What causes DNS cache poisoning? A summary of solutions.

　　What causes DNS cache poisoning? This is a common question many website owners encounter when their websites suddenly malfunction, domain name redirects incorrectly, or HTTPS certificate errors occur. Even though the server is working properly, the program hasn't been modified, and even the domain name resolution records haven't been altered, user access still fails. This situation is often related to DNS cache poisoning. For novice website owners, DNS cache poisoning sounds complicated, but in reality, understanding the principles allows for quick problem identification and resolution.

　　To understand DNS cache poisoning, you must first understand the role of DNS caching. The essence of DNS resolution is converting domain names into IP addresses. To speed up access and reduce the pressure of repeated queries, DNS servers, routers, operating systems, and even browsers cache the resolution results. When a domain name is resolved for the first time, the result is saved for a period of time, and subsequent accesses don't require a re-query but directly use the cached data.

　　The DNS caching mechanism itself is designed to improve performance, but if cached records are maliciously tampered with or forged, DNS cache poisoning occurs. DNS cache poisoning refers to an attacker injecting incorrect resolution records into the DNS server or caching system, causing the domain name to point to an incorrect IP address. When users access the domain, they are redirected to a server specified by the attacker. So, where exactly does DNS cache poisoning originate? It can occur at multiple levels.

　　The first level is a local computer cache problem. The operating system itself caches DNS resolution results. If the local cache is tampered with by malware, it will lead to abnormal access. This usually affects a single device.

　　The second level is a router cache problem. Many home or office routers forward and cache DNS requests. If the router is compromised or has vulnerabilities, and the cache records are poisoned, the entire local area network will be affected.

　　The third level is a problem with the ISP's DNS server. If the upstream DNS server cache is attacked, a large number of users will experience abnormal resolution. This situation has a wider impact.

　　The fourth level is a vulnerability in authoritative DNS or recursive DNS servers. If the DNS server does not have adequate security protection, such as not opening random ports or not verifying transaction IDs, attackers may inject cache by forging response packets.

　　The fifth situation is hijacking in public Wi-Fi environments. Some insecure networks may actively tamper with DNS responses, redirecting users to advertising pages or phishing websites.

　　After understanding the source of the problem, let's look at the common manifestations of DNS cache poisoning.

　　The most typical symptom is that domain access redirects to an unfamiliar website. For example, accessing your own website but opening an advertising page or a fake page. The second situation is HTTPS certificate errors because the accessed IP is not the original server. The third manifestation is different results for different regions; some users experience normal access, while others experience abnormal access. The fourth situation is frequent changes in the resolution results within a short period.

　　As a website owner, how can you determine if you've encountered DNS cache poisoning? You can follow these steps to troubleshoot:

　　First, use commands to check the resolution results. Check if the returned IP matches the server's real IP.

　　Second, compare the resolution results using different DNS servers. For example, use 8.8.8.8 and 1.1.1.1 respectively. If a DNS server returns an abnormal IP, the problem may lie with that DNS node.

　　Third, clear the local DNS cache and test again. Then access the website again to see if it returns to normal.

　　Fourth, try changing your network environment, such as using mobile data to access the website. If it works normally on other networks, then it's very likely that the current network's DNS cache is poisoned. Once DNS cache poisoning is confirmed, targeted solutions can be implemented based on different levels of vulnerability.

　　If the problem is with the local computer's cache, the simplest method is to clear the DNS cache and perform a full system scan. Ensure no malware has tampered with system settings. Also, check the local hosts file to confirm no abnormal DNS records have been added.

　　If the problem is at the router level, it is recommended to immediately log in to the router's admin panel and check the DNS settings. If abnormal DNS addresses are found, restore factory settings and reconfigure the network. Simultaneously, change the administrator password, disable remote management, and upgrade the firmware.

　　If the problem is with the ISP's DNS, you can manually configure the public DNS. In a server environment, you can also specify a static DNS in the system to avoid using the default assigned DNS.

　　For businesses or website operators, it is recommended to use a professional DNS service provider and enable the DNSSEC function. DNSSEC can digitally sign and verify DNS responses, effectively preventing forged data from being accepted and technically defending against cache poisoning attacks.

　　Additionally, consider using DoH or DoT; these encrypted DNS technologies can prevent man-in-the-middle tampering of DNS responses, improving overall security.

　　To reduce the risk of DNS cache poisoning, website owners can establish the following long-term protection mechanisms:

　　First, choose a secure and reliable DNS service provider.

　　Second, regularly check domain name resolution records.

　　Third, enable DNSSEC.

　　Fourth, use HTTPS and enable HSTS.

　　Fifth, monitor website access logs and investigate any anomalies promptly.

　　Sixth, regularly update server and router systems.

　　Many novice website owners tend to overlook DNS-level security, focusing only on server configuration and website programs. However, domain name resolution is the first step in the access chain; if this step fails, even the most powerful server performance will be ineffective.

　　In summary, DNS cache poisoning can occur at multiple levels, including local devices, routers, ISP DNS, or public recursive servers. Essentially, it involves the tampering of cached resolution records. The solution should follow the principle of "layered investigation and step-by-step verification," gradually locating the source of the problem from the local machine to upstream. With the right methods, even website owners without extensive technical backgrounds can quickly resolve the issue.

　　Once you truly understand the principles behind DNS cache poisoning, you'll find it's not a mysterious technical issue, but rather a fundamental aspect of network security. Whether you're an individual user or a website owner, proper DNS security management can significantly reduce the risk of hijacking and attacks, ensuring stable website operation.