DNS is a system that translates domain names into IP addresses. It acts like the internet's "phone book," helping browsers find target servers. When DNS fails, users may encounter problems such as inaccessible websites, extremely slow access speeds, inaccessibility in certain regions, and inability to send or receive emails (MX record errors). Simply put, a DNS failure is an abnormality in domain name resolution, preventing users from accessing websites or using related services.
Common causes of DNS failures:
1. Incorrect Domain Name Resolution Configuration
The most common mistake made by novice website owners is improper DNS record configuration, such as incorrect A or CNAME record entries, unreasonable TTL settings (too long a TTL results in slow changes taking effect, while too short a TTL increases DNS query pressure).
2. Domain Name Not Resolved or Not Effective
Newly purchased domain names need a certain amount of time (usually a few minutes to 24 hours) to become effective globally. This can be due to DNS server cache not being refreshed, or the domain name not being bound to the correct DNS server (NS record error).
3. DNS Server Failure
DNS service provider outages or network problems, unstable DNS service on Hong Kong or overseas servers, or abnormal local ISP DNS servers can all lead to resolution failures.
4. Domain Expiration or Locking
Expired domains will have their DNS resolution stopped by the registrar, resulting in the domain being locked or its service suspended.
5. Network Problems or Firewall Blocking
The local network cannot access the DNS server, the server firewall or security policies are blocking DNS requests, or there are abnormal CDN or reverse proxy configurations.
6. Caching or Local Configuration Issues
The browser's DNS cache is not refreshed, the operating system's hosts file has been tampered with, or there are abnormalities in the router or local DNS cache.
Diagnostic Methods for DNS Failures:
Before resolving DNS failures, first identify the source of the problem. Common diagnostic methods are as follows:
1. Use the ping command to test the domain name. If the returned IP address is correct, DNS resolution is normal. If it displays "Unable to resolve hostname," DNS resolution has failed.
2. Use the nslookup or dig command to view information such as the IP address, DNS server, and TTL for domain name resolution. 3. Online DNS testing tools can be used to check DNS resolution status worldwide and determine if there are regional issues.
4. Check local configuration: clear browser cache, clear operating system DNS cache, and check if the hosts file has been accidentally modified.
Troubleshooting DNS problems:
1. Correct DNS records: ensure A and CNAME records point to the correct IP address or domain name, avoid having A and CNAME records for the same subdomain, and set a reasonable TTL (Time To Live), generally 600 seconds or the default value.
2. Switch or upgrade DNS servers: if the registrar's default DNS is failing, switch to a stable third-party DNS. For enterprise websites, use a DNS high-availability solution to avoid single points of failure.
3. Renew and unlock domain names: confirm the domain name has not expired, and check if it has been locked or suspended by the registrar.
4. Network and firewall checks: ensure the server firewall allows DNS requests (port 53), check for DNS hijacking on the local network or by the ISP, and for slow cross-border access, use CDN or Hong Kong CN2 lines for acceleration.
5. Clear cache: Clean operating system and browser caches, restart router, refresh local DNS cache, and wait for TTL to expire so that global DNS updates take effect.
6. Contact your service provider: If the problem cannot be located, contact the technical support of your domain registrar or server provider, providing specific error information such as ping/nslookup results and the time of the failure for quick troubleshooting.
Techniques to prevent DNS failures:
1. Use a stable DNS service provider and configure a secondary backup DNS. If the primary DNS fails, the secondary DNS can take over resolution, ensuring uninterrupted access.
2. Regularly monitor DNS status using monitoring tools (such as Zabbix or Pingdom) to periodically check if domain name resolution is normal.
3. Configure TTL appropriately: Set longer TTLs for stable domains and shorter TTLs for frequently modified records.
4. Manage your domains effectively: remind domains to renew, check lockout status, and ensure accurate WHOIS information.
FAQs
Q: Will DNS failures affect website SEO?
A: Yes. If a website is inaccessible for an extended period, search engine crawlers will be unable to access it, impacting indexing and ranking. It is recommended to resolve the issue promptly.
Q: How long does DNS resolution take to take effect?
A: Generally a few minutes to an hour, but can take up to 24 hours, depending on the TTL and the DNS service provider's caching.
Q: Why can I access the website locally but not from other regions?
A: This could be due to regional DNS caching or ISP hijacking. It is recommended to use a third-party DNS or CDN for acceleration.
Q: Can DNS failures be completely avoided?
A: They cannot be completely avoided, but the risk can be significantly reduced through stable DNS services, multi-node backups, regular monitoring, and proper TTL settings.
Q: Does DNS need to be modified after the server IP changes?
A: Yes. If an A record points to the server IP, the record must be updated when the IP changes; otherwise, access will fail.
DNS failures are common problems in website operation, but most can be resolved. For novice website owners, understanding the causes and diagnostic methods of DNS failures is crucial. By properly configuring DNS, conducting regular monitoring, and employing backup strategies, the risk of failure can be effectively reduced, ensuring stable website access.
