Many people have encountered a similar situation: a website works fine overseas, but no matter how hard you try, it won't open, even after switching browsers. Some call it DNS poisoning, while others call it network blocking. While the two sound similar, the principles and solutions are completely different. Failing to understand the difference can lead to significant troubleshooting errors.

　　DNS poisoning is a problem that occurs during domain name resolution. When a user enters a URL into a browser, the system must first convert the domain name into an IP address before accessing the target website. This process relies on DNS. If the DNS response is tampered with, the user will receive an incorrect IP address, preventing them from connecting to the actual website. DNS poisoning is characterized by a false resolution result. While the IP address appears to be an invalid one, it will either redirect to an error page. This results in the webpage failing to open or being redirected to a completely unrelated location.

　　Network blocking is a different matter. It doesn't rely on DNS manipulation; instead, it directly blocks communication between you and the target server at the network level. In other words, even if you know the website's real IP address, and even bypass DNS and enter the IP address directly, you still won't be able to connect. Network blocking can occur in many ways, such as dropping packets at the routing layer, denying certain IP addresses in firewall rules, or simply blocking entire IP ranges. The end result is that connection requests are forcibly blocked, manifesting as the website failing to load or even timing out.

　　The difference between the two can be more clearly illustrated with a real-life example. If you want to go to a restaurant, DNS pollution is like someone giving you an incorrect address. You follow it, but it turns out it's not the restaurant, so you can't get to the restaurant. Network blocking, on the other hand, is like knowing the restaurant's location, but the police have blocked the intersection. No matter which way you go, you'll be stopped, so you can't reach the restaurant. Both methods have in common: they prevent you from reaching your destination, but the former involves a problem with the "address lookup" stage, while the latter involves a problem with the "road accessibility" stage.

　　Symptoms of DNS pollution often manifest as a website resolving an IP address, but this IP address isn't the actual server. For example, if you ping a compromised website, an invalid address might be returned. However, if the network is blocked, the connection is often impossible to establish at all; it will either time out or be directly disconnected, and the ping command might not even work. This is a key difference when troubleshooting the two.

　　The solutions are also quite different. If you encounter DNS poisoning, the solution is usually to circumvent the tampered resolution results. The most common method is to switch to a public DNS, such as Google's 8.8.8.8 or Cloudflare's 1.1.1.1, which can sometimes circumvent local poisoning. Furthermore, you can use DoH (DNS over HTTPS) or DoT (DNS over TLS) to encrypt DNS requests, making it difficult for third parties to insert spoofed results. If the poisoning is more severe, you can directly modify the Hosts file to hard-code the correct IP address on the local system, eliminating the browser's reliance on external DNS queries.

　　However, if the network is blocked, the above methods are largely ineffective. No matter how many real IP addresses you obtain, if the data packets can't reach the target server, the result will be a failure. A more common approach to circumventing network blocking is to use a proxy to route traffic through an unblocked transit node, which then reaches the target server. This prevents local network-layer blocking from affecting the connection between you and the target site. However, this approach often increases latency and isn't always feasible in certain environments.

　　Furthermore, the two methods differ in their causes. DNS poisoning is sometimes a hacker attack, such as DNS cache poisoning, where attackers inject false data to redirect users to malicious websites and steal passwords. It can also be a manually implemented filtering policy to block access to specific websites. Network blocking is more often driven by administrative or policy requirements, or for security reasons, such as preventing attacks from malicious IP addresses or restricting cross-border access. In other words, DNS poisoning can be malicious or part of an administrative policy, while network blocking is typically a simple denial of access.

　　For the average user, the most immediate effect is simply being unable to access the website, but the different mechanisms behind this mean different countermeasures. Sometimes you think your network is blocked, but it's actually just DNS pollution. In this case, changing the DNS or modifying the hosts setting can resolve the issue. Sometimes, even if you repeatedly change the DNS and still can't access your website, it could be a complete network block, requiring a proxy to circumvent. Understanding this is crucial; otherwise, you can easily waste a lot of time on the wrong track.

　　The biggest difference between DNS pollution and network blocking is that the former involves tampering with the resolution process, causing you to misdirect your traffic, while the latter involves a forced blockage of the transmission process, preventing you from reaching your destination. One involves a "wrong address," while the other involves a "blocked path." While both can prevent you from accessing websites, the strategies and potential risks are completely different. Understanding this not only helps us troubleshoot issues more quickly but also improves internet security to a certain extent.

　　Some FAQs about DNS pollution:

　　Q1: How can I tell if it's DNS pollution or a network block?

　　A1: The easiest way is to directly ping or use nslookup. If you can resolve the IP address but can't access it, and the IP is clearly incorrect, it's likely DNS pollution. If the connection times out immediately or all IP addresses are unreachable, it's more likely that the network is blocked.

　　Q2: Can modifying the Hosts file resolve a network block?

　　A2: No. Modifying the Hosts file only resolves DNS pollution, as it bypasses the resolution process, but it has no effect on actual network blocking.

　　Q3: Which is more dangerous?

　　A3: From a security perspective, DNS pollution is more dangerous because it could potentially lead you to phishing websites without you realizing it.

　　Q4: What is the relationship between DNS pollution and carrier hijacking?

　　A4: Carrier hijacking is often accomplished by tampering with DNS and is essentially a form of DNS pollution.

　　Q5: Can I experience both DNS pollution and a network block at the same time?

　　A5: It's possible. Some websites are both polluted during resolution and blocked during transmission. In these cases, a combination of measures is required to access them.