Causes and solutions for router DNS poisoning

　　DNS pollution in routers is a common problem encountered by many website owners and ordinary users, but it's often difficult to troubleshoot immediately. Especially when a website suddenly becomes inaccessible, domain name resolution is abnormal, or access redirects to unfamiliar pages, many people tend to suspect server or program failures, neglecting the underlying DNS resolution process. Once DNS pollution occurs at the router level, not only will all devices on the network be affected, but it can also lead to website traffic loss, a degraded user experience, and even security risks. This article will systematically explain the principles, causes, typical symptoms, troubleshooting steps, and complete solutions for router DNS pollution, making it easy for even novice website owners to understand and handle themselves.

　　DNS is essentially a "Domain Name System," responsible for translating easily remembered domain names into server IP addresses. When you enter a website address in your browser, the device first queries the DNS server for the corresponding IP address before establishing a connection. If the DNS returns an incorrect IP address, access will naturally fail. DNS pollution occurs when, during the resolution process, the correct IP address that should have been returned is tampered with, hijacked, or replaced with an incorrect address.

　　In home or office networks, routers typically act as DNS forwarders. Terminal devices (computers, mobile phones) obtain their DNS addresses from the router by default, and the router then queries the upstream DNS server. If the router's DNS settings are tampered with, or the router itself is compromised, all devices connected to that router will be affected. Therefore, router DNS poisoning is characterized by its "centralized" and "hidden" nature.

　　Common causes of router DNS poisoning include the following:

　　First, malicious intrusion into the router. This is one of the most common causes. If the router's management backend uses a weak password, such as admin/admin, a hacker can brute-force it to gain access and change the DNS server address to a malicious one. Afterward, any website accessed by the user will first pass through the attacker-controlled DNS server, resulting in hijacking or the insertion of advertisements.

　　Second, router firmware vulnerabilities. Some older router models have not been updated in a long time and have remote execution vulnerabilities, allowing attackers to directly tamper with DNS configurations. Many home networks do not upgrade their router systems for several years, a very common occurrence.

　　Third, DNS interference at the ISP level. In some regional network environments, ISPs may redirect or hijack DNS for advertising push or traffic management purposes. Technically, this situation doesn't fall under the category of "router attack," but the user experience is consistent.

　　Fourth, malware tampering. Some Trojan programs automatically modify the router's DNS, and even automatically scan and modify the configurations of routing devices within the local area network. Once a computer is infected, the entire network will be affected.

　　Fifth, cache pollution. If incorrect records are injected into the cache server during DNS lookups, it can also lead to short-term access anomalies. This often occurs in public DNS or intermediate forwarding nodes.

　　After understanding the causes, let's look at the typical manifestations of DNS pollution.

　　The most obvious symptom is that the website is inaccessible, but the server is actually functioning normally. For example, the server can be pinged, and direct IP connection works, but accessing it via the domain name is impossible. A second symptom is being redirected to an advertising page or phishing website. A third situation is unstable resolution results, with the same domain name displaying different IPs on different devices. A fourth is HTTPS certificate errors because the website being accessed is actually linked to an incorrect IP.

　　If you are a website owner, when users report that "the website is inaccessible," it is recommended to perform the following troubleshooting steps immediately.

　　Step 1: Use local commands to check the DNS resolution results. In Windows, open the command prompt and type:

　　nslookup yourdomain.com

　　Check if the returned IP address matches the server's real IP address. If they don't match, DNS poisoning is likely.

　　Step 2: Change the public DNS for testing. Change your computer's DNS to 8.8.8.8 or 1.1.1.1, then resolve again. If the problem disappears, the original DNS was faulty.

　　Step 3: Access the website directly via the IP address. If the IP address opens normally but the domain name doesn't, it's likely a DNS resolution issue.

　　Step 4: Log in to your router's admin panel and check the DNS settings. Check if the WAN port DNS has been changed to an unfamiliar IP address, such as an uncommon foreign address or a strange combination of numbers.

　　Once you confirm DNS poisoning on your router, follow these steps to completely resolve the issue:

　　First, immediately change the router administrator password. The password should contain uppercase and lowercase letters, numbers, and symbols, and be at least 12 characters long. Avoid using the default username.

　　Second, restore factory settings. If your DNS has been maliciously tampered with, the safest method is to press and hold the router's reset button for more than 10 seconds to reset it, and then reconfigure your network.

　　Third, manually configure a trusted DNS server.

　　Fourth, upgrade your router firmware. Download the latest firmware version from the official website and patch any potential vulnerabilities.

　　Fifth, disable remote management. If you don't need remote access to the router's admin panel, it's recommended to disable the WAN port's remote management port to reduce the risk of attack.

　　Sixth, check your DNS settings regularly. It's recommended to log in to your router's admin panel monthly to confirm that the DNS address has not been tampered with.

　　For website administrators, you can also strengthen protection at the server level. For example, enable HTTPS forced redirection, deploy DNSSEC to prevent resolution tampering, and use a multi-node DNS service provider to improve stability. You can also enable access log monitoring on the server; if a large number of abnormal IP accesses are detected, further investigation can be conducted.

　　Another advanced solution is to use encrypted DNS technology, such as DoH (DNS over HTTPS) or DoT (DNS over TLS). This method can prevent intermediate nodes from tampering with the resolution results, improving security. Modern browsers such as Chrome and Firefox support enabling secure DNS features.

　　From an SEO perspective, persistent DNS pollution directly impacts website indexing and ranking. Search engine crawlers will access the site abnormally, reducing crawling frequency and potentially marking the website as unstable. Therefore, stable and reliable DNS resolution is a fundamental infrastructure for website operation.

　　Many novice webmasters neglect network infrastructure, focusing only on program and server configurations. However, the stability of domain name resolution is equally crucial. This is especially important when using overseas servers or cross-border bandwidth, ensuring a smooth DNS connection.

　　In summary, the essence of router DNS pollution is the tampering with the resolution link, affecting the entire local area network. Common causes include router intrusion, firmware vulnerabilities, malware, and ISP interference. The core solution involves three steps: confirming the resolution anomaly, restoring router security settings, and using trusted DNS and strengthening protection.

　　With the correct troubleshooting methods, even novice webmasters without technical backgrounds can locate and resolve the problem within ten minutes.

　　Frequently Asked Questions

　　Q: Why is the server working normally, but the website is inaccessible?

　　A: It may be a DNS resolution error, with the domain name pointing to the wrong IP address. It is recommended to use nslookup to check the DNS resolution results.

　　Q: Will DNS poisoning affect all devices?

　　A: If the poisoning occurs at the router level, then all devices connected to that router will be affected.

　　Q: Will changing to a public DNS always solve the problem?

　　A: If the source of the poisoning is at the local router or ISP level, changing the DNS can usually alleviate the problem. However, if the router is maliciously controlled, a factory reset is required.

　　Q: Will DNS poisoning affect SEO?

　　A: Yes. Frequent search engine failures may reduce crawling frequency and even affect rankings.

　　Q: How to avoid DNS poisoning in the long term?

　　A: Regularly changing router passwords, upgrading firmware, disabling remote management, and using encrypted DNS are the most effective long-term protection measures.

　　Once you understand the principles of DNS resolution, you'll find that many "websites won't open" problems are actually not complicated. The key lies in having a clear troubleshooting approach and prioritizing underlying network security. For website owners, server performance is important, but DNS stability is equally fundamental to stable website operation.