The stability of domain name resolution directly determines the accessibility of websites and services. Once DNS malfunctions, users may still be unable to access the site, regardless of whether the server is functioning normally. Therefore, real-time monitoring and alerting of DNS resolution in production systems is crucial for ensuring stable business operations. However, DNS itself is a distributed, cross-regional system. Resolution status is affected not only by domain name service providers but also by factors such as DNS caching in multiple locations globally and the network environment of ISPs. Therefore, monitoring cannot rely solely on local testing but should employ comprehensive monitoring across multiple nodes, protocols, and methods to identify potential problems.

　　Before building a domain name resolution monitoring system, it's essential to understand the multi-level structure of DNS resolution, including root name servers, TLD servers, authoritative DNS, recursive DNS, and local caching. Each plays a different role, and delays or errors in any link can lead to resolution problems for end users. Therefore, monitoring logic should not only check whether a domain name can be resolved but also whether the resolution is correct, stable, and consistent. For example, a domain name might resolve correctly with Beijing Telecom but encounter errors overseas or on mobile networks; or the resolution result might be correct, but excessive processing time leads to slow user access; or the DNS TTL setting might be too short, causing frequent resolution fluctuations. These problems all require timely detection through real-time monitoring. To achieve more accurate detection, a combination of manual testing and automated tools is typically used, rather than relying solely on a single script or service.

　　Regarding manual testing, the most basic method is to use tools like `dig` or `nslookup` to check if domain name resolution is as expected. `dig` provides more detailed output, making it suitable for troubleshooting complex issues. For example:

dig +trace example.com

　　This command can resolve DNS servers level by level, starting from the root server, and observe whether DNS propagation is normal. If you need to check a specific region or ISP, you can test a specific DNS server.

dig @8.8.8.8 example.com
dig @223.5.5.5 example.com
dig @1.1.1.1 example.com

　　It can also check for hijacking or contamination, for example, by comparing whether the parsing results of different nodes are consistent:

dig AAAA example.com +short
dig A example.com +short

　　If records show drift, abnormal IPs, or inconsistencies, it could be due to CDN misconfiguration, intelligent DNS resolution malfunction, or external interference. Another common monitoring method is to periodically test DNS resolution time, for example:

dig example.com | grep "Query time"

　　If DNS resolution times suddenly surge over a period of time, it's likely due to a DNS service provider malfunction or excessive load. While manual methods are suitable for temporary troubleshooting, real-time monitoring and alerts require specialized tools and automated systems.

　　Many teams use self-built scripts to perform high-frequency DNS resolution checks on multiple domains. For example, they might use a cron job to check DNS results every minute and send anomalies to WeChat, email, or DingTalk. A typical script logic is as follows:

#!/bin/bash
domain="example.com"
expected_ip="1.2.3.4"
result=$(dig +short $domain)

if [[ "$result" != "$expected_ip" ]]; then
    echo "DNS abnormal：$domain return $result" | mail -s "DNS alert" ops@example.com
fi

　　While simple and effective, this method has limitations. For example, it cannot monitor status across multiple regions, nor can it detect latency, hijacking, or propagation. Therefore, it is usually only used as a supplementary method.

　　For a more professional monitoring experience, mature DNS monitoring platforms are available. These typically offer global multi-node detection, intelligent alerts, real-time resolution history recording, and DNS propagation monitoring, making them ideal for online business use. The following are some commonly used tools in the industry:

　　The first type is global node DNS monitoring platforms, such as UptimeRobot, Pingdom, and Uptrends. They can perform DNS queries from multiple countries and ISPs and provide real-time alerts based on the detection frequency. These tools can detect regional resolution failures, such as situations where resolution is abnormal only in certain countries. Due to their multi-point detection, they are well-suited for monitoring domains in intelligent resolution and multi-CDN environments.

　　The second type is professional DNS monitoring and security platforms, such as DNS Spy, Constellix, and NS1 Monitoring. These platforms offer more advanced features, such as DNS propagation monitoring, record change tracking, authoritative DNS response speed analysis, and DNSSEC status checks. For large enterprises, DNS Spy continuously monitors the availability of authoritative DNS servers and provides immediate alerts when records are tampered with, which is crucial for preventing hijacking and supply chain attacks.

　　The third type is automated monitoring tools with APIs. These tools can monitor whether DNS resolution meets expectations through custom rules and automatically trigger failover or IP switching in case of anomalies. For multi-node architectures, automatic switching reduces manual intervention and improves system resilience. They also provide DNS resolution logs, which are very helpful for operational troubleshooting.

　　Besides third-party platforms, building a self-built real-time DNS monitoring system is also a common approach for large-scale businesses. For example, using Prometheus + Blackbox Exporter allows for unified monitoring of domain name resolution, TCP connections, and HTTPS status, visualized through Grafana. An example DNS monitoring configuration is shown below:

modules:
  dns_check:
    prober: dns
    dns:
      query_name: "example.com"

　　Prometheus periodically crawls DNS resolution results and triggers alerts whenever a record changes or resolution fails. This approach is suitable for internal systems, APIs, and microservices, with high controllability and scalability being its main advantages.

　　After establishing a DNS monitoring system, it's also crucial to focus on DNS propagation and caching strategies. A DNS TTL that is too short can cause continuous fluctuations in resolution, while a TTL that is too long can lead to delayed record updates. In high-concurrency applications, the TTL should be adjusted appropriately based on business needs. During migrations and changes to DNS records, DNS propagation checking tools, such as DigWebInterface or Global DNS Checker, should be used to ensure synchronized updates across multiple regions and prevent access splitting issues.

　　Monitoring is not the ultimate goal; establishing an alerting and response mechanism is even more critical. All DNS monitoring tools should be configured with multi-channel alerting, such as email, SMS, WeChat Work, DingTalk, Slack, and Webhooks. When the monitoring system detects increased resolution time, record changes, or resolution failures, it should immediately notify operations personnel for handling. Especially for e-commerce, financial, and portal websites, DNS failures often render the entire site inaccessible. Therefore, establishing minute-level alerts and automated recovery mechanisms is crucial.

　　To ensure the stable operation of the DNS monitoring system, regular self-checks should be performed. These checks include verifying DNS service provider SLAs, clearing expired or invalid records, confirming that DNS resolution is consistent with the business architecture, ensuring correct synchronization of IPv4 and IPv6 records, and checking whether authoritative DNS servers have redundancy mechanisms. For systems using multiple authoritative DNS providers, records need to be synchronized to avoid regional failures caused by inconsistencies.

　　By building a comprehensive DNS monitoring system, website administrators can not only monitor domain name resolution status in real time but also detect signals before failures occur. Employing a global multi-node detection, multi-channel alerting, and a monitoring strategy combining self-built and third-party monitoring can significantly improve website stability.