When you find that your domain name is being resolved to an incorrect IP address in certain regions or network environments (such as an irrelevant advertising page or a politically sensitive statement), while access is normal in other regions, your domain name has likely suffered from "domain poisoning," technically known as DNS cache poisoning or DNS hijacking. This isn't your server being hacked; rather, someone has "polluted" the "roadmap" system leading to your domain. Understanding its principles and mastering the handling process is key to restoring service and ensuring user access.

The Principle of Domain Poisoning: Tampering with the "Internet Phone Book"

To understand poisoning, you must first understand how the Domain Name System (DNS) works. When a user enters your domain name (e.g., `www.example.com`) into their browser, their device doesn't directly know where the server is. It needs to query the globally distributed DNS system to obtain the corresponding IP address. This process is like a multi-level routing: the user's device first queries the local DNS server (usually provided by the ISP). If the local server doesn't know, it recursively queries upstream through the root DNS, top-level domain DNS (.com), and authoritative DNS (your domain provider), eventually returning the correct IP address to the user and caching it for a period to improve subsequent query speeds.

Domain name poisoning occurs within this query chain. Attackers or certain network devices inject fake responses at some point in the DNS query process, redirecting your domain name to an incorrect IP address. The main poisoning methods include:

1. Man-in-the-middle attack: Malicious devices forge DNS response packets in the link between the user and the local DNS server, returning an incorrect IP address before the genuine response.

2. Recursive DNS server poisoning: Attackers use techniques (such as predicting DNS transaction IDs and port numbers) to send a large number of forged responses to upstream public or ISP-controlled recursive DNS servers, poisoning their caches and causing all users querying that server to receive incorrect results.

3. Authoritative DNS attack or tampering: Directly attacking your domain registrar or authoritative DNS server to modify DNS records; this is the most thorough but relatively rare form of poisoning.

Poisoning is characterized by its regional and network specificity. Because it relies on polluting specific DNS server caches, it may only affect users of a particular ISP, a specific province, or even a specific city, while users using clean DNS (such as `8.8.8.8`) will access the service normally.

Pollution Diagnosis and Evidence Collection: Identifying the "Pollution Source"

The first step in dealing with pollution is to identify and locate the scope of the pollution. You need to probe from different perspectives.

1. Global DNS Resolution Comparison: This is the most crucial diagnostic method. Use online DNS propagation checking tools (such as `viewdns.info`, `dnschecker.org`) to query your domain's A records from dozens of different locations and networks worldwide. If most locations return the correct IP, but specific regions (such as a certain ISP in China) generally return a different, incorrect IP, this is typical evidence of regional pollution.

2. Command-line Deep Probing: Use the `dig` or `nslookup` commands locally, specifying different public DNS servers for queries, and compare the results. By tracing the entire DNS resolution process, you can determine which hop the pollution occurred at.

# Use the `dig` command to query and specify a clean public DNS (such as Google DNS)

dig @8.8.8.8 www.example.com A

# Trace the complete DNS resolution path

dig +trace www.example.com

# Check if authoritative DNS records have been tampered with

dig www.example.com NS

3. Collect user reports: Through customer service channels, ask affected users to provide the following information:

The IP address obtained by pinging your domain name.

The network operator they use (such as China Telecom, China Unicom).

The results of the query using the `nslookup` command (instruct them to run `nslookup www.example.com` and `nslookup www.example.com 8.8.8.8` and take screenshots).

Handling process and countermeasures: Repair the "roadmark system"

After confirming contamination, a systematic process should be followed for handling.

Phase 1: Emergency Containment and Information Verification

Immediately log in to your domain registrar or DNS hosting provider's console to check if A records, NS records, etc., have been modified without authorization. If modified, correct them immediately and enable two-factor authentication. Check your servers and office network for malware that has tampered with local hosts files or DNS settings.

Phase 2: Technical Countermeasures and Mitigation

If the authoritative records are correct, but the DNS resolution is poisoned, the poisoning occurs at the recursive layer. You need to implement technical countermeasures; the core goal is to help clean DNS servers and users bypass the poisoned recursive servers.

DNSSEC verifies the authenticity of DNS responses using digital signatures. While it cannot prevent poisoning, it allows recursive servers with DNSSEC verification enabled (such as `8.8.8.8`) to automatically reject tampered responses, making it a fundamental defense. Enable and configure DNSSEC on your authoritative DNS servers.

Enable DNS over HTTPS/TLS: Encourage users to configure DoH or DoT in their applications. These protocols encrypt DNS queries, preventing man-in-the-middle attacks and tampering. They are supported by most major browsers and operating systems. If the contamination is caused by a DDoS attack targeting your authoritative DNS, consider migrating to a professional high-defense DNS service provider with strong DDoS protection and an Anycast network. Their globally distributed nodes and powerful cleaning capabilities can effectively resist attacks.

For affected users, guide them through social media, announcements, and other channels to manually set the DNS servers on their devices or routers to uncontaminated public DNS servers.

Phase Three: Monitoring and Reporting

Use monitoring tools to continuously check the domain name resolution status from various monitoring points and plot the recovery time curve. Compile detailed evidence of contamination (error IP, occurrence time, affected ISPs, traceroute, and dig logs) into a report. If it is determined to be a malicious attack, report it to agencies such as the National Computer Network Emergency Response Technical Team/Coordination Center of China (CNCERT/CC). If it is a problem specific to a particular ISP, try contacting their technical department (although the response is uncertain).

Effect Evaluation and Procedure Summary

The success of the handling can be evaluated using the following indicators:

Core Indicator: Global DNS propagation checks show that the resolution results at all monitoring points have returned to normal and are consistent.

User Feedback: Users in affected areas report that access has returned to normal.

Long-term monitoring: No recurrence occurred within the following 72 hours.

To mitigate future risks, long-term procedures should be established:

1. Prevention Procedures: Mandate DNSSEC for all critical domains; use reputable, secure enterprise-grade DNS hosting; enable strong passwords and two-factor authentication for domains and DNS hosting accounts.

2. Detection Procedures: Deploy automated DNS monitoring, checking resolution results multiple times daily from the global network, setting alerts, and immediately triggering support tickets upon detecting abnormal resolutions.

3. Response Procedures: Establish a clear internal response flowchart, clearly defining the responsibilities of the security, operations, and customer service teams. Prepare a list of technical countermeasures and user communication templates for rapid action in the event of an incident.

Domain poisoning is essentially an attack and defense against information accuracy. By understanding the principles of its distributed poisoning and adopting a combined strategy from authoritative hardening (DNSSEC), communication channel encryption (DoH/DoT), to endpoint guidance (DNS switching), you can effectively defend against most poisoning attacks, ensuring your users can always find your service through the correct "landmarks." Staying vigilant, proactively monitoring, and responding quickly are the only ways to manage these invisible threats.