Many people may have encountered this situation: others can access the same website normally, but you can't. Or, after entering a certain domain name, the page redirects to a strange website. Sometimes, web content loads abnormally, and you can't find the cause. Besides problems with the website itself and local network instability, the most common cause of these problems is DNS pollution. DNS pollution, also known as DNS spoofing, essentially tampering with DNS resolution results, causing users to receive incorrect IP addresses, preventing them from accessing the actual website and potentially redirecting them to phishing or malicious sites. Therefore, learning to detect DNS pollution can not only help you quickly identify network issues but also protect your online security to a certain extent.
What is DNS pollution?
The Domain Name System (DNS) is the internet's "phone book." When you enter a URL into your browser, the system first uses DNS resolution to convert the domain name into its corresponding IP address before accessing the target server. If the IP address returned during this process is tampered with, then the address you receive is not the actual address of the target website. This is DNS pollution. Poisoning can be caused by network equipment errors or human intervention. Regardless of the cause, it directly affects users' ability to access internet services.
How to detect DNS pollution:
The most intuitive way to detect DNS pollution is to compare the results returned by different resolution sources. Typically, a user's DNS request first passes through the local router, the carrier's DNS server, and then the authoritative DNS. If any link is tampered with, the returned IP address may be incorrect. Therefore, you can use the following methods to determine:
A common method is to use command-line tools such as nslookup or dig. For example, enter nslookup www.example.com 8.8.8.8 in the command line to force the use of Google's public DNS servers for resolution. If the returned IP address differs from the address obtained by the carrier's default DNS resolution, and the address returned by the carrier is inaccessible, while the address returned by the public DNS works properly, it is likely that the local DNS is polluted. The dig tool is more powerful and can display detailed information during the query process, making it easy to compare.
Another method is to compare the results of multiple DNS resolutions. In addition to Google's 8.8.8.8, common public DNS servers include Cloudflare's 1.1.1.1 and Quad9's 9.9.9.9. Users can query these DNS servers simultaneously to check whether they receive the same results. If a particular type of result differs significantly, especially if an unreachable IP address is returned, it's generally a good indicator of DNS pollution.
In addition to command-line tools, users can also use online testing services. Many websites offer DNS testing features, which simultaneously call multiple DNS servers for resolution and compare the results. If the local resolution differs significantly from the authoritative resolution, the likelihood of DNS pollution is high. These tools are very user-friendly for users unfamiliar with command lines and provide a more intuitive presentation.
Sometimes, DNS pollution manifests not as complete inaccessibility, but rather as extremely slow access speeds and incomplete content loading. This situation is more complex to detect. Users can try testing the resolved IP address using the ping or tracert (Windows)/traceroute (Linux, Mac) commands. If the IP address itself is unusual, pinging may not be possible at all; or if tracert shows unusual packet hops within the network, these signs indicate that the DNS resolution result may be inaccurate.
Another approach is to combine actual access conditions. For example, if a well-known international website is accessible almost everywhere but inaccessible only locally, after ruling out local network fluctuations, DNS pollution is a possible cause. This is especially true if the IP address a domain name resolves to is clearly incorrect, such as a foreign website resolving to a random domestic IP address or a reserved address. These are typical signs of DNS pollution.
In addition to proactive monitoring, operations personnel or technical enthusiasts can also implement longer-term monitoring solutions. For example, they can regularly perform multi-point resolutions on key domain names, record the results, and create a comparison table. If unusual IP addresses are returned during certain time periods, an alert can be immediately issued. This approach is often used in enterprise network security protection, as DNS pollution not only impacts user experience but can also cause business systems to malfunction.
Temporary Solutions for DNS Pollution:
After confirming that DNS pollution has occurred, users can try switching DNS servers as a temporary solution. For example, manually modifying local network settings to change the DNS to 8.8.8.8 or 1.1.1.1 can often circumvent the problem. However, this isn't always effective, as the problem can occur at the higher network transport layer, causing data packets to be tampered with during queries even when using a public DNS. In this case, you can consider using an encrypted DNS protocol, such as DoH (DNS over HTTPS) or DoT (DNS over TLS). These encrypt DNS requests to prevent tampering during transmission. Modern browsers such as Chrome and Firefox already support DoH; users simply need to enable it in their settings to mitigate DNS pollution to a certain extent.
It's important to note that DNS pollution isn't the same as a DNS server failure. A server failure typically causes resolution failures for all users, while pollution is often localized or targeted, sometimes targeting specific websites or parts of the network. Therefore, if you suspect DNS anomalies, it's best to first ask other users around you or use a proxy network to access the same website. This will make it easier to identify the root cause.
FAQ:
Q: What's the difference between DNS pollution and a DNS server failure?
A: DNS pollution occurs when resolution results are manipulated by human or unauthorized means, resulting in incorrect IP addresses being returned. A server failure, on the other hand, occurs when the DNS service itself is unavailable, typically causing all resolution requests to fail.
Q: Can I avoid DNS poisoning by switching to 8.8.8.8?
A: Sometimes you can, but if the poisoning occurs at the transport layer, even with 8.8.8.8, the returned results can still be tampered with. A more effective approach is to use an encrypted DNS protocol.
Q: How can I tell if the returned IP address is poisoned?
A: You can check the IP's location. If the resolution result doesn't match the website's location at all, or points to a reserved address (such as 127.0.0.1), it's almost certainly poisoned.
Q: Does DNS poisoning affect all websites?
A: No, it usually only targets certain domains, and most domains will still resolve normally.
