DNS pollution refers to the tampering or forgery of DNS query results, leading to inaccurate domain name resolution and affecting normal network access. When a user visits a website, DNS pollution causes a domain name that should resolve to the correct IP address to return a malicious IP address, redirecting the user's request to the wrong target or even a malicious site. The main forms of DNS pollution include cache poisoning, DNS redirection, and DNS spoofing. To quickly and effectively troubleshoot webpage loading failures caused by DNS pollution, we can follow these steps:

　　Step 1: Confirm the Existence of DNS Pollution

　　When a user cannot load a webpage, the first step is to confirm whether the resolution error is due to DNS pollution. This can be done manually using tools like nslookup or dig to check if domain name resolution is normal. These tools can bypass local DNS caching and directly send query requests to the specified DNS server, helping users confirm whether it's a DNS problem.

　　For example, using nslookup to query the IP address of a domain name:

nslookup www.example.com

　　If the returned IP address is inconsistent with the expected one, it may indicate DNS pollution. For example, if a user visits www.example.com and receives an IP address that does not match the website, it's almost certainly a DNS pollution issue.

　　Another commonly used tool is `dig`, which provides more detailed DNS resolution information to help pinpoint the problem. For example, running:

dig www.example.com

　　If the query results are inconsistent with public DNS resolution, or if the returned IP address is abnormal (e.g., from a suspicious IP range), it indicates a possible DNS poisoning.

　　Step Two: Verify the DNS Resolution Server's Functionality

　　DNS poisoning typically occurs when using the default DNS resolution server provided by your ISP, as these servers may be attacked or tampered with by the operator. In this case, users can try switching DNS servers to use a reliable public DNS service to verify if the problem is resolved. After changing the DNS server, use nslookup or dig to check domain name resolution again. If the domain name is resolved correctly after changing the DNS server, then the problem can be identified as DNS poisoning, and the source of the poisoning is at the default DNS server.

　　Step Three: Check the Local DNS Cache

　　If the local machine has cached incorrect DNS resolution results, even if the DNS server recovers, the browser will still incorrectly resolve the domain name to an incorrect IP address. Therefore, clearing the local DNS cache is another important step in troubleshooting.

　　On Windows systems, the command-line tool ipconfig can be used to clear the DNS cache. Run the following command:

ipconfig /flushdns

　　On Mac OS or Linux systems, you can clear the cache using the following command:

sudo dscacheutil -flushcache

　　After clearing the cache, try accessing the website again to check if the problem is resolved.

　　Step Four: Analyze DNS Server Logs

　　If you manage your own DNS server or use a custom DNS resolution service, you can further investigate the cause of the pollution by analyzing the DNS server logs. DNS logs typically record all query requests, response results, and related error messages. Examining these logs can help us identify abnormal query behavior.

　　For example, when using DNS servers such as Unbound or Bind, the relevant log files (usually located in the /var/log/ directory) record all query activity. We can use the grep command to view the resolution records for a specific domain name:

grep "example.com" /var/log/named.log

　　If you frequently encounter warning messages such as "unexpected answer" or "cache poisoning," it indicates that your DNS server may have been poisoned.

　　Step 5: Check ISP and Carrier DNS Services

　　In some cases, DNS poisoning may be caused by ISPs (Internet Service Providers) or other upstream carriers. Carrier-level DNS poisoning is relatively common, especially in certain countries or regions. Carriers may tamper with or hijack DNS responses to control information flow, preventing users from accessing certain websites.

　　In such cases, users can use proxy services to bypass the carrier's DNS resolution and check if they can access the target website normally. To avoid such problems, users can switch to a trusted public DNS server or directly use DNS services based on encrypted DNS protocols such as DoH or DoT to prevent carrier interference.

　　Step 6: Prevent DNS Poisoning

　　If you frequently encounter DNS poisoning, you can take a series of preventative measures. First, enabling DNSSEC can effectively prevent DNS spoofing and tampering. DNSSEC prevents DNS poisoning by adding signature information to DNS responses to ensure the integrity of domain name resolution.

　　Additionally, using DoH or DoT can encrypt DNS requests, preventing man-in-the-middle attacks and DNS hijacking. These encrypted DNS protocols effectively protect privacy and security during the DNS resolution process, avoiding the effects of DNS poisoning.

　　Step 7: Analyze with Packet Capture Tools

　　For more complex DNS poisoning issues, packet capture tools (such as Wireshark) can be used to capture packets during DNS communication. This helps us understand detailed information about requests and responses, and detect if any abnormal DNS responses have been injected. For example, Wireshark can help us view the source IP address, destination address, and returned DNS response content of DNS query requests, thereby determining if there are malicious DNS responses.

　　When capturing with Wireshark, selecting the "DNS" protocol for filtering allows for a clear view of the DNS request and response packet content. If the returned IP address does not match expectations, or if the response content contains abnormal domain names or IP address information, the source of the poisoning can be further identified.

　　DNS poisoning is a common but easily overlooked problem in network security. It can cause websites to fail to load, affecting users' normal internet browsing experience. By following the correct troubleshooting steps, including verifying DNS servers, clearing DNS caches, analyzing DNS logs, and switching to public DNS services, users can quickly identify and resolve problems caused by DNS poisoning. Meanwhile, adopting security measures such as DNSSEC and DoH/DoT can improve website security in the long run and reduce the risks associated with DNS poisoning. By continuously strengthening DNS monitoring and protection, the security of the network environment will be effectively guaranteed.