When you try to access a website but can't open it, or when others say they can't connect even though the server is online, technicians often say, "Check the DNS." DNS testing might sound like a black box, but it's actually a complete inspection process designed to verify that each component of the domain name resolution system is working properly. Like checking for a leaky pipe, you need to check each section one by one to find the problem.

DNS testing starts with the most basic resolution function. This is equivalent to asking, "Can this domain name be translated into an IP address?" The most direct tools are `nslookup` or `dig`. In the command line, type:

nslookup example.com

The system will return the IP address corresponding to this domain name. If it returns a "non-authoritative response" and displays the IP address, it means the basic resolution is normal; if it displays "not found," it means it simply cannot be resolved. But this is just the beginning, because DNS resolution involves multiple components: local caching, recursive resolvers, authoritative name servers, etc. A more detailed check involves using the `dig` command to trace the entire DNS resolution process: `dig +trace example.com`. This displays the complete path from the root name server (.), through the top-level domain server (.com), to the authoritative server hosting the domain, helping you pinpoint where the resolution failed.

After basic DNS resolution is successful, you need to test its consistency. This addresses the question, "Why do different people see different websites?" Because there are thousands of DNS servers on the internet, caching strategies and network optimizations can cause the same domain name to resolve to different IPs in different regions and networks. You can use online DNS lookup tools (such as dnsspy.io) or write your own script to query the DNS resolution results of the same domain name from multiple monitoring points worldwide. Compare these results; if some return US server IPs and others return Hong Kong server IPs, this may be normal CDN scheduling; however, if completely unrelated or even incorrect IPs are returned, you may have encountered DNS hijacking or poisoning. A simple way to check if local DNS resolution has been tampered with is to use `dig @8.8.8.8 example.com` to specify the use of Google's public DNS for the query, and then compare it with the local resolution results. Next, we'll conduct specialized tests on DNS record types. The Domain Name System (DNS) doesn't just convert domain names into IP addresses (A records); it also manages mail routing (MX records), aliases (CNAME records), text descriptions (TXT records), and more. Emails failing to send? This might be due to incorrect MX record settings. You can use `dig MX example.com` to query the mail server's priority and address. Is your website using encryption? You need to use `dig TXT example.com` to check if the correct SPF, DKIM, and DMARC records are configured to prevent email spoofing. For modern websites, you also need to test IPv6 support (AAAA records) and DNS security extensions (DNSSEC).

Performance testing is a crucial part of DNS testing, as it affects website loading speed. We primarily test two metrics: response time and TTL policy. When using `dig example.com`, pay attention to the "Query time" at the end of the output; this is the response time from your local machine to the DNS server. A time exceeding 200 milliseconds can slow down the user experience. However, a single measurement is inaccurate. You can use `for i in {1..10}; do dig example.com | grep "Query time"; done` to perform 10 consecutive measurements and take the average. A more professional tool is `dnsperf`, which can simulate high-concurrency queries and test the performance of a DNS server under pressure. The TTL (Time to Live) value determines how long the results can be cached; too short a value increases the query burden, while too long a value results in untimely updates. Check the TTL using `dig example.com | grep TTL`.

Security testing is becoming increasingly critical. DNS hijacking and poisoning attacks can silently redirect users to phishing websites. A basic detection method is to compare the results returned by the authoritative DNS and the local DNS to see if they are consistent. DNSSEC can prevent this type of tampering; you can use online verification tools or the `dig` command to check if the DNSSEC signature chain of a domain name is complete. Another common problem is DNS amplification attacks, where attackers use misconfigured DNS servers to launch DDoS attacks. You can use `dig +short test.openresolver.com TXT` to check if your DNS server has recursion enabled. If it returns a result, it means your server may be being exploited.

For domains you manage, you also need to test the correctness and redundancy of the configuration. Check for any missing record types and ensure that `www.example.com` and `example.com` can both resolve (the latter requires an A record or URL forwarding). Use `dig NS example.com` to check the authoritative name server settings; there should be at least 2-3 servers distributed across different networks to prevent single points of failure. If you are using a third-party DNS service (such as Cloudflare), you also need to test whether the configuration synchronization of its API or control panel takes effect promptly.

In actual operation and maintenance, DNS problems are often not singular. For example, if users report that a website is inaccessible, possible causes include: local DNS caching of old IPs, zone DNS server failure, authoritative server record errors, firewall blocking of port 53, or even domain expiration. Systematic troubleshooting should follow an inside-out order: first check the local hosts file and DNS settings, then test the local DNS server (e.g., 192.168.1.1), next test the upstream ISP's DNS, and finally test public DNS and authoritative servers. Automated testing scripts can improve efficiency, for example, by periodically checking the resolution of critical domain names:

#!/bin/bash

DOMAINS=("example.com" "www.example.com" "mail.example.com")

for domain in "${DOMAINS[@]}"; do

ip=$(dig +short $domain)

if [ -z "$ip" ]; then

echo "$(date): $domain resolution failed" >> dns_monitor.log

else

echo "$(date): $domain -> $ip" >> dns_monitor.log

fi

done

Adding such a script to a scheduled task will continuously monitor the DNS status. When encountering cross-regional issues, online DNS monitoring platforms (such as ThousandEyes and DNSChecker) can provide a global perspective, displaying differences in resolution across different countries and ISPs.

The ultimate goal of DNS testing is to ensure fast, accurate, secure, and stable domain name resolution. It is not only a troubleshooting tool but also a preventative measure. Regular, comprehensive DNS testing can avoid many potential service interruptions. With the widespread adoption of new technologies such as HTTP/3, DoH (DNS over HTTPS), and DoT (DNS over TLS), DNS testing is constantly evolving, but the core objective remains unchanged: ensuring that users can successfully reach the correct destination after entering a domain name.