolve this domain" or "DNS_PROBE_FINISHED_NXDOMAIN".

　　2. The subdomain is inaccessible, but the main domain works normally.

　　3. Accessing the server using the IP address works fine, but accessing it using the subdomain fails.

　　4. Different network environments produce different results; some can access it, some cannot.

　　5. Pinging the subdomain returns no response, but pinging the main domain works normally.

　　These phenomena all point to the same core problem: the subdomain has not been correctly resolved to the server.

　　Main Causes of Subdomain DNS Resolution Errors

　　In actual operation and maintenance, the most common causes fall into the following categories. 1. No DNS records were added for the subdomains. Many beginners only configure the main domain, neglecting subdomains like www and api.

　　2. Incorrect record type selection. Using a CNAME record when an A record is needed, or vice versa, will cause access problems.

　　3. Incorrect IP address entered. A typical scenario is when the server changes its IP address without updating the DNS.

　　4. The CNAME record points to a non-existent or incorrect target domain.

　　5. The TTL (Time To Live) is too long; changes take effect for an extended period, appearing "unfixable."

　　6. The domain's NS (Network Name) record has been modified, and the DNS resolution is no longer effective on the current DNS platform.

　　7. Wildcard DNS was mistakenly enabled but incompletely configured, affecting the specified subdomains.

　　Understanding these common causes will provide a clear direction for troubleshooting.

　　How to quickly determine if it's a DNS resolution error?

　　When a subdomain is inaccessible, check in the following order: First, directly access the relevant service using the server's IP address. If the IP address is accessible, the server itself is fine. Then use `nslookup` or `dig` to query the subdomain resolution results and compare them with the IP address in the DNS console. Next, check if the corresponding subdomain record exists in the DNS panel and if its status is normal.

　　Next, confirm that the record type is correct. For example: pointing to the server IP → A record, pointing to another domain → CNAME record.

　　If using a CDN, also confirm that the CNAME record provided by the CDN is entered correctly.

　　Finally, check if the NS server currently used by the domain is the platform configured for your resolution, to avoid the situation of "configured record but not effective".

　　Solutions for DNS resolution errors causing subdomain inaccessibility

　　After confirming the problem, you can handle it step by step as follows:

　　Step 1: Fill in the missing subdomain resolution. If the subdomain record does not exist, simply add one.

　　For example:

　　Enter the subdomain prefix (e.g., www, api) in the host record

　　Select A as the record type

　　Enter the server's public IP address in the record value

　　Save and wait for it to take effect.

　　Step 2: Correct the record type. If the current subdomain points directly to the server IP, use an A record. If it points to a CDN or other domain, use a CNAME record. Do not mix them, otherwise circular resolution or failure may occur.

　　Step 3: Confirm the IP is up-to-date. After reinstalling the system or migrating the instance on a cloud server, the public IP often changes. Be sure to update all subdomain resolution records synchronously.

　　Step 4: Check the CNAME link. When using a CNAME, ensure the target domain actually exists and can be resolved; otherwise, the subdomain will also be inaccessible.

　　Step 5: Lower the TTL and wait for a refresh. Before modifying the resolution, you can lower the TTL (e.g., to 600 seconds) to avoid caching old results for a long time. After modification, patiently wait for DNS refreshes in various locations.

　　Step 6: Check if the NS is correct. Confirm the NS used by the current domain in a WHOIS or DNS lookup tool to ensure it matches the actual configuration platform. If inconsistent, you need to modify it at your registrar.

　　Step 7: Combine with CDN or multi-line resolution. For high-traffic or geographically dispersed sites, it is recommended to use a CDN or intelligent DNS resolution. This improves speed and reduces subdomain unavailability caused by single-point DNS failures.

　　How to Prevent Subdomain Resolution Issues from Recurring

　　Solving a problem once is not difficult; the key is long-term stability. It is recommended to develop the following habits:

　　Create a subdomain list for unified management of all business domains.

　　Update DNS synchronously when server IP changes.

　　Set up monitoring alerts for important subdomains.

　　Use CDN or backup DNS for core business operations.

　　Regularly export DNS configuration backups.

　　Reduce TTL before modifying DNS.

　　These small details can greatly reduce the probability of production incidents.

　　DNS resolution errors are one of the most common causes of subdomain inaccessibility, but it is not complex. As long as you master the core points of "checking if the record exists, if the type is correct, if the IP is up-to-date, and if the NS is consistent," most problems can be quickly resolved. For novice website owners, it is recommended to treat DNS as infrastructure and manage and monitor it properly, rather than waiting for access anomalies to occur before troubleshooting.